My last post on this blog [LINK] about my intuitions for climates was over 1 year ago, in 2024. Since then, I have continued to read more about climates of the world. Later in 2024, I was particularly more careful to look at maps of mean surface-level wind velocities. This led me to start to carefully catalog the climates of the world and attempt to explain them based on mean surface-level pressures & wind velocities along with qualitative ideas about the differences between air masses at different temperatures & humidity levels. I felt satisfied doing so for Oceania as well as for Africa in the southern hemisphere. I did so for South America at the middle latitudes (which is entirely within the southern hemisphere) too, and I thought of continuing through tropical latitudes in South America, near-equatorial latitudes in Africa, and thereafter all tropical, subtropical, middle, and subpolar latitudes in the northern hemisphere. However, as I looked more carefully at these maps and compared them to actual climate data from various locations, I started to think that my understanding of these climactic processes is too limited, especially by my focus on qualitative understanding of surface-level phenomena, to be able to come up with accurate explanations. (Even looking back at the post linked at the beginning of this paragraph and even older posts linked within that post, I can see how many things I have said in those posts that I know now to be inaccurate.) Because of that, I shelved the idea of continuing with these detailed explanations until much more recently, when I started looking more carefully at maps of sea/ocean surface temperatures and at calculations of air density at various pressure levels, humidity levels, and temperatures. This made it possible for me to reinforce my intuitions about temperatures & precipitation distributions at various locations in the aspects in which they were correct and fix them in the aspects in which they were wrong. Thus, this blog post is meant to be that originally-intended compendium of explanations for climates in various parts of each comment in tropical, subtropical, middle, and subpolar latitudes (excluding Antarctica).
The sources that I used were many relevant pages from Wikipedia, the Columbia University interactive maps of mean monthly wind velocities [LINK] & mean monthly sea/ocean surface temperatures [LINK], the European Centre for Medium-range Weather Forecast static global maps of mean surface-level air pressures in different astronomical seasons [LINK] (though this website has very recently started displaying a warning that the maps are now out of date), and the OmniCalc air density calculator [LINK]. Again, I am not a trained climatologist or meteorologist; I can't guarantee that this information is accurate, and I can only say that my intuitions seem through my limited understanding to align with superficial aspects of more detailed explanations. Follow the jump to see these explanations.
Equilibrium parameters of air
There are a few equilibrium parameters of air to keep in mind. These are as follows.
Dependence of air temperature on elevation
The primary driver of atmospheric dynamics, and therefore of climates on Earth, is incident electromagnetic radiation (light at various wavelengths) from the sun. The specific heat capacity of air is low enough compared to that of water & land that most atmospheric dynamics come from the heating of the water & land. This means that air will be warmer closer to the surface and its temperature will decrease with increasing elevation until it reaches a local (in the mathematical sense with respect to elevation as the independent variable for that column of air) minimum; above that elevation, the air temperature will increase to some extent because the air's greater distance from the surface matters less than its lesser distance from the sun, though it will decrease again with increasing elevation when its density becomes so small as to prevent it from absorbing significant heat from the sun. To recapitulate, under typical conditions, air temperature will be highest (as a global maximum in the mathematical sense for that air column) near the surface & lowest in what is essentially outer space, but with respect to increasing elevation, it will decrease to a local minimum, then increase to a different local maximum, and finally decrease again to the temperature of marginal matter & radiation in outer space.
Although the surface temperature drives most air temperature gradients with elevation, there is a contribution from the effect of gravity on the air column too. As the gravitational field of the Earth pulls an air column down, air pressure & density decrease with increasing elevation (as there is less air above pushing down with increasing elevation). The Earth's gravitational field does work on the air column, and the approximate absence of further heating upon the system (as the heating of air directly by solar radiation close to the surface is negligible compared to indirect heating by the surface as well as the work done by the Earth's gravitational field on the air column) makes this an adiabatic process, meaning that the internal energy of the gas changes only because of the work done on or by it. The effects of this contribution to the internal energy upon the temperature can usually be neglected for a static air column, but these adiabatic processes are much more important for air masses moving up or down hills or mountains or otherwise rising or falling through the atmosphere. In particular, adiabatic expansion means that when other thermodynamic parameters apart from pressure, density, and temperature are held constant, a column of air that adiabatically expands or rises will see a decrease in temperature, and a column of air that adiabatically contracts or falls will see an increase in temperature.
It is possible, under certain conditions & in smaller areas, for the global maximum (in the mathematical sense) temperature of an air column with respect to elevation to emerge some distance above the surface and for temperature from the surface up to that point to increase. This is known as a temperature inversion. There are a few different ways that this situation can arise, and these will be discussed in later sections of this post as appropriate, though it is worth noting that inversions are usually on much smaller length/area scales than synoptic scales (length scales of around 1,000 kilometers or more).
Dependence of air density on pressure, temperature, and humidity
Holding all other variables constant, increasing pressure increases air density. This can be seen from the ideal gas law \( P = \mathrm{k}_{\mathrm{B}} T\rho \) where \( P \) is the pressure, \( T \) is the absolute temperature, and \( \rho = N/V \) is the number density (and \( \mathrm{k}_{\mathrm{B}} \) is the Boltzmann constant, which is not a thermodynamic variable). This can be important for air movement over synoptic scales, but it is usually less important for local air movement because pressures at much smaller scales are usually close to uniform, so more significant differences in density between different air masses at smaller length scales are usually due to other factors.
Holding all other variables constant, increasing temperature decreases air density. This can also be seen from the ideal gas law.
Finally, holding all other variables constant, increasing relative humidity decreases air density. This is because water molecules have lower mass than most other atomic or molecular constituents of air, including diatomic nitrogen, diatomic oxygen, [monatomic] argon, and carbon dioxide. That said, there is a slight subtlety in making this statement, as the air density relevant in this statement is the mass density, while the ideal gas law uses the number density \( \rho = N/V \).
What happens when air masses of different densities meet
If multiple fluids (gases or liquids) of different densities meet in an approximately constant gravitational field (which is taken to point down), they tend to assort themselves such that the fluid of the highest density is at the bottom and every fluid above is of a lower density. This is a similar behavior as what can be seen in air columns which can otherwise be taken to be homogeneous, as the lessening of air higher in the column leads to a decrease in the density with increasing elevation. If different fluids are miscible, meaning that they can dissolve into each other, then they will indeed dissolve into each other, but during the transient period where the mixing is still happening, the denser fluids will sink toward the bottom and there will be a density gradient; this can be seen when isopropyl alcohol is added to water (with the former being less dense than the latter) around room temperature. If different fluids are immiscible (meaning "not miscible"), then they will separate into layers in which each layer will be less dense than the layer below it and more dense than the layer; this can be seen when water is poured over oil (or vice versa) at room temperature.
Air masses of different temperatures & humidity levels are miscible. That said, because these air masses may have significant speeds relative to the ground, they might not fully mix before one partially goes past the other. In any case, as an idealization for the sake of intuition, this post discusses the mixing of different air masses as if they were immisicible fluids, as it is easier to picture certain atmospheric phenomena as if these air masses separated clearly into different layers.
Dependence of moisture content in air on temperature
This post involves analyses of air masses of different humidity levels. For simplicity of intuition in these analyses, this post assumes that a moist air mass coming from over a sea/ocean will have the same temperature as that sea/ocean and will have a relative humidity of 100% and that a dry air mass over land at a given temperature will have a relative humidity of 2% (as it was a little annoying in the OmniCalc air density calculator to keep switching between moist & dry air mass settings to compare densities, so I approximated dry air masses as having relative humidity levels of 2%); of course, this does not account for the fact that realistic humidity levels even of dry air masses are more like 10-20% and that humidity levels can be more like 50-70% when moist air masses travel inland.
However, there are subtleties to be aware of when considering relative humidity. Holding external atmospheric pressure constant, the absolute amount of water vapor present in an air mass at a fixed relative humidity will increase with increasing temperature because the equilibrium partial pressure of water vapor increases with increasing temperature. For example, 1 kilogram of air at 100% relative humidity contains approximately 0.008, 0.015, and 0.028 kilograms of water at temperatures of 10, 20, and 30 degrees Celsius, respectively (showing the superlinear scaling of absolute humidity with increasing temperature). The absolute humidity can be quantified by the dew point (or frost point, if it is less than 0 degrees Celsius) of an air mass, which is always less than or equal to the temperature of the air mass itself and is the temperature to which the air mass must be brought for water vapor to begin to condense (or deposit directly as ice from the vapor, if it is less than 0 degrees Celsius). An air mass whose dew point is equal to its temperature has a relative humidity of 100%, and an air mass whose dew point is somewhat close to its temperature has a somewhat high relative humidity, though the preponderance of water on Earth means that it is practically impossible for a natural air mass to have a relative humidity of exactly 0%. This also means that if the temperature of an air mass increases but its dew point remains the same, then its relative humidity decreases.
Causes of precipitation
There are 3 major causes of precipitation; these are not mutually exclusive in their occurrence, as two or all three of them can occur simultaneously. These are frontal, convective, and orographic lifting precipitation, and are explained as follows.
