Clearing Up Definition of Degree Days
I received an email to the Ask-A-Met inbox today asking the question:
What is a Degree Day?
Looking it up in the AccuWeather Glossary yields this definition:
Generally, a measure of the departure of the mean daily temperature from a given standard, one degree day for each degree (C or F) of departure above (or below) the standard during one day. Degree days are accumulated over a "season." As used by the U.S. Army Corps of Engineers, degree days are computed above and below 32F, positive if above and negative if below.
The first part of the definition is the most important: "a measure of the departure of the mean daily temperature from a given standard".
The question is... what is "the standard," i.e. what is the number that we compare the mean (average) against?
The definition goes on to say: "As used by the U.S. Army Corps of Engineers, degree days are computed above and below 32F." Not only is this information mere trivia in the world of meteorology, but it's also not correct. What we meant to say was: "As used by the U.S. Army Corps of Engineers, freezing degree days are computed above and below 32F." This is confirmed indpendently by this Army site, which says: "A freezing degree-day is the amount that the average temperature for a given day is below 32F." Our glossary was borrowed (with permission) from the Novalynx Glossary, so I'll put in a "bug fix request" to get this corrected on our site and theirs.
The most commonly used "standard" is 65 degrees, and this is used to calculate "Heating Degree Days" and "Cooling Degree Days." These terms are used by statisticians at your local Power Company to estimate future power usage. The theory is that if it’s under 65 outside, you’ll need to heat your house, so they talk about heating degree days. If it’s over 65, you’ll need to cool your house, so they talk about cooling degree days.
A couple of examples:
High 80, Low 70. A warm day! Average = 75. 65-75 = -10, so 10 Cooling Degree Days, i.e. you need to cool your house to bring it back down to 65.
Another example: High 66, Low 34. A chilly day. Average = 50. 65-50 = 15, so 15 Heating Degree Days, in other words you need to heat your house.
Maps plotting Heating and Cooling Degree days (along with daily stats for towns nationwide) are available on Pro site (an example is shown at right).
Another type of Degree Day, calculated off of the 50 F degree standard, is the "Growing Degree Day," which agricultural folks use to determine when to plant and harvest different crops. There are also specific versions of this, such as "Corn Degree Day", "Summer Wheat Degree Days," etc., which are based on different temperatures other than 50.
You should be able to see past and forecast measurements of Heating, Cooling, and Growing degree days on the John Deere weather site, which is powered by AccuWeather, but the links don't seem to be working this morning. I'll drop them a note.
To summarize:
Degree Days: Difference Between Daily Mean Temp and A Standard Temperature
Heating & Cooling Degree Days: Difference Between Daily Mean Temp and 65 F
Growing Degree Days: Difference Between Daily Mean Temp and 50 F
Freezing Degree Days: Difference Between Daily Mean Temp and 32 F
