With snow this weekend and another round of cold temperatures expected in coming weeks, it feels like spring will never get to Illinois. But it will arrive someday – I promise.
Earth’s orientation to the sun on the equinox. Click to enlarge.
In coming weeks, many will point to the spring equinox as the start of spring, which is March 20 this year. The equinox is an astronomical event when the Earth’s axis is at a 90 degree angle to the sun (see figure). In theory, the length of day and night are the same around the world. However, due to how we calculate sunrise and sunset, and how the earth is not a perfect sphere means that the days and nights are not exactly equal in length on the equinox. However, that’s a story for another day.
The bigger problem with the equinox definition is that the spring and fall equinox*, as well as the winter and summer solstice, do not line up well with the annual march of temperature (see figure below for Chicago). With spring, temperatures on average are already warming significantly by March 21, compared to temperatures in January and February. By the arrival of the summer solstice, temperatures are already getting very close to their annual maximum.
As a result, many climatologists and meteorologists use calendar month definitions that line up better with the average annual changes in temperature. It works best for summer (June-August) and winter (December-February) in capturing the warmest and coldest periods of the year. In spring (March-May) and fall (September-November), it works well at capturing the strong transition in average temperatures. Several professional have called the period March-May “meteorological spring”, but it is based on climatological data so the better term is “climatological spring”.
Of course, all of this is a bit academic. We all carry our own definition of spring that is probably more in line with nature – the first signs of green grass, the buds swelling on trees and shrubs, increased activity in wildlife (especially birds). I think the one thing winter does best is to make us appreciate spring even more.
*the use of words like winter, spring, summer, and fall, are in reference to the Northern Hemisphere.