Frontal precipitation
Frontal precipitation is so named because it arises from clashes of air masses at synoptic scales with differences in temperatures or humidity levels, which are known as fronts. These are often warm fronts or cold fronts. Warm fronts arise from warmer air moving relative to the surface toward a mass of colder air stationary over the surface, while cold fronts arise from colder air moving relative to the surface toward a mass of warmer air stationary over the surface.
Frontal precipitation is also known as stratiform precipitation because air masses of different temperatures or humidity layers are likely to have different temperatures, so upon colliding, they will likely form different horizontal layers (strata), and any condensed moisture (in the form of clouds) will likely be similarly layered. This is seen most commonly in warm fronts, because the typically greater density of colder air compared to warmer air makes it harder for a warm air mass moving in to fully dislodge the stationary cold air mass; instead, the warm air mass gradually rises above the cold air mass and then gradually mixes into it from above, and that boundary between air masses is horizontal or gently sloped (implying more clearly defined strata). This is less commonly seen in cold fronts, because the typically greater density of colder air compared to warmer air means that a cold air mass moving in can more abruptly wedge under a stationary warm air mass, forcing it to rise quickly along a steeper slope. If the warm air mass is moist, then a warm front will feature drizzles of rain (or ice pellets, also known as sleet in the US, or light & fluffy snow if the air temperature below the cloud is less than 0 degrees Celsius) from low-lying flat stratus or nimbostratus clouds (showing again why this is known as stratiform precipitation), while a cold front will feature quicker & more intense downpours (and occasionally quick & heavy snow, if the air temperature below the cloud is less than 0 degrees Celsius) from narrow vertically extended cumulonimbus clouds.
In some cases, fronts can have very large widths (from one end of the boundary between air masses to the other). In other cases, fronts may be narrower but have more well-defined horizontal velocities as they are sustained by transient systems of low pressure that draw in air of a given temperature from the locations over which they pass at any given moment, colder air from the direction of the pole, warmer air from the direction of the equator, and moist air of less predictable (without knowing more details about the location) temperature if near a coast. Such systems of low pressure may be tropical depressions, tropical cyclones, or extratropical cyclones, and feature rotation that is counterclockwise in the northern hemisphere or clockwise in the southern hemisphere. Such systems of low pressure are most likely to follow trade winds, prevailing westerlies, or other prevailing winds that are well-defined & change less within a season; these can be called storm tracks, and it is worth knowing in this context that the name "trade wind" came from an older use of the word "trade" meaning "track".
Frontal precipitation can happen with transient systems of low pressure for any combination of temperatures & humidity levels for dry & moist air masses, as long as the moist air mass density is not the highest among air masses entering the system of low pressure. Frontal precipitation can happen when the moist air mass is warmer than the dry air mass, whereas convective precipitation cannot (for reasons that will become clearer in the explanation of convective precipitation), but it is more likely to penetrate deeper into the dry air mass if carried by a well-defined transient system of low pressure than a bigger boundary with a weaker system of low pressure.
Convective precipitation
To understand convective precipitation, it is necessary to more carefully understand how densities of air depend on temperature & relative humidity simultaneously. The density of air increases with temperature or relative humidity. This means, taking a dry air mass at any temperature to have 2% relative humidity (for convenience when using tools like the OmniCalc air density calculator, as the density of an air mass at 2% relative humidity at fixed pressure & temperature for typical pressures & temperatures anywhere at any time of year differs by less than 0.05% from the density of perfectly dry air at the same pressure & temperature, whereas the density of an air mass at 2% relative humidity at fixed pressure & temperature for typical pressures & temperatures anywhere at any time of year usually differs by at least than 1% from the density of an air mass at 100% relative humidity at the same pressure & temperature) and a moist air mass at any temperature to have 100% relative humidity, that a moist air mass could have a lower temperature but still a lower density than a dry air mass. Note that even inland deserts at tropical, subtropical, and equatorward middle latitudes that reach extremely high daytime temperatures during the summer half of the year and get essentially no precipitation still have relative humidity levels around 10%, so the assumption of dry air masses having relative humidity levels of 2% is a computational convenience illustrating extreme cases.
Convective precipitation will occur if a moist air mass is less dense than a dry air mass but the density of the dry air mass is low enough to create a localized system of low pressure that draws in the moist air mass. This in turn implies that the moist air mass is initially at a higher pressure, and then as the moist air mass is drawn over land, it expands in the localized system of low pressure, adiabatically cools as it expands (which in part means that it is rising in elevation too), condenses as it rises as the adiabatic cooling pushes the temperature down to the dew point, and leads to precipitation.
Strictly speaking, convective precipitation refers to local heating of the land that causes local moisture to evaporate from bodies of water or be transpired from plants, rise in a local system of low pressure over land, condense as it rises & adiabatically cools, and lead to precipitation. This happens mostly at tropical latitudes in rainforests as well as in farmland in middle latitudes that get warm moist air, as will be discussed in later sections in this post, but not in most other locations. It is more common for something like convective precipitation to occur by heating of the land to lead to a slightly lower pressure that draws in relatively cooler moist air masses, which are then heated and may rise to the point where the moisture condenses & falls as precipitation, but this still requires the moist air mass from over the ocean to be less dense than the warmer dry air mass over land. Because this is still a relatively small-scale but not purely local effect, I have come to call it quasi-convective precipitation, but be advised that this is not standard terminology, so a reader probably will not find it when looking online. If the moist air mass is warmer than the dry air mass, then the dry air mass will be at a slightly higher pressure than the moist air mass, so the moist air mass will not be drawn in over land. This means that convective precipitation will not happen if the moist air mass is warmer than the dry air mass.
At this point, it is necessary to give some concrete numbers, assuming an air pressure of 1,000 millibars for simplicity. For a dry air mass to be less dense than a moist air mass if the temperature of the moist air mass is 10, 15, 20, 25, or 30 degrees Celsius, then the dry air mass temperature must be at least 11.3, 16.9, 22.6, 28.6, or 34.9 degrees Celsius, respectively. This shows that the range of temperatures in which convective precipitation is likely, due to the moist air mass being cooler than the dry air mass, increases in absolute magnitude with increasing temperature of the moist air mass. Moreover, absolute humidity in moist air masses increases with increasing temperature. These two reasons individually and together make clear why convective precipitation is less likely to happen at lower temperature and, when it does, produces lower total amounts of precipitation on average. In particular, convective precipitation & quasi-convective precipitation usually do not happen much for sea/ocean surface temperatures below 20 degrees Celsius and are much more likely to occur in significant quantities for sea/ocean surface temperatures above 25 degrees Celsius (the latter as long as the dry air mass above land has a temperature that is not so high as to make the dry air mass less dense than the moist air mass).
Heating of land is not a magical cause of convective precipitation, because if the land becomes too hot, then the lesser density of dry air masses will ensure that moist air masses cannot rise above dry air masses and lead to precipitation. Instead, a temperature inversion would occur, implying atmospheric stability. Additionally, there may be situations where temporary, seasonal, or permanent systems of high pressure caused by air falling from higher in the atmosphere & adiabatically warming as it compresses will lead to temperature inversions and to the land becoming very hot but unable to support the upward motion of moisture needed to lead to convective precipitation; instead, cool moist air masses will keep the air dry but allow extremely high temperatures to be reached, while warm moist air masses will marginally moderate the temperature but lead to oppressively hot & humid weather. Thus, conditions need to be right for convective precipitation to happen even when temperatures are high.
Orographic lifting precipitation
Orographic lifting precipitation occurs when moist air masses move up the slopes of mountains or hills. That process makes those moist air masses expand due to less air being above (as the elevation increases) and thus adiabatically cool to the dew point (or frost point), leading to condensation (or deposition) and thus precipitation. When a moist air mass carried by a prevailing wind has its moisture condense (or deposit) on the windward side of a mountain, the latent heat of condensation (or deposition) makes that air mass warmer than it was before, and then as the air mass moves onto the leeward side of the mountain, it adiabatically warms further due to contraction from gravity as it falls, so its relative humidity drops further and more water can evaporate or ice can sublimate into water vapor that is then carried by that air mass. This is the phenomenon of a foehn wind (also called a Chinook wind in some parts of the US, the latter of which is called by indigenous tribes a "snow eater" as the wind on the leeward sides of mountains can lead to the sublimation of snow & fast warming of the area). More broadly, this is a significant reason for locations on the leeward sides of mountains having drier climates than locations on the windward sides of the same mountains at the same latitudes (the rain shadow effect).
Places that are at the foothills of mountains on the windward sides relative to moist air masses carried by prevailing winds get more precipitation on average than places where such mountains are absent, even if the remaining atmospheric & oceanic conditions are identical. Such precipitation covers somewhat smaller than synoptic area scales.
