Sources used in this document:

San Francisco is at latitude 37.4 north, and its elevation is 7 feet. The maximum temperature for the years 1961-1990 was 106 degrees Fahrenheit, and the minimum low temperature for the same period was 24 degrees Fahrenheit. Rain was more common than snow during this time, with a yearly average of 19.7 inches of rain compared to .1 inches of average yearly snowfall. In contrast, Denver is at nearly the same latitude, 39.5, but its elevation is 5333 feet. Denver's maximum temperature for 1961-1990 was 103 degrees Fahrenheit, with a minimum of -25 degree Fahrenheit. Snow was far more common than rain in Denver, with only 15.4 inches of yearly average rainfall compared to an average of 60.4 inches of yearly snowfall.

Although San Francisco and Denver are at nearly the same latitude, they have dramatic differences when it comes to low temperatures and yearly snowfall. Denver has a minimum temperature nearly fifty degrees lower that San Francisco's, even though their maximum temperatures are only 3 degrees apart. In addition, Denver sees an average of roughly 60 inches of snowfall a year, compared to San Francisco's average of .1 inches per year (although San Francisco does get slightly more rain than Denver). In addition to their obvious elevation difference, the distinct differences in climate are likely influenced by the two cities vastly different geographic features. Perhaps most importantly, San Francisco sits directly next to the ocean, and thus its climate is especially affected by the formation of water vapor over the ocean. The ocean serves to moderate San Francisco's temperature, so it rarely if ever gets cold enough to snow. Denver, on the other hand, sits on the eastern side of the Rocky Mountains, which blocks most water vapor coming in the ocean and so feels the moderating effects of the ocean far less immediately than San Francisco. In fact, the water vapor which does eventually reach it from the ocean makes it way through Canada before heading south, and so it is likely far colder than it would be without the barrier the mountains provide. Thus, Denver gets huge amounts of snowfall each year, spending nearly half the year with temperatures cold enough for snow. San Francisco almost never gets that cold, and even if it does, never with enough frequency to produce the massive amounts of snowfall Denver sees.

At 5:36 PM on May 1, 2011, the GEOS (Geodetic Earth Orbiting Satellite) showed a thin layer of water vapor trailing from a larger pattern circulating off the Canadian coast while making its way inland, and San Francisco was just catching the southern-most part of its tail. The current view within San Francisco is a bright, sunny sky with no visible clouds or water vapor, so the pattern of water vapor shown by GEOS is not visible (although a location directly on the ocean would likely have a different perspective). The current weather seems largely consistent with the climate data, as May receives very little rain on average, although it is slightly warmer than average for the month.