Today I celebrate the first first day of spring. Given what we’ve had, the +8º F walk in to work this morning might have actually felt spring if we’d had a little less wind. But never mind the temperature. Today marks my first of many celebrations welcoming spring. How, you ask, can I possibly celebrate January 24 as the first day of spring here in the frozen, frozen north? Well, at least I am going to pretend that you asked that.
The mechanical characteristics of the seasons, like temperature and length of daylight, result from our coupled orbital and rotational motion. Because our orbit wraps around each year, the seasons come and go on the familiar annual cycle. The characteristics we associate with the seasons are, thus, periodic functions of time with peaks in one season and troughs in the opposite season or ascending values in one season and descending values in the opposite season. Plotting the amount of daylight (photoperiod) as a function of the day of the year yields a graph that looks a lot like a sine wave with a period of one year. It’s not exactly a sine wave but it is similar to one. Now, for many characteristics of the seasons it makes sense to me to call the minimum of this curve midwinter and the maximum midsummer. Winter becomes the 13 weeks centered on this minimum and the first day of spring arrives 6 and a half weeks after the minimum. In practical terms we like to talk about “slow” parts of sine-like functions and “fast” parts. In the slow parts, near maximum and minimum, the characteristic that is being plotted changes relatively slowly with time; in the fast parts that same characteristic changes relatively rapidly. Winter and summer are the seasons of slow change; fall and spring are the seasons of rapid change. I like to think of the first day of spring and the first day of fall as our slipping onto the fast part of the curve as shown below.
Now we have a mechanism in place to recognize seasonal transitions in many ways. Average, or “normal”, temperature may be the most obvious. When we slip onto the fast part of the curve of temperature versus time we reach the first day of “temperature” spring. That will happen a few weeks after we have slipped onto the fast part of the photoperiod curve, when we celebrate the first day of “daylight” spring. But now we can break it down even further. Locally, sunset time reached its earliest point on about December 9. That was my “evening daylight” midwinter. That makes today the first day of “evening daylight” spring. Don’t overdo the celebrating because we get to do it all again in a couple of weeks when we reach the first day of “daylight” spring. Then come “morning daylight” spring, “temperature” spring, meteorological spring and astronomical spring. Just for the heck of it I celebrate the day our average high temperature gets back above freezing as a first day of spring as well. Then there are the less predictable first days of spring like the first wood duck I see, the day the flowers on the west side of my house pop through the ground and the first morel I find. With so much celebrating of the arrival of spring it’s easy to forget that here in Northeast Iowa we have a winter at all, even when it is -20º F.