Other terminology about latitudes to keep in mind
There are a few other bits of terminology that I will consistently use. For both hemispheres, I will use the term "tropical latitude" for any latitude between 0-23 degrees (where 0 degrees in latitude is the equator), "subtropical latitude" for any latitude between 23-30 degrees, "middle latitude" for any latitude between 30-60 degrees, "subpolar latitude" for any latitude between 60-67 degrees, and "polar latitude" for any latitude between 67-90 degrees (where 90 degrees in latitude is the pole of that hemisphere). The tropics and polar circles are well-defined by the Earth's rotational axis being tilted by 23 degrees relative to its orbital plane. However, the ideas of 30 & 60 degrees in latitude separating middle latitudes from subtropical & subpolar latitudes is a matter of empirical convenience. There will be times when I refer to the equatorward middle latitudes, which I define to be 30-45 degrees in latitude, and the poleward middle latitudes, which I define to be 45-60 degrees in latitude. Similarly, there will be times when I refer to the equatorward tropical latitudes, which I define to be 0-15 degrees in latitude, and the poleward tropical latitudes, which I define to be 15-23 degerees in latitude. There will be other times where I will specify the latitude range because the atmospheric behavior is not as aligned with the term that would usually be assigned to that latitude range; examples include the climates of the deserts of the Southwest in the US being more subtropical than middle latitude despite being in the "middle" latitudes of 30-35 degrees (as the heating of land is enough to support the ITCZ, leading to a wet summer monsoon there) and the climates of the Scandinavian Peninsula being more similar to poleward middle latitude climates along west coasts despite being in the "subpolar" latitudes of 60-65 degrees (as the Gulf Stream is unusually warm compared to other typical ocean currents in that latitude range).
General temperature and precipitation trends for generic continent arrangements
There are general trends in temperatures, precipitation amounts, and precipitation seasonality that can be observed across continents on Earth and can be intuited for hypothetical simplified continent shapes, positions, and topographies. The following subsections will consider extreme cases and then cases that are closer to the continents that actually exist on Earth, though for all cases, the oceanic & atmospheric temperatures will be assumed to be similar to those actually observed on Earth unless otherwise specified.
Islands or narrow continents
Under current atmospheric & oceanic conditions but in an alternate history where land levels were low enough even in the absence of global warming such that no land existed above sea level, then the issue of climates in land-based locations would be moot. The ITCZ would be a largely uniform band around the Earth (though the ocean would be hotter where it is shallower) that largely tracks the range of tropical latitudes in a given month that have the sun directly overhead in the middle of the day, the subtropical ridges in each hemisphere would be largely be uniform bands at latitudes of approximately 30 degrees away from the ITCZ in each direction at any given time, and as there would be no well-defined centers of subtropical ridges about which winds rotate to move ocean surfaces, then oceans would no longer feature gyres and would instead have circulation patterns more dependent on overturning.
If the only lands present above sea level were to be small islands or narrow (with respect to longitude or latitude) continents with no significant topographical features, then those lands would not significantly affect atmospheric circulation; this is like the idea in physics of an infinitesimal test mass or test charge whose gravitational or electromagnetic field is negligible compared to that of the big system in question, so it is affected by the system but does not significantly change the system. Because the subtropical ridges would continue across different continents & would not produce significant ocean gyres, there would not be significant upwelling that would cool the eastern edges of oceans in middle & subtropical latitudes. Thus, almost every part of every land would get significant precipitation through the year, with the most precipitation coming at locations that are closest to the ITCZ through the year, locations at tropical & subtropical latitudes that always get ordinary easterly trade winds getting relatively more precipitation in the summer half of the year than in the winter half of the year, locations under the most equatorward location of the subtropical ridge being fairly dry in the winter half of the year, locations between the most equatorward & poleward locations of the subtropical ridge getting relatively uniform but somewhat lower levels of precipitation through the year (getting moist prevailing westerlies in the winter half of the year & moist easterly trade winds in the summer half of the year), locations under the most poleward location of the subtropical ridge being fairly dry in the summer half of the year, and locations being even more poleward of that getting prevailing westerlies that consistently bring moist air but that bring more precipitation during the winter half of the year than during the summer half of the year. After accounting for the presence of mountains, these behaviors can variously be seen in the islands of Southeast Asia that are close to the ITCZ through the year, the islands of the Caribbean Sea that get ordinary easterly trade winds through the year, and Tasmania & New Zealand which get the prevailing westerlies through the year.
The assumption of small islands or narrow continents implies that these islands or continents could not become hot enough to support convective or quasi-convective precipitation, unless the ocean surface temperature is high enough and the atmosphere is hot enough for land temperatures to be in the right range to support convective or quasi-convective precipitation. Thus, in the absence of mountains, all precipitation would be frontal. In this scenario, rainforests would be present at almost every location over land. That said, if the moist air masses are dominant over the whole Earth, then it is unlikely for air masses over land to significantly differ in temperatures or relative humidity levels, so there might not be that much precipitation despite the consistency of prevailing winds & high humidity levels. This does not necessarily make the existence of rainforests impossible, as temperate rainforests exist or used to exist on Earth around 50 degrees in latitude along & somewhat inland of the west coasts of continents in each hemisphere despite precipitation levels sometimes being a little low (though the existence of a rainforest could so dramatically increase the capacity for storing water that the resulting increase in humidity in air parcels over land, which may have different temperatures from air parcels over the ocean, could subtly make precipitation more likely).
No oceans or seas
If there were no surface water, such that Earth would be more like Mars, then there would be no precipitation, and the whole surface of the Earth would be a desert. Weather patterns would still exist, but all air masses, irrespective of temperature, would be completely dry. If there were only small seas, then there would only be marginal frontal or convective precipitation (assuming no mountains to lead to orographic lifting precipitation) near those sea coasts, and the overall planet-wide desert atmospheric dynamics would be largely unchanged.
Bigger continents
If continents are bigger than just small islands or narrow strips of land, then the difference between the specific heat capacities of land versus water can lead to continents significantly changing weather patterns. Assuming that the continent has a large width with respect to longitude but is not so wide as to squeeze out oceans on either side and that it has a large width with respect to latitude, the effects of the continent on weather patterns will depend on its position with respect to latitude (as position with respect to longitude is irrelevant unless other continents are present, breaking the symmetry of the Earth's surface). This effectively constitutes a third phase of climates, where the two extreme phases are purely oceanic climates (broken only by small islands or flat continents that span extremely narrow ranges of longitude or latitude) & purely desert continental climates (the latter in the absence of major oceans or seas), as those two extreme phases show climates that depend almost entirely on latitude (though in very different ways, contrasting on the actual Earth the moderate temperatures in oceanic climates with the wildly swinging temperatures seen in inland deserts), whereas this third phase shows much more dependence of a climate at a given location on the continent's broad position with respect to latitude, the continent's size, the continent's topography, and the longitude of that location (in the sense of how close it is to a west or east coast). The following sub-subsections consider the progression of climates in a single continent that is the only significant landmass on Earth, unaffected by islands and having no significant topographic features (like mountains or mountain ranges) unless otherwise specified.
Landmass only at equatorward tropical latitudes
A big continent that is only present within equatorward tropical latitudes, possibly in both hemispheres, would have its land heated to a large degree through the year and would be surrounded by oceans that are also heated to a large degree through the year. This means that within each hemisphere where it has significant landmass at equatorward tropical latitudes, it would support a large strong system of low pressure, which is the ITCZ, through the year, pulling hot moist air parcels from over the ocean at the equator to the east poleward & slightly to the west, somewhat warm moist air parcels from over the poleward ocean slightly equatorward & mostly to the west, hot moist air parcels from over the ocean at the equator to the west poleward & more to the east, and more dry hot air parcels over land to the east on the equatorward side (which is opposite the usual convergent easterly flow over an ocean at the equator). It would probably get significant locally convective, quasi-convective, and frontal precipitation through the year. This can be seen in the rainforests at inland tropical latitude locations of South America & Africa in both hemispheres close to the equator, though in both cases, mountain ranges aligned more with lines of longitude prevent full convergence of ordinary easterly & reversed westerly trade winds over the landmass. The continent would probably not affect or be affected by the details of the subtropical ridges (as the trade winds would be consistent through the year and would turn only because of the ITCZ), which would exist as continuous bands over oceans significantly poleward of the poleward coast(s) of the continent in each hemisphere where it has significant landmass; additionally, each subtropical ridge would largely block incursions of colder air from the pole in the same hemisphere.
Landmass only at tropical & subtropical latitudes
If the continent has significant landmass not just at equatorward tropical latitudes but also at poleward tropical & subtropical latitudes, then there would be some changes but not significant changes to the progression of climates. The ITCZ would still be over most of the continent through the year, and locations at equatorward tropical latitudes would still consistently get somewhat high temperatures & levels of precipitation through the year (mostly as convective or quasi-convective precipitation but also potentially as frontal precipitation). Furthermore, the subtropical ridges in each hemisphere would continue to be continuous bands (with respect to longitude) over the ocean and block incursions of cold air from the poles. This means that the poleward coast(s) of the continent would consistently get easterly trade winds through the year.
