What are cooling degree days?

A degree day is a measure of heating or cooling. A degree day is defined as a departure of the mean daily temperature from a given standard: one degree day for each degree of departure above (or below) the base temperature during one day. The degree day is related to the outside temperature and is not related to time.

A cooling degree day is every degree that the mean temperature is above 65 degrees during a day. So, if the high temperature for the day is 95, and the minimum is 51, the average temperature for the day is 73. That would be 8 cooling degree days (73-65).

The amount of energy required to maintain a building’s temperature in the summer is proportional to the accumulated cooling degree days. Cooling degree days, or CDD for short, are used to estimate fuel consumption needs.

The typical accumulated CDD for Madison is 664. The recent hot summer has, so far, registered 1065 CDDs — well above normal.

Of course, cooling a building depends on total energy gains and losses of the building and not just the mean temperature outside. Other factors, such as cloudiness, insulation and the number and efficiency of electrical appliances running will influence energy needs.

We also compute growing degree days, GDD, which is used by farmers to predict plant growth such as when a flower will bloom or when a crop should reach maturity. During winter, we keep track of heating degree days, or HHD. Each degree that the mean temperature is below 65 degrees is one heating degree day.

The Wisconsin State Climate office publishes this data at go.madison.com/madisonclimate.

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