This post is essentially a follow-up to a recent post [LINK] about the Köppen & Trewartha categorizations of climates; that post was in turn a follow-up to a recent post [LINK] about my intuitions of various climates. This post will discuss, more systematically & in more detail, the climates that are impossible or not typically observed in the Trewartha categorization even when consistently using the third & fourth letters to specify the hottest & coldest mean monthly temperatures respectively, the pros & cons of the Trewartha categorization, and a proposal that I thought of to address some of the cons of the Trewartha categorization. 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 more.
Some climates are not possible in the Trewartha categorization by definition. These are as follows.
Climates clearly impossible in the Trewartha categorization
1. Climates with the first letter 'A' cannot have the fourth letter (mean temperature of coldest month) be 'l' or lower by definition because those would make the first letter 'C', 'D', or 'E' climates. By the same token, climates with the first letter 'C', 'D', or 'E' cannot have the fourth letter (mean temperature of coldest month) be 'b' or higher because that would make the first letter 'A'.
2. Climates with the first letter 'C', 'D', or 'E' cannot have the third letter (mean temperature of the hottest month) be 'k' or lower because that would make the first letter 'F'. By the same token, climates with the first letter 'F' climates cannot have the third letter be 'l' or higher because those would make the first letter 'C', 'D', or 'E'. Moreover, climates with the first letter 'F' where the second letter is 'i' cannot have the third letter be 'k' because then the second letter would be 't'.
3. Climates with the first letter 'D' or 'E' climates cannot have the fourth letter be 'l' because that would make the first letter 'C'.
Climates not usually observed in the Trewartha categorization for reasons that the the Köppen categorization makes clear
In the Trewartha categorization, climates with the first letter 'E' typically do not have the third letter as 'a' or higher because the labels Cfc, Csc, Cwc, Dfc, Dfd, Dsc, Dsd, Dwc, and Dwd in the Köppen categorization, which all assume 1-3 months with mean temperatures of at least 10 degrees Celsius, implicitly assume that the hottest month must have a mean temperature below 22 degrees Celsius (though the label 'a' increases slightly to 22.2 degrees Celsius in the Trewartha categorization) because the labels Cfa, Csa, Cwa, Dfa, Dsa, and Dwa which have the hottest month to have a mean temperature of at least 22 degrees Celsius also assume at least 4 months with mean temperatures of at least 10 degrees Celsius. This may be because it is basically impossible under the current global climate for a place to be consistently cold enough to only get 1-3 months with mean temperatures of at least 10 degrees Celsius but have one of those months swing wildly enough to have a mean temperature of at least 22 degrees Celsius.
Climates not usually observed in the Trewartha categorization for less clear reasons
1. Climates with the first letter 'C' or 'D' where the third letter is 'i' and climates with the first letter 'D' where the third letter is 'h' do not seem to exist. This may be because those extreme conditions might only be possible in the arid/semi-arid climates where the first letter would be 'B'.
2. Climates with the first letter 'B' where the third letter is 'h' or 'i' but the fourth letter is 'o' or below do not seem to exist. This may be because high levels of heat in the summer seem to be incompatible with freezing mean monthly temperatures in the winter, even in the extreme conditions of arid climates.
3. Climates with the first letter 'C' where the fourth letter is 'o' or lower do not seem to exist. This may be because it is basically impossible under the current global climate for a place to be consistently cold enough to only get 1-3 months with mean temperatures below 10 degrees Celsius but have one of those months swing wildly enough to have a mean temperature below 0 degrees Celsius, which is the logical complement of the above argument against climates with the first letter 'E' having the third letter be 'a' or higher.
Where the Trewartha categorization does at least as well as the Köppen categorization
Within the US, the Trewartha categorization seems to do a better job of consistently categorizing many climates in the Southwest as hot deserts and the edges of those deserts as hot semi-arid climates (including in the southern part of the San Joaquin Valley, which has been fertile enough for long enough that saying that it was a desert before the start of agricultural settlement there seems misleading). It labels many climates directly on the coasts of Washington, Oregon, and northern California, facing west to the Pacific Ocean, as temperate oceanic with cold summers (Dolk) in a relatively predictable way based on latitude (though this is arguably a weak point in another way, as I will discuss below). It labels many climates in the Mountain West as hot or cold semi-arid climates in a relatively predictable way based on latitude or altitude. It labels many climates in the Midwest, New England, and inland northern Mid-Atlantic region as temperate continental with hottest & coldest months varying somewhat predictably by latitude. It labels many climates in the southern Mid-Atlantic, upper South, and southern Midwest as temperate oceanic with hot summers. It labels many climates in the lower South & Florida as humid subtropical. Finally, it labels and many climates in California as dry-summer subtropical with much variability in temperatures by location, due to the many parallel sets of mountains in California that run from north to south and also in some cases from east to west, that need to be characterized by the third & fourth letters to show that variability (including those that the Köppen categorization overzealously labels as semi-arid). These positive features extend to other parts of the world too.