There are only a few differences in the progression of climates for a continent that exists at tropical & subtropical latitudes versus a continent that exists only at equatorward tropical latitudes. West coast locations at poleward tropical & subtropical latitudes, which get the easterly trade winds through the year that would have lost their moisture upon crossing the poleward or east coasts, would be more dry through the year than east coast locations at poleward tropical & subtropical locations. The assumption in this scenario is that the poleward coast of the continent in a given hemisphere does not come close enough to the subtropical ridge to be able to modify it, so those locations may merely be dry in a relative sense. Additionally, in the winter half of the year, all locations at poleward tropical & subtropical latitudes would be more dry than they are in the summer half of the year, as the decrease in land & ocean temperatures would be more pronounced than at equatorward tropical latitudes and would therefore reduce the probability of convective or quasi-convective precipitation at that time of year.
This progression of climates is much more sensitive to the existence of other big continents (or the same continent at more poleward latitudes) because of the implications for the subtropical ridge & ITCZ, as the fact that this sort of continent is not purely at equatorward tropical latitudes means that it is not guaranteed to have a climate dominated by hot land & hot oceans all the time. There is no continent on Earth as it actually is that approximately satisfies these criteria, although many aspects of this climate progression can be seen in eastern at locations tropical & subtropical latitudes in South America & Africa in the southern hemisphere.
Landmass at subtropical & middle latitudes (and possibly tropical & subpolar latitudes)
If a big continent has significant landmass in a given hemisphere at subtropical & middle latitudes, then its presence will change the shape of the subtropical ridge over the oceans to the east & west of the continent. If the continent is more like a rectangle with its west coast largely following lines of longitude, then the subtropical ridge over the ocean to the west will be at a more poleward latitude than the subtropical ridge over the ocean to the east. Additionally, the subtropical ridges, being restricted in the range of longitudes, will create ocean gyres following those prevailing winds, leading to warm currents along the east coast at subtropical & equatorward middle latitudes, east-moving currents at poleward middle latitudes that become colder toward the east, cold currents (especially reinforced by upwelling if the ocean is deep there) along the west coast at most middle latitudes, and west-moving currents at tropical latitudes that become warmer toward the west. This means that the continent will be surrounded by water that is consistently fairly warm (though warmer in the summer half of the year than in the winter half of the year) along any equatorward coast that may exist, warm over a big range of latitudes during the summer half of the year but over a much smaller range of latitudes closer to the equator during the winter half of the year along the east coast, consistently fairly cold along the poleward coast, and cool over a big range of latitudes through the year (becoming comparable in temperature to the water along the equatorward coast only at subtropical & tropical latitudes) along the west coast. This effect can be seen with the oceans respectively surrounding the mainland of Australia & North America, and the eastern part of this effect can be seen with the oceans surrounding Asia.
If a big continent is present in a given hemisphere at subtropical & middle latitudes, then the subtropical ridge over the ocean to the west will be at middle latitudes near the west coast of the continent in question, That subtropical ridge will have a more poleward latitude in the summer half of the year & a more equatorward latitude in the winter half of the year. The high pressure & atmospheric stability from air warming as it falls from that subtropical ridge could be supported by upwelling along the west coast equatorward of the subtropical ridge if the ocean is deep enough that upwelling would bring up much colder water. There will also be a subtropical ridge over the ocean to the east at similar latitudes, though it will at all times be more equatorward along the east coast than the one along the west coast (and it will be farther away from the east coast of the continent during the summer half of the year than during the winter half of the year). The big difference from a continent at only tropical & subtropical latitudes is that in addition to the continent in inland areas supporting a seasonal system of low pressure (which may be a poleward extension of the ITCZ) during the summer half of the year, the continent can in similar inland areas support a seasonal system of high pressure from the settling of cold air during the winter half of the year. This system of high pressure would shed prevailing winds clockwise in the northern hemisphere & counterclockwise in the southern hemisphere, making westerly winds on the poleward side colder, sending cold winds equatorward along the east coast, making easterly winds on the equatorward side somewhat warmer, and sending somewhat warm winds poleward along the west coast.
In the summer half of the year, the climates of locations at subtropical & middle latitudes would be as follows. The poleward middle latitude locations along or near the west coast (though with the effect decreasing farther inland) would get stabilizing cool moist air parcels from the prevailing westerlies as the ocean to the west is cold there, leading to relatively dry conditions with relatively little humidity and bigger swings between daytime & nighttime temperatures. The equatorward middle & subtropical latitude locations along the west coast would get similar but slightly warmer conditions from the lower latitudes & from sea breezes (as the ocean is still fairly cold there) instead of the prevailing westerlies while more inland locations would see hotter temperatures; locations farther inland would experience less cooling effect from sea breezes and therefore much hotter temperatures. These west coast locations may also get fog at night & during the early morning hours each day, as the lesser overall humidity would allow the land to cool more than the ocean at night, which would lead to water vapor in the sea breezes that would have come during the daytime hours condensing near the ground; this effect can be seen at many west coast locations at equatorward middle latitudes in every continent but dissipates quickly with further distance inland. Thus, west coast locations at middle latitudes generally cannot support quasi-convective precipitation in the summer half of the year. The east coast (and equatorward coast, if it exists) at subtropical & middle latitudes would be able to support quasi-convective precipitation from moist air parcels over the warm ocean, they may support frontal precipitation too depending on the prevailing wind directions from the subtropical ridge to the east. There could be larger-scale quasi-convective or frontal precipitation farther inland at middle latitude locations in the eastern half of the continent because the seasonal system of low pressure over the continent (which may be seen as a poleward extension of the ITCZ) would pull in warm moist air from the equatorward & eastward oceans and cool moist air from the poleward & westward oceans to mix in with the hot dry air over land. If the continent has significant landmass at poleward middle or subpolar latitudes, then systems of low pressure from the heating of land or from fronts could pull in colder dry air from the pole as the landmass at poleward middle & subpolar latitudes would not become as hot, leading to more precipitation upon collision with moist air parcels from the ocean to the east or equatorward. These effects can be seen in North America, in Asia (when considered as the eastern part of a bigger continent), and to a lesser extent in the mainland of Australia.
In the winter half of the year, as the subtropical ridges move equatorward, the system of high pressure over the continent from the settling of cold air would spin such that the prevailing westerlies on the poleward side would become colder farther from the west coast, then turn toward the equator along the east coast and bring air that warms as it moves toward the equator, then turn toward the west as air that is warm & dry, then turn toward the pole along the west coast and bring air that cools as it moves toward the pole, and finally rejoins prevailing westerlies from over the ocean to the west. This means that the winter half of the year will generally be colder in the eastern half of the continent than in the western half of the continent at the same latitudes.
In the winter half of the year inland and along the east coast of the continent at middle latitudes, if there are prevailing winds/storm tracks that are strong enough to overcome this seasonal system of high pressure along the east coast or in the inland eastern parts of the continent, then those could bring frontal precipitation from relatively warmer oceans closer to the equator or along the east coast from subtropical latitudes, helped by the contrast in densities of air parcels between warm moist air from over the ocean and cold dry air (moved by this seasonal system of high pressure) over the continent (as the latter is more dense, so uplift of warm moist air in a warm front or cold front would lead to condensation/deposition followed by precipitation). Such frontal precipitation would help to moderate the temperatures of locations in the eastern half of the continent at middle latitudes, though those locations will still be somewhat colder than locations along the west coast of the continent at the same latitudes. This effect can be seen in North America. If instead the seasonal system of high pressure from the settling of cold air over the continent is strong enough to "repel" storm tracks, then the eastern portion of the continent will get prevailing winds blowing offshore from that system of high pressure and will thus experience consistently very cold, dry weather. This effect can be seen in Asia. In general, further inland from the east coast, locations will be more cold & dry due to greater distance from the moderating effects of moist air parcels from any coast. These effects can be seen in North America & Asia. In locations at even the equatorward middle latitudes, the winter half of the year will be more cold & dry if the continent has significant landmass in the poleward middle & subpolar latitudes, as there can be more incursions of colder air from the seasonal system of high pressure supported over land there.
