WRAL WeatherCenter Blog

Meteorological spring? It's a real thing

Posted March 2, 2015

The end date of the coldest 91-day period based on daily climatology from NCDC. (Source: Iowa State University)

After the last couple of weeks, we are looking for any hint, any sign, of hope that winter is behind us and warmer days are ahead.

For some, we will get that this weekend when we “spring forward” by setting our clocks ahead one hour. The “extra hour” of daylight in the evening helps get many of us out of the winter doldrums; however, that has basically nothing to do with how we determine seasons.

Meteorologists like to talk about meteorological seasons.

There’s meteorological winter – climatologically, the coldest three calendar months of the year: December, January and February, meteorological summer – the three hottest: June, July and August, and the transitions between them: Spring is March, April, May and Fall is September, October, November.

By that reckoning, spring began on Sunday, March 1.

Of course, you wouldn’t have known it based on the widespread freezing rain we woke up to. (Thankfully, and with apologies to the Bard, "one day does not a meteorological spring make.")

Defining seasons this way is clean, convenient, and easy to use – who can remember when the equinoxes and solstices are, especially when they change from year to year?

But for some reason, the idea of meteorological seasons always ruffles the feathers of Tony Rice, our resident astronomy expert.

Tony’s argument is that our seasons are caused by, and therefore should be determined by, the combination of the earth’s orbit around the sun and the 23.5 degree tilt of its axis relative to that orbit. Astronomical spring begins this year on Friday, March 20, at the Vernal Equinox, the time when the sun is directly over the equator on its way northward.

That’s all well and good, of course, but we normally think about seasons more in terms of the typical weather that’s associated with them than the placement of the sun, the earth and our tilted axis.

That’s where the concept of meteorological seasons comes from, and with all respect to Tony, it’s anything but “hooey." And to address Tony’s criticism, you can, in fact, measure it.

If you divide the year up into four roughly equal periods based on the coldest (winter), warmest (summer), and transitional (spring and fall) times of the year, Monday – yes, Monday, March 2 – is the last day of winter, the last day of the coldest 91-day period of the year for us in central North Carolina.

That means tomorrow, March 3, is the first day of spring, based on climatological data. There’s bad news for folks along the coast, however. They see the coldest 91 days start a little later in December and their tide turns a little later. By this method, you’ll have to wait a couple more days for spring to arrive, but it will be here before the end of the week.

So, where did the whole “meteorological spring is March, April, and May” bit come from?

As it turns out, if you average these meteorological season start and end dates across the country, March 1 becomes a pretty good approximation for the end of winter and the beginning of spring.

Likewise for the start dates for the other three meteorological seasons. Hence, meteorologists have adopted the three-calendar-month method of determining seasons. Meteorological summer begins on June 1, autumn on Sept. 1 and winter on Dec. 1.

So, Tony, we appreciate and agree with your desire to have something you can measure to determine seasons. And as it turns out, our meteorological seasons are based on something we can, and indeed do, measure.

Regardless of how you want to mark its beginning, though, a long-term trend toward warmer weather from now through August is here. Get your swimsuits ready!


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  • Chris Holder Mar 3, 2015
    user avatar

    It's just a convenience thing. You'll see it all the time in meteorology, climatology, air quality, etc. Much easier to assess seasons as whole-month groups (MAM, JJA, SON, DJF) than to splice it on the equinoxes and solstices.

  • John Lobenstein Mar 2, 2015
    user avatar

    What is so difficult of associating Spring with March 21'st +/- , June 21'st with Summer, September 21'st with Fall and December 21'st with Winter winter? Using the 21'st of these four months more accurately reflects the real "world" astronomically than "feel good" mental laziness and/or or "feel good" conversational convenience?

  • Floyd Bridges Mar 2, 2015
    user avatar

    Tony Rice is correct. It may be the last day of the coldest 91 day period in central North Carolina, but central NC is a small place with about .00001% of the world's population. It means nothing for the rest of the earth's inhabitants.