Separate from climate categorizations, it is useful to note that the
reason why most of Argentina is arid or semi-arid while most of the US
east of the Mississippi River is neither arid nor semi-arid is as
follows. The east coast of the US follows the curve of the Atlantic
Ocean current in the northern hemisphere, tending to the pole and toward
the east, so warm moist air can easily follow and enter the east coast
of the US, making those areas humid & wet. By contrast, the east
coast of Argentina goes away from the curve of the Atlantic Ocean
current in the southern hemisphere (instead tending to the pole and
toward the west even as the current tends to the east at those
latitudes), so it is much more affected by the rain shadow of the Andes
Mountains except for cities like Buenos Aires that are closer to both
the equator and the east coast. The same arguments applicable to the US & the Atlantic Ocean can also be applied to China & the Pacific Ocean, though the greater landmass of elevated desert in central Asia creates conditions for a more noticeable monsoon and therefore a greater difference in precipitation between the summer versus winter halves of the year along the east coast of China.
Where neither the Trewartha nor the Köppen categorizations do well
1. It should be noted that climates that in the Trewartha categorization are Cfbk or Cfbl don't exist in North America, and I could only find three cities in South America that have such climates, of which two (Salta in Argentina & Curitiba in Brazil, but not Mar del Plata in Argentina) are because of high elevation, and none of the three are near the Pacific Ocean (west coast). This is likely because the Pacific Ocean in both hemispheres is not warm enough, when bringing moisture and air to the west coast of America at latitudes outside of where the subtropical ridge would produce dry summers, to ensure that more than 7 months have mean temperatures of at least 10 degrees Celsius, so temperate climates on the west coast of America outside of the subtropical ridge would be Dobk or Dolk in the Trewartha categorization (but not Dcbo, Dclo, or anything like those until one more closely approaches the poles, because the moisture and air are still warm enough to ensure that no month has a mean temperature below 0 degrees Celsius). The key point is that the climates labeled Dobk or Dolk are on the other side of the continent in each hemisphere from those labeled Doak in the Trewartha categorization; the Köppen categorization has the same problem with climates labeled Cfb having no geographic continuity with those labeled Cfa in America. The only exceptions are Curitiba being somewhat close to Porto Alegre and Mar del Plata being moderately close to Buenos Aires & Montevideo, as all of the latter cities in each comparison have climates labeled Cfal in the Trewartha categorization.
It is possible to find climates in Europe labeled Cfbk or Cfbl in the Trewartha categorization. These are present in the northern part of Portugal, the west and north coasts of Spain, and the southern part of the west coast of France. However, in Europe, climates labeled Cfak in the Trewartha categorization are only present in the cities around (or facing from inland) the northern part of the Adriatic Sea (with a few exceptions, like Istanbul and Sochi which nevertheless exhibit similar dynamics with the Black Sea), while climates labeled Cfal in the Trewartha categorization are only present in the Azores, again showing no geographic continuity with climates labeled Cfbk or Cfbl. Similarly, climates in Europe labeled Dobk in the Trewartha categorization are present in most of the continent, but those labeled Doak in the Trewartha categorization are only present in the more inland areas around the Adriatic Sea as well as a few cities (Bratislava in Slovakia, Constanța in Romania, and Budapest in Hungary) in the inland Carpathian Basin, and the only continuity with climates labeled Dobk is when going from lower to higher elevations.