In the winter half of the year, locations along the west coast at middle latitudes would get frontal precipitation from transient systems of low pressure generated along the prevailing westerlies. Until one reaches the latitudes of the polar front (where mild moist air from the subtropical ridge collides with cold dry air from the seasonal system of high pressure formed by cold air settling over the continent, leading to a spatially extended seasonal system of low pressure with frontal precipitation), more precipitation would fall upon more poleward locations in the middle latitudes, as they are farther from the subtropical ridge and therefore less subject to its falling air which would stabilize the atmosphere and therefore suppress precipitation (along with the formation of transient systems of low pressure moving along the prevailing westerlies), and they are more subject to incursions of colder air from the pole that would be much denser than incoming moist air parcels as the ocean temperature varies very little over middle latitudes along the eastern edge of an ocean (next to the west coast of a continent); poleward of the polar front, west coast locations have dry climates in the winter half of the year due to cold dry prevailing winds blowing offshore. Thus, equatorward of the polar front, as one moves poleward away from the subtropical ridge along a west coast at middle latitudes, the average temperature in the winter half of the year decreases much more gradually with increasing latitude, but because that moderation can only occur because of moist air parcels coming from over the ocean to the west, the compensating effects are significant increases in cloud cover & precipitation with increasing latitude (more than can be accounted for by the individual effects in isolation of more frequent transient systems of low pressure carried by the prevailing westerlies forming more poleward of the subtropical ridge and of greater chances of collisions between the mild moist prevailing westerlies and cold dry continental air generated by more poleward systems of high pressure). Closer to the subtropical ridge along the west coast in the winter half of the year, average temperatures increase and the amounts of cloud cover & precipitation decrease due to the stabilization of the atmosphere from the subtropical ridge. These effects in turn allow the land to become warmer during the daytime hours, and that warming can draw in further stabilizing cool sea breezes. These west coast locations at middle latitudes have climates which are dominated by the prevailing westerlies in the winter half of the year, though the prevailing westerlies are not guaranteed to consistently bring transient systems of low pressure that would in turn bring precipitation; these locations are thus subject to medium-term (longer than 1 year) cycles of precipitation & drought, with the latter occurring when fewer warm fronts from over the ocean to the west come to the continent. These west coast locations have climates that are overall much less influenced by the seasonal system of high pressure over the continent in the winter half of the year than by the prevailing westerlies from the ocean to the west, but the seasonal system of high pressure over the continent in the winter half of the year can lead to subtle changes in climate. Generally, that high pressure over the continent means that warm fronts from the prevailing westerlies cannot penetrate large distances inland from the west coast, and this blockade strengthens more poleward (reducing the inland penetration distances of westerly warm fronts). Additionally, that high pressure over the continent brings relatively warm dry air over west coast locations at equatorward middle latitudes, which can further stabilize the atmosphere and make the climate more dry at west coast locations at latitudes closer to the subtropical ridge even in the winter half of the year. Such dry conditions can lead to wildfires following the hot dry summer season, as has been seen in many fall & winter seasons along west coast locations around 30-35 degrees in latitude (most recently & notably in the Greater Los Angeles area earlier this year); thus, there is an irony that colder conditions over the rest of the continent can lead to hotter & more wildfire-prone conditions along the west coast at equatorward middle latitudes. However, because air from that system of high pressure over the continent by definition does not have moderation from moisture, it cools much more quickly with increasing latitude along the west coast, so the wildfire threat in the fall season decreases a lot along the west coast with increasing latitude (though it is still present in the summer season at those latitudes), and that colder dry air can actually contribute to more precipitation due to the greater density contrast with moist air parcels carried by the prevailing westerlies. If the continent has more landmass at poleward middle & subpolar latitudes, then the seasonal system of high pressure from cold air settling over the continent will be stronger especially more poleward and will have even colder air, so the greater probability of incursions of cold air from the pole means that temperatures in the winter half of the year even along the west coast will be a little colder, there will be somewhat more precipitation along the west coast at each latitude than there would be without that additional landmass at poleward middle & subpolar latitudes, and transient systems of low pressure bringing precipitation along the prevailing westerlies will penetrate less far inland from the west coast at poleward middle & subpolar latitudes. Similar effects can be seen in North & South America compared to Europe, though the presence of tall mountain ranges close to the west coast in North & South America make it a little harder to separate precipitation shadow effects from the natural gradient of precipitation & temperature farther inland.
Generally, a continent having significant landmass at middle & potentially subpolar latitudes will not significantly change the climates along the continent's east coast at subtropical latitudes. This is because those locations during the summer half of the year will still get significant local quasi-convective precipitation as well as frontal precipitation drawn from the subtropical ridge over the ocean to the east into the poleward extension of the ITCZ in the continent and during the winter half of the year will get less precipitation because of the subtropical ridge extending over the eastern part of the continent at subtropical latitudes & stabilizing the atmosphere. However, a continent having significant landmass at middle & potentially subpolar latitudes will significantly change the climates along the continent's east coast at subtropical & poleward tropical latitudes, because the deformation of the subtropical ridge over the ocean to the west means that locations along the west coast equatorward of the subtropical ridge through the year at subtropical & poleward tropical latitudes will get equatorward prevailing winds parallel to the coast over relatively cool waters through the year and will therefore be deserts, being subject to atmospheric stabilization from the subtropical ridge & sea breezes. Deserts at subtropical & poleward tropical latitudes along west coasts can be seen in North America & South America (in both cases modified by the tall mountain ranges slightly inland), Africa in both hemispheres, and the mainland of Australia. Additionally, the lesser effects on the climates of locations along east coasts at subtropical latitudes can be seen in North America, South America, Africa in the southern hemisphere, and the mainland of Australia.
Counterintuitively, in contrast to the situation along east coasts at subtropical latitudes, a continent having significant landmass at middle & potentially subpolar latitudes will significantly change the climates of locations at equatorward tropical latitudes. This is because if the continent can support a seasonal system of low pressure in the summer half of the year at subtropical or middle latitudes, which may be a poleward extension of the ITCZ, then that system will draw in ordinary easterly trade winds on the poleward & eastern sides and reversed westerly trade winds on the equatorward & western sides. This means that locations along the west coast (and the equatorward coast, if present) between the equator & ITCZ will get significant quasi-convective & frontal precipitation aided by reversed westerly trade winds carrying warm moist air parcels from the ocean to the west during the summer half of the year, and they will get much cooler & drier ordinary easterly trade winds during the winter half of the year, leading to a much more pronounced seasonality in precipitation. In contrast, locations along the east coast (and the poleward coast, if present) between the equator & ITCZ will get hot dry air from over the continent during the summer half of the year (though with the possibility of a little local quasi-convective precipitation, but that probability decreases farther from the coast), and it will get significant quasi-convective & frontal precipitation aided by ordinary easterly trade winds carrying warm moist air parcels from the ocean to the east during the winter half of the year, again leading to a much more pronounced seasonality in precipitation but in the opposite way. The dry winter effects can be seen even without intervening mountain ranges in the western part of Africa in both hemispheres at equatorward tropical latitudes and the equatorward coast of the mainland of Australia. The east coasts of South & Southeast Asia are the only significant continental (not island) landmasses at equatorward tropical latitudes that show dry summers & wet winters because the ITCZ moves poleward of it during the summer half of the year, and even these are more modulated by surrounding mountain ranges.
Landmass only at poleward middle & subpolar latitudes
If a big continent has significant landmass only within one hemisphere at poleward middle & subpolar latitudes, then the subtropical ridge is likely to remain a continuous band around the Earth over the ocean through the year and its seasonal movement is unlikely to be significantly disrupted by the presence of the continent. The ocean surrounding the continent is likely to be fairly cold throughout the year, and as the prevailing westerlies will drive eastward ocean currents along the poleward & equatorward coasts of the continent throughout the year, there may be upwelling along the equatorward coast making that area even more cold & dry if the ocean is deep enough there and downwelling along the poleward coast maintaining the already relatively cold temperature of the ocean there. During the summer half of the year, the continent may heat up to some degree and draw in moist air parcels from over the ocean, but it will not support as strong of a system of low pressure as the ITCZ or something like it over a continent at subtropical or equatorward middle latitudes, any moist air parcels that are drawn in are unlikely to lead to quasi-convective precipitation because the air over the continent may be warmer and therefore less dense (despite being dry) than the air over the ocean, and any quasi-convective precipitation that does happen will be in lesser amounts because the equilibrium vapor pressure of water decreases with decreasing temperature. There could be some transient systems of low pressure carried by the prevailing westerlies from the ocean to the west over the continent which bring a little more frontal precipitation, due to mixing of colder air closer to the pole, than in locations at equatorward middle latitudes. During the winter half of the year, the continent will likely support a strong seasonal system of high pressure from the settling of cold air. This means that transient systems of low pressure bringing relatively warmer (compared to air over the continent) moist air parcels via the prevailing westerlies will bring most frontal precipitation to the west coast and be guided along the poleward coast, bringing a little more frontal precipitation there. Additionally, clashes of cold dry air masses moving equatorward from the continent with relatively warm moist air masses moving poleward from the subtropical ridge over the ocean will lead to significant frontal precipitation near the equatorward coast in the polar front; this can be seen along the coast of Antarctica through the year as well as near the Aleutian Islands, Greenland, and Iceland in the winter half of the year. Overall, this means that the western part of the continent will get more precipitation than the eastern part of the continent, and there may be a slight skew of precipitation toward the winter half of the year. That said, if the polar front moves seasonally (poleward from the winter solstice to the summer solstice and equatorward from the summer solstice to the winter solstice), then west coast & equatorward coast locations will get cold dry offshore prevailing winds during the winter half of the year and more onshore frontal precipitation during the summer half of the year.
There is no continent that fulfills all of these criteria. The landmasses that are most similar to these criteria are western & central Europe as well as the British Isles, which experience the prevailing westerlies through the year and are surrounded by seas & oceans with very fractal shapes, so they experience cooler temperatures through the year and moderate amounts of precipitation (though much less than locations at equatorward middle latitudes in the eastern part of North America) through the year. Additionally, South America in the poleward middle latitudes shows a trend of decreasing precipitation from west to east and in the summer half of the year compared to the winter half of the year, though it experiences a significant precipitation shadow effect east of the Andes Mountains, and that part of that continent covers a big range of latitudes but a small range of longitudes.
