Weird Science Fact: Cooling Degree Days

If you've heard the term "Cooling Degree Days" you may have dismissed it a unnecessarily complicated science speak.  If so, I wouldn't blame you; scientists are the worst at naming stuff.  In this particular case, though, Cooling Degree Days are actually quite useful for measuring energy consumption.

If you use electricity you've no doubt seen "kilowatt hours" (kWh) on your energy bill and, even if you don't know exactly what a kWh is, you know the higher the number the more energy you used.  Cooling Degree Days (CDDs) are similar, in that the higher the number the more energy a community is likely to spend on cooling homes and businesses.

Unlike kilowatt hours, though, Cooling Degree Days are a predictor of energy usage based on how hot or cool it's been; they do not change based on actual energy usage.

Here's an example of how CDDs are calculated.  If the high temperature for a particular day was 90°F, and the low was 65°F, the mean temperature for that day was (90°F + 66°F) ÷ 2 = 78°F.  The number of CDDs on that date would be the difference between 78°F and 65°F, or 13.

Put another way, CDDs are based on the assumption that, when the outside temperature is 65°F, we don't need any air conditioning to be comfortable.

Over the course of a year or season, the sum total of CDDs will be higher for "hot" years or summers, and lower for "cool" years or summers.  Iin theory, if everyone wants to be comfortable, more energy will be used on air conditioning during high-CDD years than in low-CDD years.

Credit: Climate Central. Click to enlarge.

A Climate Central analysis of CDDs, completed in 2018 using publicly available NOAA temperature data from 1970-2017, shows that the annual number of CDDs has trended up in 93% of the 244 cities analyzed.  In Minneapolis-St. Paul, the number of CDDs is up about 30% since 1970.

For more, read the High Cost of Hot report here.  Stay cool, my friends.