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Cold Winters in Boulder
As we did with hot summers, we consider several measures of winter weather. First, does cold weather sometimes arrive early, so that the average temperature over the December-February period is lower than that in calendar winter (Dec 21 - March 21)? Then, there's the measure of the number of very cold days, which we define as minimum temperature below 10 degrees. The table below compares the coldest winters by each of these measures. We found that the June-August period in the hottest years had about the same average temperature as the summers of the hottest years by that measure. In contrast, the calendar winter is substantially colder than the December-February period.
Of course, it's also interesting to see which were the warmest winters by the same measures. The table below provides those comparisons. Although we sometimes complain about "early winter" when we get some cold days in early December, it's clear from these two tables that the first part of March, on average, is several degrees colder than the first part of December. Note that 2000 was the only year to make the 10-warmest list in all three categories.
Finally, we have a graph of the number of 10-degree days, with the different measurement locations noted. Each point represents a winter that includes parts of two calendar years. That is, the 2006 value includes the period September 2005 through April 2006. It's interesting to note that the number of 10-deg days didn't vary much with location until the station was moved to the NIST grounds. The NIST campus is closer to the base of the Flatirons and at somewhat higher elevation than the earlier measurement sites. Much of the difference may be due to being slightly above the coldest part of the inversion layer that is often present on cold days.