Degree days: Do you need a degree?

Like baseball, weather is a prodigious generator of numbers, some elegant, some wonderfully useless, some arcane.

In the latter category, we would have to place "degree days." We think of baseball because our old colleague Steve Lopez once likened the "degree day" to the game-winning RBI of the 1980s, a statistic deemed so confounding that baseball eventually dropped it.

Although it is not our favorite measure of anything, and the term is almost as baffling as "normal," we here at Weather or Not would like to come to the defense of the degree day.

Just what is a degree day? It's the total number of degrees in which the aveage temperature deviates from 65 -- the theoretical boundary between the need for indoor heating and cooling -- on any given day.

If the totals are below 65, they count as "heating degree days," since theoretically they would merit the use of heaters; if above, "cooling degree days," for air-conditioning.

For example, if the high is 80 and the low 70, the average is 75, producing 10 cooling degree days. If the high is 60 and the low 50, the average is 55, yielding 10 heating degree days.

We have readers who track these numbers diligently, since they constitute a simplified way to monitor energy use. (We have others who might question those readers' sanity.)

This summer has been one of the hottest on record, but the degree-day data shows that based on temperature, overall air-conditioning use would have been considerably less than last year's.

At a glance, the degree days show that June and August were significantly less demanding-- about 20 percent each -- on cooling energy.  In June 2011, Philadelphia accumulated 321 cooling degree days; June 2010, 404. August 2011, 354; the previous August, 441.

Scanning spring and summer, the 2011 cooling energy required so far should have been about 11 percent less than last year's.

We could have done all this by hand using the daily average temperatures and adding and subtracting them from 65, but the National Weather Service does it for us.

It tabulates and posts the degree-day daily, monthly, and seasonal tallies every day, and you'll find them on this site in the Daily Climate Report, the Preliminary Monthly Climate Data and the Monthly Weather Summary. They also appear in The Inquirer Weather Report.

Philadelphia has quite a varied climate, and typically both heating and cooling degree days  show up in nine months of the year.

The National Weather Service starts keeping score on Jan. 1 for cooling degree days, although the first ones usually don't show up until March. The heating-degree tab starts July 1.

July isn't a big month for heaters, but it is not uncommon for homes and businesses to end up switching back and forth between heating and air-conditioning in May and September.

In May 2009, the average temperature for the month was exactly 65 in Philadelphia, yet it resulted in 86 cooling degree days and 79 heating.

The degree day has outlived the game winning RBI, and we suspect it will out-last its critics.