The existence of climates labeled Cfbk or Cfbl in Europe but not in North America may be because there is no analogue in the Pacific Ocean to the Gulf Stream in the Atlantic Ocean. If this reason is correct, I think that in turn is because the Atlantic Ocean is smaller than Pacific Ocean and the
large landmass of the Sahara Desert, which lies to the east of the Atlantic Ocean and closer
to the equator, can effectively heat the air going over the Atlantic
Ocean a lot more, whereas the landmass at same latitudes in Mexico, though a
desert and elevated (creating effective additional landmass), is still a
lot less (and elevation leads to a lot more precipitation through
orographic lift) than the the landmass of the Sahara Desert. Thus, the Pacific Ocean is a lot
cooler when it circulates to reach the west coasts of Canada, the US, or
Chile than the Atlantic Ocean is when it circulates to reach the west
coasts of Spain or France at the same latitudes relative to the equator (and in those cases, the presence of the
Atlantic Ocean to the north of continental Europe around latitudes of 50
degrees is like a miniature version of the temperate ocean current
circulating around Antarctica). (On a related note, the heating of Atlantic Ocean by
Sahara Desert could lead to a similar explanation for why Somalia is
mostly a desert despite being on the east coast of Africa at the
latitudes of the trade winds. In particular, the trade winds there may come more from over
the Arabian Desert than over the Indian Ocean.)
The only places with continuous transitions from climates labeled Cfbk or Cfbl to those labeled Cfal are in South Africa, going from Gqeberha & East London (both Cfbl) on the south coast to Durban (Cfal) on the east coast, and in the country of Australia, going from Melbourne (Cfbk) in the southeastern corner on the coast to Sydney (Cfal) in the southern section of the east coast. This is because those continents do not go closer to the pole, so the south coasts are warmed in the winter & cooled in the summer by the ocean current that circulates from west to east around Antarctica (and is not blocked by more landmass, which cannot be the case in the northern hemisphere). Additionally, Johannesburg in South Africa and Mbabane in Eswatini have climates that are respectively Cfbk and Cfbl despite not being too far from Durban, but that has more to do with their higher elevations.
Thus, the splitting of climates in the Trewartha categorization labeled Cfak/Cfal from Cfbk in Eurasia & South America and Dfak
from Dfbk in North America on opposite coasts (which are in many cases
thousands of miles apart), with the former being considerably closer to
the equator than the latter is to the equator, compared to the continuity of those climates in
South Africa & Australia, is an example of a phase transition when
adding landmass in the latitudes of 30-70 degrees. For example, in South Africa and the country of Australia, when going south along the southern part of the west coast (like Cape Town or Perth), then east along the south coast (like Gqeberha or Melbourne), and finally north along the east coast (like Durban or Sydney), it is possible to see sporadic intense thunderstorms in the summer half of the year and somewhat regular drizzles in the winter half of the year in all of these places in different mixtures showing continuity with geographic progression, with the regularity of the latter increasing toward the west and the probability of the former increasing toward the east. By contrast, in the combination of Chile & Argentina at similar latitudes, no such geographic or climactic continuity exists, with regular drizzles in the winter half of the year confined to the west coast of Chile and sporadic intense thunderstorms in the summer half of the year confined to the east coast of Argentina.
(One can argue that there is continuity of these climate groups across oceans. For example, a path from Norfolk, Virginia (Cfak) across the Atlantic Ocean to La Rochelle, France (Cfbk) or from Durban, South Africa (Cfal) across the Indian Ocean to Auckland, New Zealand (Cfbl) goes east and toward the pole in each case. However, this is a very strange way to argue for continuity, given that both categorizations otherwise (apart from the issues discussed in this post) do a much better job of showing continuity of climates within continents.)
2. It seems problematic to make Nantucket and Vancouver have the same climate labeled Cfb in the Köppen categorization or Dobk in the Trewartha categorization. Although their temperature patterns & precipitation levels are similar, Nantucket is much less vulnerable to wildfires as there is no drying trend at all in the summer, whereas Vancouver just barely escapes a Csb label in the Köppen categorization despite a marked drying trend in summer that has left it very vulnerable to wildfires. (This vulnerability to wildfires will come up below too.)
3. It seems problematic to label, respectively in the Köppen or Trewartha categorizations, Skagway as Dsb or Dclo, Flagstaff as Dsb or Dcbo, and Salt Lake City as Dsa or Dcao. These cities are mostly on high plateaus surrounded by mountains with their rain shadows, making the trends over the year much less regular compared to other climates, so the wettest & driest months both occur in the summer half of the year even as the driest month is very dry in an absolute sense & significantly drier relatively speaking compared to the wettest month. All of these cities exhibit effects like monsoons at times of year that do not neatly divide into the winter versus summer halves of the year, so perhaps both categorizations could use a new label like 'm' for monsoon, such that Skagway is Dmb or Dmlo, Flagstaff is Dmb or Dmbo, and Salt Lake City is Dma or Dmao, though details of what the relative difference between the driest and wettest months in the same half of the year must be to qualify for this label remain to be determined.
