I agree Probably true--I can't find anything conclusive on that. It stands to reason that the diffusivity of water vapor should increase with temperature, and therefore warmer air should dissipate moisture more quickly. It would depend on the direction of the wind on the days in question, but typically I would agree that the humidity on Sněžka is higher, spoken purely from personal experience. The area of Yosemite tends to be rather dry, particularly as compared to the Czech Republic in general. There are no obstacles between the Pacific and Mammoth either (not that I'm aware of, at least), and the Pacific is much closer to Mammoth than the North Sea to Sněžka. I would guess that the direction the wind was blowing from would be the main source of any difference in humidity (air temperature and amount of sunshine along the path of the air mass would also contribute). An interesting factoid, to be sure, but not really relevant to this discussion, given that we have already established temperatures in both places. As far as I can tell from the link you provided, the Köppen climate classification only deals with absolute air temperature, rather than apparent temperature. If we had the exact dates, yes, it might be possible to get that info from an almanac for the area of Mammoth as well. My naive guess, having not been either place on these days, is that probably both humidity and relative sunshine played a role in the large discrepancy in apparent temperature.