Seasons marked by the June and December solstices and the March and September equinoxes are the astronomical seasons.
You might wonder about this because for most people in the United States summer begins for all practical purposes with Memorial Day and ends with Labor Day. These dates are close to “meteorological summer, June 1 through August 31.
For most of the mainland United States the meteorological seasons, agree more with temperatures than the astronomical seasons.
Is there a way to define the seasons that track better with temperatures?
In 1993 as I was working on the “USA TODAY Weather Almanac”(Vintage Books, 1994), I came up with one way to define seasons based on temperatures.
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. Spring and fall are the quarters with temperatures between the winter and summer figures.
While the idea of temperature-based seasons is fun to play with, it has absolutely no practical use that I can imagine.
The meteorological seasons are the most practical since they make it easy to calculate seasonal averages. That’s tricky with the astronomical seasons since the dates of the solstices and equinoxes can differ by a day in different time zones and years. Also, you don’t have to look up the dates of the solstices and equinoxes.
For the “USA TODAY Weather Almanac,” we calculated the temperature-based seasons for a dozen cities including Washington, using U.S. National Climatic Data Center (NCDC) daily temperature normals.
“Normals” are 30-year averages. Those used to calculate temperature-based seasons are the average of each day’s high and low temperature, the “daily average” temperature. The NCDC recalculates these normals each decade using figures from the previous 30 years.
Washington’s temperature-based seasons
The current normals are for the period from 1980 through 2010. These are available on the NWS Sterling, Va., office website.
Using these, I’ve worked out Washington’s current temperature-based seasons:
* Coldest time of the year: Jan. 7-22, daily average 35°
* Warmest time of the year: July 6-22, daily average 80°
* Winter: daily average temperatures: 46° to 35° to 46°, Nov. 25-March 13, 109 days
* Spring: daily average temperatures: 47° to 68°, March 14-May 25, 73 days
* Summer: daily average temperatures: 69° to 80° to69°, May 26-Sept. 23, 121 days
* Fall: daily average temperatures: 68° to 47°, Sept. 24-Nov. 24, 62 days
How Washington’s temperature ‘seasons’ have changed since 1961-1990
Here are the highlights of the similar “seasons” in the “USA TODAY Weather Almanac.”
The USA TODAY Weather Almanac used the 1961-1990 figures. For Washington they were:
* Coldest time of year: Jan. 7- Feb. 1, daily average 35°
* Warmest time: July 23-25, daily average 80°
* Winter Nov. 22-March 18, 46° to 35 to 46° 117 days
* Spring March 19-May 24, 47° to 68° 67 days
* Summer May 24-Sept. 23, 69° to 80° to 69°, 122 days
·* Fall Sept. 24-Nov. 21, 68° to 46°, 59 days.
Source: USA TODAY Weather Almanac, p. 124
The two most obvious differences between Washington’s 1961-1990 and 1991-2010 climates is that while winter’s coldest period and summer’s warmest period have the same average temperatures, winter is eight days shorter and summer is a day shorter. Spring, which is now six days longer and fall, which is three days longer use the days lost by winter and summer.
I’m interested in what climate scientists think about these figures.