4. Many cities in northeastern Europe have climates labeled Dfb in the Köppen categorization or Dcbo in the Trewartha categorization, but they are much more dry than cities with the same labels in the Northeast or Midwest of the US. This is because cities with those labels in the US tend to lie relatively close to the Atlantic Ocean or one of the Great Lakes, in either case getting large amounts of moisture through the year from those bodies of water as well as from the Gulf of Mexico, whereas cities with those labels in Europe tend to lie far from large bodies of water. (The same effect occurs when comparing Bratislava to Philadelphia, both of which have climates labeled Cfa in the Köppen categorization or Doak in the Trewartha categorization, or Bucharest to Toronto, both of which have climates labeled Dfa in the Köppen categorization or Dcao in the Trewartha categorization, but there are far fewer cities in Europe than in North America with those labels, so this is more of a trivial point.) It isn't clear to me how big of a problem this is because I'm not so familiar with the vegetation of those parts of Europe or North America. The rationale in both categorizations may be that if neither sets of climates are dry enough to have the first letter be B, then the large differences in annual precipitation don't actually matter as much. However, I could see this overlap becoming a bigger problem as the global climate warms, because those cities in North America will likely get more precipitation, while those cities in Europe will likely get less precipitation and be more vulnerable to drought.
Where the Trewartha categorization does worse than the Köppen categorization
1. In the US, cities like Yreka, Boise, Medford (in Oregon), Elko, Bend (in Oregon), Roseburg (in Oregon), Spokane, Leavenworth (in Washington), and Coeur d'Alene are in mountainous or other highland areas and are in the western part of the country, so they exhibit significant drying trends in the summer that make them experience frequent droughts and wildfires. Despite this, the Trewartha categorization makes no allowance for distinguishing climates with dry summers if less than 8 months have a mean temperature of at least 10 degrees Celsius. The rationale for this choice may be that colder climates cannot support as much precipitation anyway due to the equilibrium partial pressure of water decreasing with decreasing temperature, though this rationale at once falls apart when seeing extremely wet and cold climates like those in the panhandle of Alaska. In any case, this choice becomes even harder to justify when some of these cities have the same labels in the Trewartha categorization as New York City (Doak) or Boston (Dcao) even though those cities, whose labels do exhibit continuity with their geographic surroundings in the Northeast, do not exhibit dry summers and are generally much more wet through the year. It is worth noting that the Trewartha categorization has optional labels for highland climates that corrects each mean monthly temperature according to elevation, but journal articles, including the one linked by the Wikipedia article about this elevation correction, make clear that the exact implementation in labeling is ambiguous (whether only the first and second letters change according to the correction or the third and fourth letters change too). Thus, I thought of a possible solution by replacing the Trewartha categorization labels Dc and Do with Df and Ds, such that under the Trewartha categorization, Ds has the same temperature profile as Dc or Do but with the same precipitation profile as Cs, Df has the same temperature profile as Dc or Do but with the same precipitation profile as Cf, and whether the lowest monthly mean temperature is below 0 degrees Celsius is determined just by the fourth letter. With this change, only the aforementioned cities and a few other similarly-situated cities around the world have a label that starts with Ds, suggesting more continuity between lowland Cs and highland Ds climates (similar to the Köppen categorization but to a lesser extent). (I also defined Es and Ef climates analogously to replace Ec and Eo, but I could not find any cities with Es climates.)
2. In Italy, the cities of Genoa and Naples have the label Csa under the Köppen categorization but the label Cfak under the Trewartha categorization. This is because the Trewartha categorization, while having the same conditions on the absolute amount of precipitation in the driest month of the summer half of the year and its relation to the wettest month in the winter half of the year as the Köppen categorization, adds the restriction that the total precipitation in the year must be less than 890 millimeters (which is the threshold that I have used, though some people choose 900 millimeters instead, which ultimately makes a difference only for a few cities). The rationale for this definition may be that any place that gets more rain than this threshold cannot experience frequent drought or vulnerability to wildfires even with a drying trend in the summer. I'm not sure what the situation is in Genoa or Naples in that regard, so I can't speak to that further. It is worth noting though that some journal article authors have defined a "wet Cs" label in the Trewartha categorization that has the same precipitation threshold for the driest month in the summer half of the year and the same ratio inequality relating it to the wettest month of the year but does away with the annual precipitation threshold, essentially restoring the definition of the label Cs to what it is under the Köppen categorization; however, I'm not sure how common this practice is.