Temperature and precipitation trends for actual continents: going beyond generic explanations
In the previous section, I laid out the general principles of continental positions & weather patterns that can explain most of the broad weather patterns seen on most continents. There are other details of climates of various locations that require further explanation in the following subsections.
Coastline orientation, ocean size, and upwelling effects
Beyond the generic explanations, differences between climates in different continents can be explained in parts by differences in coastline orientation. Relevant differences include but are not limited to the following sub-subsections.
Arid or semi-arid climates of the east coast of South America in the southern hemisphere at latitudes between 40-55 degrees
The east coast of South America in the southern hemisphere at latitudes between 40-55 degrees has arid & semi-arid climates. This is in marked contrast to the east coasts of North America & Asia at similar latitude ranges ranges. This occurs for two related reasons, both of which have effects (albeit with different implications) in each half of the year.
In the summer half of the year, the east coasts of North America & Asia have humid summers with significant precipitation. In North America, this is because the subtropical ridge over the Atlantic Ocean to the east brings prevailing winds as well as warm ocean currents parallel to the coast, allowing for quasi-convective as well as frontal precipitation. In Asia, this is because the subtropical ridge over the Pacific Ocean to the east brings easterly trade winds feeding directly into the ITCZ over the continent, similarly allowing for quasi-convective as well as frontal precipitation. In both cases, the east coasts at those latitudes curve along the direction of the air flow around the subtropical ridge, making such precipitation more likely. By contrast, in South America, there is too little landmass at those latitudes to support significant systems of low pressure over land in the summer half of the year, the subtropical ridge over the Atlantic Ocean to the east sends air & warm ocean currents away from the continent at latitudes that are relatively closer to the equator, and the continent at latitudes of 40-55 degrees curves away from the subtropical ridge's prevailing wind direction at those latitudes, so warm ocean currents & transient systems of low pressure do not flow to east coast locations at those latitudes. Instead, cold ocean currents from the Antarctic Ocean flow equatorward along the east coast of South America, so sea breezes would further stabilize the atmosphere and suppress precipitation in the summer half of the year.
In the winter half of the year at those latitudes, the east coast of North America lies on storm tracks that cross the continent from west to east as well as storm tracks that go poleward along the coast, so transient systems of low pressure carried along those storm tracks bring significant frontal precipitation due to the ocean to the east being relatively warmer than the land. Additionally, more poleward, the east coast of North America lies near seasonal systems of low pressure near Greenland & Iceland, arising from collisions of mild moist air masses from the subtropical ridge over the Atlantic Ocean and cold dry air masses from the seasonal system of high pressure over Greenland, so this brings more frontal precipitation. Although most of the east coast of Asia gets cold dry offshore winds from the seasonal system of high pressure from the settling of cold air over the continent and therefore does not lie along a storm track, the east coast of the Kamchatska Peninsula is close to the seasonal system of low pressure around the Aleutian Islands that arises from the from collisions of mild moist air masses from the subtropical ridge over the Pacific Ocean and cold dry air masses from the seasonal system of high pressure over Kamchatska & Alaska, so this brings significant frontal precipitation. By contrast, although the east coast of South America at those latitudes does get a bit more precipitation (in the winter half of the year compared to the summer half of the year) from transient systems of low pressure carried by the prevailing westerlies from the Pacific Ocean to the west & over the Andes Mountains and mixing cold dry air over the continent with relatively warmer (though still cool) moist air over the Atlantic Ocean to the east, it is far from the seasonal system of low pressure caused by the collision of mild moist air from the subtropical ridge over the Atlantic Ocean with cold dry air from the permanent system of high pressure from the settling of cold air over Antarctica, so that cannot bring frontal precipitation to those latitudes, and the Andes Mountains makes those moist air parcels from the west significantly drier (and a bit warmer than they were in the west) to the east, promoting atmospheric stability that would tend to suppress precipitation. There is not enough landmass in the southern hemisphere at those latitudes to support seasonal systems of high pressure from the settling of cold air over that landmass to then lead to collisions of air that would move the seasonal system of low pressure equatorward from subpolar latitudes in the winter half of the year. Thus, the east coast of South America at those latitudes has arid or semi-arid climates with relatively more precipitation in the winter half of the year than in the summer half of the year.
Arid or semi-arid climates along the north coast of South America, the east coast of Somalia in the northern hemisphere, and the Arabian Peninsula
Upwelling is often described as happening along the eastern edges of ocean gyres next to west coasts of continents, bringing to the surface deeper cold water that promotes temperature inversions, increases in air pressure, and atmospheric stability that together reduce the chances of precipitation. However, upwelling is not limited to those areas. It can occur at subtropical & tropical latitudes too, including along east coasts, if the current along the east coast is not part of a closed ocean gyre (even though the current may be driven by a bigger circulation around a subtropical ridge).
The north coast of South America along the Caribbean Sea, especially including the north coasts of Colombia & Venezuela as well as the dependent island territories of Aruba, Bonaire, and Curaçao, consistently receives ordinary easterly trade winds from the subtropical ridge over the Atlantic Ocean in the northern hemisphere. These easterly trade winds and the resulting warm ocean currents are not part of closed loops (gyres) around the subtropical ridge, so unlike the warm ocean currents that are the western edges of ocean gyres which are warm & fast-moving throughout their depth, the warm ocean current along the north coast of South America is not deep or fast-moving. This means that easterly movement along the north coast leads to upwelling of cold water from deeper in the ocean, which stabilizes the atmosphere, increases atmospheric pressure by decreasing the air temperature above it, and thereby suppresses precipitation. Thus, these locations have arid or semi-arid climates.
Somalia as well as the Arabian peninsula get reversed southwesterly trade winds in the summer half of the year & ordinary northeasterly trade winds in the winter half of the year. The arid climates of most inland locations in those regions can be explained by the fact that those trade winds would have traveled mostly over land & be mostly dry upon reaching those regions. However, the east coast of Somalia in the northern hemisphere as well as the southeast coast of the Arabian Pensinsula should in principle get more quasi-convective or frontal precipitation carried over those storm tracks with the warm Indian Ocean to the east. This does not happen in the summer half of the year because the ocean currents driven by the reversed southwesterly trade winds along the coast are not part of a closed gyre, so any warmth does not go deep, and the reversed southwesterly trade winds steering those ocean currents lead to upwelling of deep cold water which suppresses precipitation for the aforementioned reasons. In the winter half of the year, the ordinary northeasterly trade winds originate over cold dry land and do not travel for long distances over the Arabian Sea, so there are few transient systems of low pressure that would bring frontal precipitation. Thus, these coastal locations have arid or semi-arid climates.
Relative warmth in winter of western, central, and northern Europe
In popular discourse (among people who only remember small bits of climate science that was presented at a very superficial level in grade school), Europe is said to have warmer winters than North America because of the Gulf Stream, which is the western & poleward ocean current in the Atlantic Ocean in the northern hemisphere. The fact that this statement is usually made by comparing the climates of most locations in Europe, which is mostly poleward of the US, to the climates of most locations in the eastern parts of North America at similar or more equatorward middle latitudes makes it trivially true, in the sense that the oceanic moderation of the climates of any west coast from the subtropical ridge over the ocean to the west makes it have milder winter temperatures than climates of any east coast (which are often influenced by seasonal systems of high pressure from cold air settling over the continent, if the continent is big enough) at the same latitudes. However, there is a less trivial version of this statement that requires more careful explanations. The less trivial version of this statement is that locations in western, central, and northern Europe have climates with warmer temperatures overall and, in the cases of western & central Europe but not northern Europe, less of a seasonal contrast in precipitation, than west coast locations at similar latitudes in North & South America (as Africa & the mainland of Australia do not have landmass at those latitudes).
The Pacific Ocean in each hemisphere is significantly bigger in terms of the range of longitudes (as the range of latitudes in each case is roughly similar) than the Atlantic Ocean in the same hemisphere. This means that although the warm currents on the western edge of the Pacific Ocean in each hemisphere are similar in temperature to the warm current on the western edge of the Atlantic Ocean in the northern hemisphere, the currents on the poleward edge of the Pacific Ocean in each hemisphere must traverse much longer distances and therefore become much colder upon reaching the eastern edge compared to the current on the poleward edge of the Atlantic Ocean in the northern hemisphere. Moreover, the west coasts of North & South America are curved in ways that align with the prevailing winds & corresponding ocean gyres of the subtropical ridges over the Pacific Ocean to the west in each hemisphere, so the eastern ocean currents in the Pacific Ocean in each hemisphere become even colder as the prevailing winds parallel to the coast at subtropical & equatorward middle latitudes lead to upwelling of deep cold ocean water along those coasts. This upwelling suppresses precipitation in the summer half of the year, leading to a greater seasonal contrast as well as cooler temperatures over the land in the summer half of the year. Additionally, North America has significant landmass poleward of its west coast at middle latitudes, with such landmass specifically present at poleward middle & subpolar latitudes, so in the winter half of the year, locations along its west coast, especially closer to or at poleward middle latitudes, can be subject to somewhat more frequent incursions of cold air from the pole, leading to more precipitation and slightly lower temperatures overall from cold fronts.
