(abbrev. HDD) A form of degree day used to estimate energy requirements for heating. Typically, heating degree days are calculated as how much colder the mean temperature at a location is than 65°F on a given day. For example, if a location experiences a mean temperature of 55°F on a certain day, there were 10 HDD (Heating Degree Days) that day because 65 - 55 = 10.

A heating degree day is a measure of how much the average of the daily high temperature and low temperature is less than a reference temperature. In other words how much heating is required. More details.

The number of degrees below 65o F for a day summed over the entire heating season.

A unit, based upon temperature difference and time, used in estimating fuel consumption and specifying nominal heating load of a building in winter. For any one day, when the mean temperature is less than 65 degrees F (18 degrees C), there exists as many degree days as there are Fahrenheit degrees difference in temperature between the mean temperature for the day and 65 degrees F (18 degrees C).

A measure of the severity of the winter in a given locality: the more heating degree days, the colder the winters. Heating degree days are the difference between 65 degrees F and the daily mean (average) temperature when the latter is less than 65 degrees F.