There is probably no day greeted with greater joy and anticipation than the first day of spring -- especially after a Minnesota winter! Sometimes, the only thing that gets us through February is knowing that better days are on the way. But when, exactly, does spring get here?
TV weathermen will tell you that spring starts on the vernal equinox -- the day when the number of hours of daylight are equal to the number of hours of night. (In 2005, this falls on March 20.) The problem is, the weathermen are wrong.
Seasons are not astronomical phenomena; they are climatological phenomena. Or, to put it in English: seasons aren't about the sun in the sky; they're about the weather on Earth. They are defined by temperature. Winter is the coldest part of the year, summer the warmest, and spring and fall the periods in-between. On average, the coldest part of the year runs from December 4 through March 4. Which makes Saturday, March 5, the first day of spring!
Now, the problem is, this date is just an average. In any given year, there will almost certainly be nicer days before it and lousier days after it. Which is probably what led the weathermen to go with the equinox. It's very predictable -- down to the minute -- and there's no arguing: the days before it are shorter, the days after it are longer. (It's also three weeks later, which means the chance of nasty weather after your "first day of spring" is diminished.)
But daylight isn't the issue. Temperature is what counts. Besides, if you make the equinox the first day of spring, then the first day of summer will be the solstice -- the longest day of the year, June 21. And, as all good Scandanavians know, the longest day of the year isn't the first summer's day; it's MID-summer's day!
The first day of each season, according to average temperature:
(To learn more about how the Sun and Earth create our seasons, visit the Weather section of the Experiment Gallery on level 3.)