By contrast, the smaller size of the Atlantic Ocean in the northern hemisphere compared to the Pacific Ocean in the southern hemisphere makes the shapes of continents play an outsize role. The curvature of the east coast of North America aligns with that of the prevailing winds generated by the subtropical ridge over the Atlantic Ocean in the northern hemisphere, and that curving part of the coastline that steers the warm current toward Europe is at the right latitude equatorward of Europe such that it does indeed go toward Europe instead of missing it by going too far poleward.
The climates of northern Europe, specifically the Scandinavian Peninsula as well as the British Isles, are affected by the Gulf Stream for these reasons. These regions exist ostly at subpolar latitudes but have climates more characteristic of poleward middle latitudes; notably, most locations along the west & poleward coasts of Norway remain ice-free throughout the year. Additionally, there is a seasonal system of low pressure around northern Europe in the winter half of the arising from the collision of mild moist air from the subtropical ridge over the Atlantic Ocean and cold dry air from seasonal systems of high pressure settling over Asia (which swing around the pole and then back equatorward, as there is no significant landmass near the north pole). This means that northern Europe gets somewhat more precipitation in the winter half of the year than in the summer half of the year.
The eastern edge of the subtropical ridge over the Atlantic Ocean in the northern hemisphere comes close to the west coast of the Iberian Peninsula in Europe. This means that locations along the west coast of the Iberian Peninsula experience climates similar to those at similar latitudes along the west coasts of other continents, especially because the equatorward turning of prevailing winds leads to upwelling of deep cold water along that coast as well as the northwest & west coasts of Africa in the northern hemisphere. However, the fact that the mainland of France & central Europe are much farther east of the Iberian Peninsula mean that they get the prevailing westerlies throughout the year instead of being subject to suppression by the subtropical ridge of precipitation, the lack of significant mountain ranges along lines of longitude to lead to orographic lifting precipitation decrease precipitation levels overall through the year compared to locations at similar latitudes in North & South America, and the fact that in the summer half of the year the moist air parcels over the ocean & dry air parcels over land can reach similarly high temperatures leads to a relatively high probability of quasi-convective precipitation along with frontal precipitation in the summer half of the year compared to most west coast locations at similar latitudes in North & South America. In particular, it is worth noting that the ocean current that moves eastward onto the west coast of France (which is largely at poleward middle latitudes) during the summer half of the year has the same temperature as the current that moves westward onto the east coast of the mainland of Australia at subtropical & equatorward middle latitudes during the winter half of the year, and the land temperatures are similar in those places during those times of year, so both places experience similar amounts of frontal & quasi-convective precipitation during those times of year. In particular, around July & August (which is the middle of the summer half of the year in the northern hemisphere and the middle of the winter half of the year in the southern hemisphere), mean land temperatures in locations along both coasts are approximately 17 degrees Celsius ranging between 12-22 degrees Celsius at from the middle of the night to the middle of the day, mean ocean temperatures in those locations are approximately 20-22 degrees Celsius, and the prevailing winds are onshore, so quasi-convective precipitation can happen, and some frontal precipitation can happen too.
Wet winters & dry summers along the poleward coast of the mainland of Australia
Locations along the poleward coast of the mainland of Australia along the Great Australian Bight, despite being at latitudes of 30-40 degrees, get wet winters & dry summers. This is because the landmass of the mainland of Australia extends poleward more on its eastern side than on its western side. Thus, the Great Australian Bight incorporates more cold water from the cold Antarctic Ocean and from the cold current along the eastern edge of the Indian Ocean than warm water from the warm current along the western edge of the Pacific Ocean. This means that the Great Australian Bight supports a smaller-scale subtropical ridge at a similar latitude as the subtropical ridges over the Indian & Pacific Oceans in the summer half of the year due to the contrast between the cold air temperatures over the ocean & hot air temperatures over land, keeping those locations dry. In the winter half of the year, the subtropical ridges over the Indian & Pacific Oceans move equatorward beyond the most equatorward latitude of the Great Australian Bight, there is no subtropical ridge over the Great Australian Bight at that time as the air temperatures over the ocean are warmer than the air temperatures over land, and the prevailing westerlies generated by the seasonal system of high pressure from the settling of cold air over land collide with the mild moist air carried by the prevailing westerlies from the subtropical ridge over the Indian Ocean to the west, leading to frontal precipitation at those locations. The ranges & distributions of temperatures & precipitation levels transition between those typical of a west coast at middle latitudes to those typical of an east coast at middle latitudes around the coast of the state of Victoria in the southeastern part of the mainland of Australia.
Rainforest effects
Plants are important stores of water, and at large enough scales, they have enough capacity to store water as well as enough water actually stored to affect the climate of a location. This can most easily be seen in the rainforest climates of South America & Africa near the equator & far inland, as well as in those of the islands of Southeast Asia, as the presence of so much water changes the temperatures and increases humidity levels to the extent that incoming moist air parcels are forced to shed their moisture. This can also be seen to a lesser extent in the inland eastern parts of North America at middle latitudes, as the large amount of farmland forces more quasi-convective or frontal precipitation from moist air parcels originating from over the Gulf of Mexico (which I refuse to call the "Gulf of America") & moving poleward. This is seen to some degree, but not as reliably, for temperate rainforests at locations around 40-50 degrees in latitude along the west coasts of North & South America, because the suppression of precipitation in the summer half of the year when evaporation would be greatest may lead to depletion of stored water, though this may be compensated for by the greater storage of water leading to even more frontal precipitation from mild moist air parcels in the winter half of the year.
Human activities adding or removing plants at large scales can have significant effects. This has already been mentioned with respect to farmland in the eastern parts of North America at middle latitudes. Europe used to have temperate rainforests, but when humans started to cut down those trees en masse to make room for bigger human settlements & agriculture, the climate changed drastically to have less precipitation & less moderate temperatures. The tropical rainforests of South America were in part created by human activity, and their recent destruction has led to less precipitation & hotter temperatures. Finally, recent conscious collective human efforts to re-green the Sahel (the region with semi-arid climates between the Sahara Desert in northern Africa and the tropical rainforests around the south coast of western Africa as well as around the equator in Africa) have already started to yield more precipitation & less extreme high temperatures than in the 20th century.
Effects of mountains and elevated plateaus on the ITCZ and on monsoon climates
Elevated plateaus surrounded by mountains can support the ITCZ in the summer half of the year especially effectively. This can be seen in the Tibetan Plateau in Asia, the Mexican Plateau along the central longitudinal (though not literally along a line of longitude) spine of Mexico, the Colorado Plateau in the US, and the Andean Plateau in South America, which draw in warm moist air parcels from all sides where warm oceans are not too far away. The Tibetan, Colorado, and Andean Plateaus, the first and second because of their middle latitudes and the third because of its especially high elevation, also supports a seasonal system of high pressure from the settling of cold air over land especially effectively in the winter half of the year, with such cold dry air being pushed out over the mountains and warming a bit into the lowlands outside. This largely explains why the west coasts of Mexico, Central America, and South America at equatorward tropical latitudes, as well as the west coasts of South & Southeast Asia experience such stark differences in precipitation over the course of the year, with very wet summers & very dry winters (the latter featuring little to no quasi-convective or frontal precipitation even if the land & ocean temperatures would otherwise favor such precipitation). That said, it is worth noting that the south coast of western Africa also gets extremely dry winters due to the ordinary easterly trade winds originating from the settling of cold dry air over the Sahara Desert.
It is worth noting too that many of these places at tropical & subtropical latitudes that are closer to west coasts than east coasts, are more inland, and get wet summer monsoons do not have symmetry in the distribution of precipitation over the course of the year (in which symmetry would imply in this case that the month with the most precipitation is in the month in the middle of the summer half of the year, namely January or July depending on the hemisphere, the month with the least precipitation is in the month in the middle of the winter half of the year (6 months earlier/later), and the increasing trend of precipitation from the winter half of the year to the summer half of the year is roughly the mirror image of the decreasing trend of precipitation from the summer half of the year to the winter half of the year). Instead, within the summer half of the year, most of the spring season is fairly dry, while only the end of the spring season, most of the summer season, and the early part of the fall season are wet. The dryness of the early part of the spring season is because these inland locations would be very hot at tropical latitudes, and the greater specific heat capacity of water would mean that the oceans would only recently have passed their minimum temperature for the year, so the big temperature difference between moist & dry air masses would stabilize the atmosphere with sea breezes. Only when the ocean has become sufficiently hot and close in temperature to that of the air over land, which would happen later in the spring season, would the wet summer monsoon "burst" over land with precipitation. Similarly, in the fall season, the land would cool down faster than the ocean, so any residual reversed westerly trade winds bringing warm moist air parcels from over the ocean to the west would would meet progressively strengthening ordinary easterly trade winds bringing colder dry air masses from poleward plateaus, leading to significant frontal precipitation.
