Which is which, & which is most useful?
Now that we’re really done with winter (but not necessarily winter-type weather)--both meteorologically (December 1-February 28) and astronomically (December 21-March 20)*--I thought I’d compare the two for the 2010-11 winter season, focusing mainly on temperatures. (No matter which definition of the season is used, snowfall is tabulated from the first trace or more to the last, regardless of date.) Bear in mind, however, that in the future, when most statistics are gathered, they will refer to the meteorological season, the official standard of the National Weather Service (NWS) as well as some other countries such as Australia and Brazil.
The above graphs paint a clear picture of how our temperatures during meteorological winter compared with the 30-year averages used by the NWS. As you can see, temperatures were well below average at the three official observing stations in December and January but considerably above average in February.
Averaging the temperatures** for all 3 months together yields:
DCA- 36.5 degrees F, compared with the 30-year average of 37.5 degrees
IAD- 32.8 degrees F, compared with the 30-year average of 34.2 degrees
BWI- 33.6 degrees F, compared with the 30-year average of 34.8 degrees
However, if we were to average all of the winter days in the same way for the 90 days of astronomical winter, Reagan Nation Airport (DCA) would have averaged 39.4 degrees F, or 1.3 degrees warmer than the 30-year average for astronomical winter (it took some work, but I calculated this) and 1.9 degrees warmer than the winter’s meteorological season average. I would expect that the other reporting stations would show similar differences.
By the way, the lowest temperature at DCA for the entire winter was 17 degrees, recorded on both January 22 and the 24th. On the latter date, under ideal radiational cooling conditions, Dulles (IAD) recorded 6 degrees and Baltimore (BWI), 8 degrees above zero.
So what does all this mean, if anything? I would say that it just verifies one of the main reasons that the NWS uses the meteorological seasons as the standard. It portrays a more accurate reflection of the seasons, since the 90 coldest and 90 hottest days of the year usually, but not always, fall closer to the meteorological seasons than the astronomical ones. The other reason, of course, is that record keeping is much easier, in that the months of March, June, September, and December are not split into 2 different seasons. Which season definition do you think is best?
As far as snowfall is concerned, I suppose you could say that, technically, the snow season began on November 25, when a trace was recorded at DCA, although the observer recorded it as “ice pellets.” The first measurable snow didn’t arrive until December 13, when .2 of an inch fell, followed by 1.5 inches on the 16th, all during “meteorological winter.” During January, a total of 7.3 inches of snow was recorded, most of that—5 inches—during the “commutageddon”storm of January 26 and only .5 of an inch fell during February Officially, only a trace has fallen so far in March. To date, then, a grand total of 9.9 inches of snow has fallen this winter at DCA, less than one-fifth of last winter’s snowfall. As the Brooklyn Dodgers used to say, “there’s always next year,” or , in this circumstance, later this week...???
*A 3rd definition of the seasons is called “traditional reckoning,” in which solar insolation is the key: the 90-day period of the year with the most is summer and the period with the least solar insolation, winter. In each case, the remaining days between the end of winter and the beginning of summer would constitute the spring season and those remaining between the end of summer and the beginning of winter, autumn.
To make matters even more confusing, some countries and cultures use still other definitions of the seasons. For example, Ireland uses the NWS meteorological definition as its standard. Yet, Irish calendars have traditionally used an old Celtic calendar system, which defines the seasons as: winter – November, December, and January; spring- February, March, and April; summer – May, June, and July; and autumn – August, September, and October.
**by adding together all of the high and low temps of each winter day, dividing by 2, and then dividing the result by 90. These figures are unofficial approximations.