These graphs of actual local measurements show observed changes in Aspen's climate over the past 50 years. They are consistent with observations of global average changes as well as with the projections of global climate models. For example, Aspen's data, global data, and model projections all show nighttime low temperatures rising more than daytime high temperatures, earlier springs and later falls, and a decline in the number of very cold nights.



This graph shows how the number of very cold nights has dropped over the last fifty years, at the rate of over five fewer below-zero days every twenty years, or thirteen fewer below-zero days over fifty years. Cold night-time temperatures are necessary for making snow for early ski season opening, and for keeping the snowpack throughout the winter.

The next graph shows how long the season is between the last frost in spring and first frost the next fall - in other words how long the summer is. The frost-free period has increased by more than 31 days over the last fifty years, or more than twelve days every twenty years.



Changes in when winter and spring arrive mean changes in the length of the ski season. The next graph shows that fall has been arriving later in Aspen - 18 days later over the last fifty years, or over a week later in twenty years. This delays the start of the winter ski season, but lengthens the summer season.



Spring has also been coming earlier, as the next graph shows. The last frost each winter has been about ten days earlier over the last fifty years, meaning an earlier end to the ski season and earlier start to summer. Spring has been arriving earlier at the rate of a little over four days over the last twenty years.