Groundhog’s Day has come and gone, and the rodents are back in their cozy little dens. Phil said “early spring,” and Wally forecasted 6 more weeks of winter. I’m not going to argue the usefulness or validity of an annual rite of late winter. You could probably guess how useful I think waking animals from a nice sleep is in forecasting the weather. Instead, let’s consider two different ways to think about the seasons in general.
First, we have the astronomical seasons based on the revolution of the earth around the sun. As our little blue planet moves around the sun over the course of the year, the sun itself seems to move around in our sky. This appearance is caused by the 23.5 degree tilt of the earth on its axis. The sun appears the farthest north in our sky in the northern hemisphere on the Summer Solstice, and the farthest south in our sky on the Winter Solstice. The farther north the sun is, the more daylight we have and the more warmth we can collect from it, which is why summer is the warmest month. The Vernal (Spring) and Autumn Equinoxes are the days when the sun appears to be over the equator and the amount of daylight and darkness are about equal. These special days are the beginnings of their respective seasons.
The other way to view the seasons is by climatological, occasionally called meteorological, seasons, and it is a bit more clear-cut on the calendar. The first day of climatological spring is March 1st, and spring runs through the last day of May. Summer runs from June through August because those are typically the warmest months of the year. Fall runs from September through November, and winter is December through February, which are usually the coldest months.
Why do meteorologists count the seasons differently? When talking about climate, we tend to look at the months grouped according to similar temperature as described above. Also, for the purpose of year over year comparisons, calendar months are much easier. The astronomical seasons’ start dates vary by a day or two from year to year. When tracking trends, keeping the same start and end dates every year (with the exception of February in a leap year) makes more sense.