It may feel like spring, but is it really spring? It depends on how you define it.
1. Astronomy-based definition (March 20 this year): The most common marker for the onset of spring is the vernal equinox, when the sun’s rays shine directly overhead the equator.
Defining seasons based on the position of the Earth in relation to the sun is the long-standing convention, but – from the standpoint of weather – isn’t necessarily a perfect fit. In many places (especially southern locations), springlike weather either arrives well before the equinox or, in a few places (northern locations), a good bit after.
2. Meteorology-based definition (March 1, every year): This alternative definition was more or less developed out of convenience, so that meteorologists and climatologists could neatly average the weather into a three-month bin – starting at the beginning of a month and ending at the end of a month.
“By following the civil calendar and having less variation in season length and season start, it becomes much easier to calculate seasonal statistics from the monthly statistics, both of which are very useful for agriculture, commerce, and a variety of other purposes,” explains the National Climatic Data Center. (The equinoxes and solstices come between the beginning and end of the month and can differ by a day in different time zones and years.)
The meteorological seasons follow the annual temperature cycle – with summer (June 1-August 31) and winter (December 1-February 28) coinciding with the warmest and coldest times of the years, and spring (March 1-May 31) and fall (September 1-November 30) coinciding with the transition months.
The onset of meteorological spring, much like astronomical spring, may not have the weather to fit in some locations.
3. The date of the last of measurable snow: Harry Enten at fivethirtyeight.com proposes the end of winter (and hence the start of spring) is marked by a simple indicator – the median date of the last measurable snowfall. In the Washington, D.C. area, that’s in the first 10 days of March.
If the snow that blanked the D.C. region on March 5 this year was our last, then it was right on schedule, and spring has begun using this definition. (But we’d caution a chilly weather pattern may grip the area at the end of March).
Of course, this “last snow” indicator won’t apply in the South where snow doesn’t fall at all in some (if not many) winters.
4. The average date of the last freeze: Many years and in many places, freezing temperatures can still occur weeks after the last snowfall.
D.C.’s average last freeze is around March 28 -about three weeks after our average last snow. In our colder suburbs, freezing temperatures linger into mid-to-late April on average.
While the date of the last freeze may be reliable indicator that winter weather is truly in the rear-view mirror, springlike weather often begins before it occurs.
5. Temperature statistics: CWG contributor Jack Williams developed a concept of defining seasons based strictly on temperature data. “My method considered winter to be the days with the year’s lowest one-quarter of the year’s daily average temperatures, summer the days with the highest one-quarter of the year’s daily average temperatures,” Williams said. “Spring and fall are the quarters with temperatures between the winter and summer figures.”
Using this method, Williams found spring in D.C. occurs between March 14 and May 25, when the average temperature rises from 47 to 68F.
6. Ground temperature of 40 degrees: David Streit’s – our weather and gardening expert – says a good indicator of spring from a botanical perspective is when (day and night) temperatures have averaged 40 degrees or higher for two straight weeks – allowing the ground to warm to this temperature. “It’s a good rule of thumb for when you start planting the early stuff [like cabages and certain greens] and the early daffodils and crocuses start coming up,” Streit says.
Close to downtown Washington, this milestone is reached usually right around March 5. In our cooler suburbs (around Dulles Airport), it’s closer to March 15.
In the Washington, D.C. area, this occurs on average around March 20.
Spring is off to a very slow start this year, apparently (see map below), based on the limited area of the South where leaves had emerged on March 7.
A limitation of all of the above definitions for the start of spring is that they are based on either fixed dates or average conditions. The weather has a mind of its own and the onset of springlike weather varies from year to year and place to place.
For this reason, here at CWG, we began last year making an announcement when we determined winter was over, based on current and forecast weather conditions. That is, we pronounced winter over when we felt there was no longer any chance of accumulating snow (of course, we tempted fate and it snowed two days later, but that’s another story) and when temperatures no longer had a chance to remain below an average of 40 degrees for two straight days.
From our perspective, despite the milder weather of late, it’s premature to rule out another sustained period of cold weather and/or chance of snow based on the forecast for the latter portion of March.