What are Degree Days?

Many of us are familiar with the concept of global warming, and the various arguments both supporting it as an environmental crisis, as well as those dismissing it as "junk science". No matter what your belief is, there is a scientific way that energy managers, facilities directors, mathematicians, and even farmers can quantify whether a particular season is warmer or cooler than a previous one. If someone claims that last winter was much colder than this winter, how can this claim be substantiated?

In order to determine such things we use a measurement called degree days. Simply put, a degree day is a way to measure "bulk" temperature. First of all, there are different types of degree days; which are used to measure different things. They all have something in common: they express how far the average temperature was above or below a certain index for a given number of days. The index can be different for measuring different things, like heating, cooling or growing season. Typically, the heating/cooling degree day index is 65ºF and the growing index is 50ºF. If a different index is used it is represented by a subscript.

A common example of degree day usage explains how the oil company determines when to deliver oil. If you recorded the average temperature every day, you could determine how far below a given level the weather was; and calculate how much oil a customer would have used.

For example, if the average temperatures for three consecutive days were: 60ºF, 58ºF and 55ºF, these would represent 5, 7 and 10 degree days respectively. The three days total for 22 heating degree days (HDD). We are basically measuring the number of degrees that the average temperature was less than or equal to 65ºF index.

The oil company uses historical records to determine how much oil each customer would consume to overcome this "amount of cold". Since each house may not have the same efficieny, they could each require a different amount of oil.

Cooling degree days (CDD) work the same way, and are often used to help relate weather to electric usage. The difference is that we measure the number of degrees that the average temperature was greater than the 65ºF index, and thus require cooling.

According to records kept at Brookhaven National Labs since 1949, the annual averages for our area are about 6,000 HDD and 580 CDD.