You see them on the news every day, but what exactly goes into the average high and low for any given day in a calendar year? Even though we compare temperatures to averages, are they really all that common?

Let’s first talk about average temperatures. The average high on any given day is calculated by taking the high temperature on that day for a 30-year period and averaging those values. Now let’s define the 30-year period these temperatures are calculated from, because that changes at certain intervals. The World Meteorological Organization or WMO recommends that every 10 years these averages get updated. This has been the schedule for the climate averages: 1981-2010 (most recent numbers disseminated to the public), 1971-2000 and 1961-1990. These averages first originated in 1956 after the numbers from the 1921-1950 period were crunched.

Note: This almanac page is not recent, just showing an example of averages

So, with that in mind, we know that on Dec. 5, for instance, the average high temperature is calculated from adding up the high temperatures each year on Dec. 5 from 1981 to 2010 and dividing by 30. For Reagan National, this equals 50 degrees.

Now, how many times, say, in the past 5 years has the high been dead on 50 degrees on Dec. 5?

Well, checking the archives for Reagan National, I see this has not occurred at all. How many times has the high temperature hit the average dead on since the start of this month? Well, the first 2 days in December featured highs that nailed the average… Dec. 1 was 52 degrees and Saturday, Dec. 3 was 51 degrees. Of course, the weather pattern was ideal for these temperatures with sunny skies and light wind.

Now let’s examine last month… how many days hit average? The answer… none! Several were close though. Here’s a table with the days that came closest to the average highs.

You have to remember that the average temperature is the middle ground of extreme values. The average high for Dec. 5 is 50 degrees, but guess what? The warmest high on Dec. 5 at Reagan National was 71 degrees in 2001.

So, the next time you see average highs and lows on the television weather report, just keep in mind that is a statistical number that represents the balance between the extremes in temperatures going back 30 years for that particular day.