3. Even using the labels Df and Ds as I've defined them above, many cities in the Northwest of the US still have the labels Dfbk or Dflk under my proposed categorization (or, respectively, Dobk or Dolk under the standard Trewartha categorization) despite the clear drying trend in the summer (which the Köppen categorization captures in the label Csb). In this case, I can more confidently call this a failure of the Trewartha categorization given how often droughts and wildfires happen in the Northwest (as well as around Vancouver in Canada), which is ironic given that the original goal of the Trewartha categorization was to separate the labels for those areas from those more applicable to California even as wildfires & droughts happen often in both California & the Northwest. Similar problems occur with the labeling of cities in the northern part of Portugal.
Part of the problem is that even using the labels Df and Ds as I've defined them above, my definition's use of the same annual precipitation threshold as the Cs labels in the Trewartha categorization (not using the "wet Cs" label) is somewhat arbitrary. One can argue that the annual precipitation threshold should be higher (so more cities qualify) because as the air cools, a moist air mass that comes through is more likely to produce more precipitation. One can also argue that the annual precipitation threshold should be lower (so fewer cities qualify) because cooler air has a lower equilibrium partial pressure of water vapor. One can even argue that Cs, Ds, and Es climates should only have conditions on the absolute precipitation in the driest month in the summer half of the year and its relation to the wettest month in the winter half of the year, as in the Köppen categorization, without any additional thresholds for annual precipitation. The last of those 3 arguments makes the most sense to me given how the drying trend in summer is so strong even in relatively wet parts of the Northwest to create vulnerability to drought & wildfire.
Perhaps a compromise would be analogous to the additional restrictions in the Trewartha categorization for climates with the first letter 'A', compared to the Köppen categorization, such that a dry season is defined as having at least 3 months with precipitation less than 60 millimeters (while not qualifying as a climate with the first letter 'B'). In this case, perhaps the definition could be that Cs, Ds, and Es climates have at least 3 months in the summer half of the year with precipitation less than 40 millimeters (which is the threshold used by some people, instead of 30 millimeters, for the precipitation of the single driest month of the summer half of the year for the labels Csa, Csb, Csc, Dsa, Dsb, Dsc, and Dsd in the Köppen categorization), and the driest of those months gets 3 times less precipitation than the wettest of the winter months, but there is no further threshold for annual precipitation. (The climates that I would label Cf, Df, or Ef would respectively meet the temperature definitions of climates with the first letter 'C', 'D', or 'E' but not the new precipitation definitions of climates labeled Cs, Ds, or Es regardless of the third and fourth letters.)
Applying these labels consistently, most cities in the Northwest would qualify for labels of these sorts, and there would be a clear continuity from Csak or Csbk in California to Dsbk in the valley or mountain regions of Oregon & Washington, from Cslk in San Francisco to Dslk along the coasts of northern California and then to Dflk along the coasts of Oregon and Washington (though the label Dslk is also used for Victoria, British Columbia and Olympia, Washington because of the stronger rain shadow effects of surrounding mountains combined with the generally cooler air of the area), and from Csak in the northern part of the Sacramento Valley in California to Dsak in the mountains & highlands of northern California & Oregon which in conjunction with Dobk around Seattle then leads east across the mountains to Dsbo in the inland highland parts of Washington (such that the presence of 'o' as the fourth letter makes clear the more extreme environment being due to the combination of elevation and increased proximity to the pole). Similar continuity is apparent when going from north to south in the Central Valley of Chile as well as when going from south to north in Portugal, as a few climates that would be labeled Cf or Do in the standard Trewartha categorization would be labeled Cs or Ds in my proposed categorization. Only a few cities, like Temuco in Chile, Marseille in France, Genoa, Florence, and Rome in Italy, Thessaloniki in Greece, and Istanbul in Turkey which are very close to Cs or Ds labels instead have Cf or Df labels only because either the third-driest month in the summer half of the year has a precipitation level marginally more than 40 millimeters or the wettest month of the year happens to be in the first or last month of the summer half of the year (even though the winter half of the year is wetter on the whole and the first or last month of the winter half of the year may have a precipitation level very close to that of the wettest month in the summer half of the year). Apart from these changes (that in my view are desired, except for the aforementioned side effects of a few cities having labels Cf or Df instead of Cs or Ds due to being edge cases), inconsistent labeling of climates for the very few highland subtropical cities experiencing monsoons, and the general use of Df or Ef as appropriate instead of Dc, Do, Ec, or Eo, no other labels change, which is a good sign that this change does not throw the baby out with the bathwater with respect to the standard Trewartha categorization.