Finally, it is worth noting that some locations close to 30 degrees in latitude (the somewhat arbitrary boundary between subtropical & equatorward middle latitudes) near but somewhat inland of a west coast may get both wet summer monsoons due to the ITCZ in the summer half of the year and a little frontal precipitation from the prevailing westerlies in the winter half of the year (more akin to locations along the west coast at middle latitudes). This happens in the Southwest in the US and neighboring areas in Mexico that are slightly to the south (equatorward) as well as locations in southern Pakistan & nearby locations in western India (constituting the western part of the subcontinent of India). These are not the only locations somewhat near west coasts that get wet summer monsoons, so the commonality among them seems to be the existence of significant precipitation shadows that give these locations arid or semi-arid climates.
Other orographic lifting precipitation effects from mountains to note
As alluded to in the previous subsection, the difference between the east coasts of Mexico & Central America getting significant precipitation throughout the year versus the west coasts of those same places at similar latitudes only getting significant precipitation in the summer half of the year can be explained by the existence of mountain ranges in between. Similarly, as Japan has mountains throughout its lands, the east coast of Japan experiences wet summers & dry winters as the ordinary easterly trade winds carrying warm moist air parcels in the summer half of the year are onshore, so quasi-convective, frontal, and orographic lifting precipitation all happen, whereas the westerly winds generated in the winter half of the year by the seasonal system of high pressure from the settling of cold air over the mainland of Asia become warm & dry upon going over the mountains in Japan & blowing offshore of the east coast; a similar effect happens in the Korean Peninsula for the same reasons. Similar effects & reasons are present in Madagascar, the east coast of Africa at subtropical latitudes in the southern hemisphere, the Ghats of India, the east coast of the mainland of Australia, and the eastern slopes of the Andes Mountains at tropical latitudes in South America, with locations leeward of those mountains getting less precipitation than locations windward of those mountains when those prevailing winds carry warm moist air parcels from over the ocean. A similar effect but with winds coming in the opposite direction occurs at equatorial tropical latitudes in Africa, as the mountains near the east coast mean that reversed westerly trade winds bring frontal precipitation into the interior of that land and further orographic lifting precipitation on the western slopes of those mountains, while the east coast remains dry with prevailing winds blowing offshore. Finally, the reversed westerly trade winds of South & Southeast Asia in the summer half of the year dump most of their precipitation on the windward slopes of mountains (especially the Himalayas). The lack of significant mountain ranges along the equatorward coast of the mainland of Australia and that coast being leeward of mountains in the islands of Southeast Asia (which support the ITCZ through most of the year) explain why that coast does not get as much precipitation in the summer half of the year as other locations that get wet summer monsoons.
Outside of regions that support the ITCZ in the summer half of the year, this orographic lifting precipitation effect can be seen on the western slopes of mountains that get moist air parcels from the prevailing westerlies in the winter half of the year, with areas to the east being much more dry. This can be seen in locations at middle latitudes in North America, South America, the Balkan Peninsula in Europe, Southwest Asia (especially Israel, Lebanon, and Syria), and West & Central Asia (particularly Turkey, Georgia, Azerbaijan, Tajikistan, and Uzbekistan). It can also be seen in locations along the west coast of Japan in the winter half of the year, as the prevailing westerlies generated by the seasonal system of high pressure over the mainland of Asia become warmer & more moist over the Sea of Japan and then collide with cold dry air over land particularly on the western slopes of the mountains of Japan; that said, the west coast of Japan also gets significant quasi-convective & frontal precipitation in the summer half of the year as even more warm & moist air from the east still retains significant moisture & characteristics of low pressure even after crossing the mountains of Japan. Moreover, locations along the west coasts of Europe, North America, and South America at middle latitudes have a precipitation maximum in the middle of the winter half of the year & a precipitation minimum in the middle of the summer half of the year, with roughly symmetric connections between them with respect to the course of the year, because when going from the winter half of the year to the summer half of the year, the land warms consistently & simultaneously with the poleward movement of the subtropical ridge over the ocean to the east, so precipitation is predictably more suppressed closer to the middle of the summer half of the year. By contrast, at locations more inland of the west coast at middle latitudes but still on the western slopes of inland mountains that get the prevailing westerlies in the winter & spring seasons, such as in Idaho in the US in North America as well as in some western locations in Georgia in Asia, the eastern part of Uzbekistan, and the western part of Tajikistan, the land may still be quite cold, so the spring season is actually almost as wet as the winter season, and the summer season is the only truly dry season.
Most of the major mountain ranges in Europe are oriented largely along lines of latitude, so they do not lead to significant orographic lifting precipitation effects. However, they along with poleward oceans do prevent incursions of cold air masses from the pole. Similarly, the Himalayas prevent incursions of cold air masses from over Asia at middle latitudes into India.
Locations along the northern edge of the Mediterranean Sea having less "Mediterranean" climates than locations along west coasts in middle latitudes
Typical definitions of a Mediterranean climate invoke the strong seasonality of precipitation, with much less precipitation in the summer half of the year than in the winter half of the year. Locations along west coasts next to major oceans at subtropical & middle latitudes are closer to those subtropical ridges and in many cases to upwelling of deep cold water, so precipitation in the summer half of the year is strongly suppressed. By contrast, the Mediterranean Sea is fairly warm (24-28 degrees Celsius) in the middle of the summer half of the year, and the subtropical ridge over the Atlantic Ocean in the northern hemisphere does not extend into the Mediterranean Sea, so locations along the northern edge of the Mediterranean Sea do get a bit of quasi-convective precipitation in the summer season, though precipitation is still usually (but not always) skewed at such locations toward the winter half of the year.
Sea effects
If there is a consistent direction of a prevailing wind for at least one season, if that prevailing wind goes across a lake or sea, and if other conditions are favorable for precipitation, then locations at the edge of that lake or sea that get onshore winds tend to get more precipitation than locations at the edge of that lake or sea that get offshore winds. This is most often seen in the winter half of the year at middle latitudes, where the lake or sea is not frozen and is significantly warmer than the land.
This can be seen at a smaller scale around the Great Lakes of North America, in which the prevailing westerlies ensure that locations along the eastern edges of the lakes get more precipitation (usually snow, termed lake-effect snow) than locations along the western edges of the same lakes. However, those lakes are usually too small and become a little too cold for the amounts or distribution of precipitation over the course of the year to significantly change on the eastern edge of the lake compared to the western edge of the lake. More significant changes in the winter half of the year are seen along both edges of the Sea of Japan (with the western edge of the Sea of Japan largely corresponding to the east coast of the Korean Peninsula & the southeast coast of Russia and the eastern edge of the Sea of Japan corresponding to the west coast of Japan) as well as locations all around the edges of the Mediterranean, Black, and Caspian Seas, as the directions of prevailing winds (usually westerly, but northerly in the case of the Caspian Sea) in the winter half of the year mean that many locations along the eastern edges of the Sea of Japan, the Mediterranean Sea, and the Black Sea, as well as locations along the southern edge of the Caspian Sea get much more precipitation & higher temperatures in the winter half of the year than locations along the opposite edges of those seas. However, if a sea or lake freezes, then this effect fails to hold because of the much greater energy needed for the sublimation of ice compared to the evaporation of liquid water.
Phenomena for which I do not know the explanation
There are still a few phenomena that I am unable to explain using surface-level climate concepts & quantities. These include the particular distribution of precipitation over the course of the year for locations along the east coasts of Mexico, Belize, Honduras, and Nicaragua as well as the northeast & southeast coasts of Brazil. Additionally, the Mediterranean & Red Seas near Egypt become very hot during the summer half of the year (around 28 degrees Celsius for the Mediterranean Sea & 30 degrees Celsius for the Red Sea), and prevailing winds on the north coast of Egypt are northerly (onshore) and along the Red Sea are parallel to the coast into the Indian Ocean; these two phenomena should suggest the existence of significant quasi-convective or frontal precipitation along the north coast of Egypt and along the edges of the Red Sea during the summer half of the year, yet those areas get no precipitation then, which I do not know how to explain.
Concluding remarks
At this point, I am largely satisfied with my understanding of why different parts of different continents have the climates that they do and why similar climates arise in some similar parts of different continents (with respect to latitude & east-west positioning on the continent). I do not anticipate writing further posts on this blog about explanations of climates, especially because when I tried to understand from maps of mean wind velocities at different pressure levels how surface-level systems of low or high pressure can be their respective opposites at high elevation, I could not see those patterns or otherwise make sense of those maps at all. The only exception is that I have considered a different categorization of climates and have been slowly putting together a list of locations by labels in that categorization of climates, as I have thought that the categorization may make clearer the reasons for transitions among neighboring climate regions, but the intuitive explanations in this blog post have satisfied me to a greater degree; thus, I am at this point continuing to put together that list mostly for my own satisfaction (as a hobby like building a model train set) and to learn GIS software better (which will help me with my current job too), and if I put together such a map and am satisfied with how it looks, then I may write a post about it.
