The question that initially motivated my desire to understand correlations of different climates to geography was why San Francisco has a much warmer winter season and cooler & drier summer season than DC despite the two cities being within 1.5 degrees of latitude of each other. A corollary to that question is why many people, including me, experience dry skin during the winter season in most of the northern half of the contiguous US but not at any point of the year in lowland California, even though California experiences very dry summer seasons. The answer has to do with dew points, but it is a little more subtle than simply saying that the dew point in the winter season is higher in lowland California than in most of the northern half of the contiguous US. Follow the jump to see more.
General Phenomenon
The extent to which skin is likely to become dry purely due to temperature & dew point can be quantified through the difference in equilibrium vapor pressures of water. Specifically, the vapor pressure difference of interest is the difference between the hypothetical equilibrium vapor pressure corresponding to a dew point equal to the skin temperature and the equilibrium vapor pressure of the air at its actual dew point.
The use of skin temperature and not the air temperature to compute the hypothetical vapor pressure constituting the first term in the difference is critical. This is because irrespective of air temperature, homeostasis implies that skin temperature tends to be in a narrow range, around 30-33 degrees Celsius. As I will demonstrate in the following sections, using the air temperature would yield a wrong result.
For simplicity, through the rest of this post, I will assume that the skin temperature is always 30 degrees Celsius. The equilibrium vapor pressure at this dew points is 42.5 millibars. For reference, for other dew points corresponding to typical temperatures in the contiguous US, the equilibrium vapor pressure is 2.9 millibars at -10 degrees Celsius, 6.1 millibars at 0 degrees Celsius, 12.3 millibars at 10 degrees Celsius, 23.4 millibars at 20 degrees Celsius, and 73.8 millibars at 40 degrees Celsius.
These reference values of equilibrium vapor pressure show that absolute humidity increases something like exponentially with dew point. This also shows the limitations of relative humidity. Relative humidity is simply the vapor pressure at a given dew point divided by the vapor pressure if the dew point were equal to the air temperature. This means that comparisons between different values of relative humidity are meaningful only if the air temperature is constant and are meaningless for different air temperatures. Examples of the latter point are as follows.
- If the air temperature & dew point are 10 degrees Celsius, then the relative humidity is 100%, whereas if the air temperature is 35 degrees Celsius & the dew point is 20 degrees Celsius, then the relative humidity is "only" 42%. The former condition may evoke a foggy spring day in San Francisco, while the latter condition may evoke a humid summer day in DC. This difference in relative humidity levels belies the more salient fact that the absolute humidity doubles when increasing the dew point from 10 degrees Celsius to 20 degrees Celsius.
- Dew points tend to change much more slowly than air temperatures. As the air temperature increases from night to day, the dew point might not change at all, so the relative humidity decreases (more quickly as the air temperature becomes even higher). However, the absolute humidity has not changed; all that has changed is the temperature allowing more moisture to evaporate.
Specific Comparisons of Interest
In DC, during the coldest part of the winter season, the dew point is typically around -5 degrees Celsius. This corresponds to an equilibrium vapor pressure of 4.2 millibars and therefore a vapor pressure difference of 38.3 millibars, meaning that in an enclosed space, evaporation roughly stops after adding 38.3 millibars more of water vapor pressure.
Outdoors, the air temperature may be around 0 degrees Celsius. Additionally, the prevailing westerlies are stronger in DC in the winter, so that wind will blow away moisture evaporating from skin & replace it with drier air (at a dew point of -5 degrees Celsius). This is why it is so easy for skin to dry out in DC outdoors during the winter season even though the absolute humidity of the air and the vapor pressure difference between the air temperature & dew point are so low.
Indoors, the air temperature may be around 20 degrees Celsius. This is commonly achieved through forced-air convection, which blows hot air into a room. This has a similar effect of ensuring consistently low dew points next to skin indoors as outdoors. This is why it is so easy for skin to dry out in DC indoors during the winter season even though the vapor pressure difference between the air temperature & dew point is much lower than that between the skin temperature & dew point.
My point of comparison in California will be Sacramento, because I used to live near there and its summer temperatures are more extreme. During the winter season, the dew point is typically around 5 degrees Celsius. This corresponds to an equilibrium vapor pressure of 8.7 millibars and therefore a vapor pressure difference of 34.1 millibars. This does indeed relieve some evaporative stress on skin, though quantitatively, the difference is not that big; anecdotally, I can attest to my skin being much less prone to dryness in Sacramento than in DC during the winter season, whether outdoors or indoors.
During the summer season in Sacramento, the dew point is typically around 15 degrees Celsius. This corresponds to an equilibrium vapor pressure of 17.1 millibars and therefore a vapor pressure difference of 25.4 millibars. This is significantly (33%) less than the vapor pressure difference in DC in the winter season, so skin will be significantly less dry in Sacramento during the summer season despite the higher air temperature; using a typical air temperature of 35 degrees Celsius instead of the skin temperature of 30 degrees Celsius to compute the vapor pressure difference would yield a vapor pressure difference of 39.2 millibars, erroneously implying that skin would be drier in Sacramento during the summer season than in DC during the winter season. Additionally, outdoors in the shade (as direct sunlight would heat up the skin much more, leading to much more dryness), there is no analogue to the continuous drying effect of air movement in DC during the winter season (whether outdoors due to natural winds or indoors due to forced convection), as winds in Sacramento during the summer season tend to be slow. The latter point is consistent with the west coasts of continents at middle latitudes being under the subtropical ridge during the summer season, as the balance of Coriolis & pressure gradient forces limit the speeds of winds circling high-pressure centers (whereas the lack of balance allows wind speeds to have no theoretical limit, to lowest order, when circling low-pressure centers).
These differences in dew points account for the majority of the differences in skin dryness in DC versus Sacramento. Additional differences come from very low temperatures in DC during the winter season suppressing natural oil production on the skin, making it easy for the chafing of clothes to strip away skin oils with slow replacement, and from promotion of sweat production in Sacramento during the summer season, which pumps more moisture within the body toward the skin. These differences contribute to why many people in the US find summers & winters to be more tolerable in California than in the eastern half of the contiguous US, as dew points in California tend to be more moderate/tolerable through the year; similar differences can be found between Europe & East/North Asia.