I should finally note that, just as in my first iteration of this proposed set of labels, I could not find any cities or towns with the label Es in my proposed categorization, which is closely related to the fact that in the Köppen categorization, climates with the labels Csc, Dsc, and Dsd are extremely rare and are confined to very high altitudes (which would therefore almost certainly not be population centers) near climates that would be labeled Dsa or Dsb, which in turn are confined to moderately high altitudes near climates that would be labeled Csa or Csb. At lower altitudes, climates that have 3 or fewer months with mean temperatures of at least 10 degrees Celsius must be close to a pole, where the prevailing winds & ocean currents would work against a trend that creates both absolutely dry conditions in the summer half of the year and relatively very wet conditions in the winter half of the year.
Implications for a changing climate
Although I originally wrote this post just to follow the previous posts, I more recently came across an article in the Washington Post [LINK] about how the First Street Foundation, which seems to be a combination of a nonprofit think tank and a consortium of other research groups that focus on climate modeling, has released a report with new data claiming to be more accurate than data from the NOAA that does not account for climate change over the last few decades, such that in many places in the US, the average peak hourly precipitation rate for a storm described by the NOAA as a 1-in-100-year event will increase considerably and the frequency of a storm whose peak hourly precipitation rate is currently listed by the NOAA as a 1-in-100-year event will increase considerably. (The linked Washington Post article didn't define a 1-in-100-year event, but from what I can tell from other NOAA reports, a precipitation amount that defines a storm as 1-in-100-year is one for which the statistical expectation value of the number of storms with at least that amount of precipitation in any 24-hour period within a 100-year period is 1.) The map in the linked Washington Post article breaks this down by county within each state, though even this illustrates problems with using counties. For example, Mercer County in New Jersey is fairly small, mostly flat, and close to the Atlantic Ocean, so the climate is fairly uniform across the county (though not constant with respect to time of year); by contrast, Fresno County in California is comparable in area to the whole state of New Jersey and includes both the Central Valley (which is far from the Pacific Ocean but close to sea level in elevation) and the windward (with respect to the prevailing westerlies) side of the Sierra Nevada (including elevations over 4,000 meters), so the climate is much less uniform across the county.
Even when considering counties that are generally uniform in geography, there are concerns with this analysis that the Trewartha categorization, without further changes, would not be able to adequately address. For example, Multnomah County in Oregon (labeled Do, or Dobk, in Portland) could see peak precipitation rates for a 1-in-100-year storm increase from 1.8 inches per hour according to NOAA estimates to 2.2 inches per hour according to the new estimates, which is an increase of 22%, and the expected rate of occurrence of a storm with a peak precipitation rate of 1.8 inches per hour increase from 1 every 100 years to 1 every 74 years, which is an increase in frequency of 35%. By contrast, Arlington County in Virginia (labeled Do, or Doak) could see peak precipitation rates for a
1-in-100-year storm increase from 3.2 inches per hour according to NOAA
estimates to 4.6 inches per hour according to the new estimates, which
is an increase of 44%, and the expected rate of occurrence of a storm
with a peak precipitation rate of 1.8 inches per hour increase from 1 every 100 years to 1 every 26 years, which is an increase in frequency of 285%. The problem is that defining the intensity of a storm by its peak hourly precipitation rate over a 24-hour period glosses over the ways that rain could be more frequent, more intense, and therefore more destructive near the west coast even without the peak precipitation rate increasing as much. In particular, as the rain over California near the end of 2022 & in the first few months of 2023 showed, the peak hourly rate was not that much more than typical, but there were many more hours of such steady rain, and that continued for many more days without letting up; similar concerns exist in Oregon & Washington but would not exist near the east coast of the US, yet this would not be reflected by pretending like all climates labeled Do in the Trewartha categorization are somehow basically the same. Furthermore, the overall drying trend in California is so strong that weird results can arise. For example, Fresno County in California will see 1-in-100-year storms with peak hourly precipitation rates that are 25% higher than the NOAA predicts, yet storms with the peak hourly precipitation rates that the NOAA predicts will somehow become even less frequent (happening on average once every 130 years). Even this doesn't capture the true danger of flooding in such places, because the storms in California earlier this year showed a lot of flooding in the San Joaquin Valley not because of the primary rainfall in the valley but because of the secondary snowmelt from the mountains. The point is that the character of increases in precipitation is very different in different climates, so the effects and therefore the responses must look very different from each other, yet I'm not convinced that the Trewartha categorization is well enough equipped to capture these important nuances (and I hope my proposed categorization does a little better than the Trewartha categorization in those areas).
