A heating degree day (HDD) or cooling degree day (CDD) is an index used to determine the amount of energy needed to heat or cool a building. The number of heating or cooling degrees in a day is defined as the difference between a base temperature – 65° Fahrenheit – and the outside temperature. If, for example, the temperature outside is 55° F, then we would say it is a ten-degree day. Organizations, such as the State of Iowa, use this information to estimate seasonal heating and cooling demands.

Heating and Cooling Degree Days - Normal for Central Iowa, 1971-2000
  Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec
HDD 1469 1151 904 501 222 44 10 31 136 462 881 1320
CDD 0 0 1 3 66 180 275 218 67 10 0 0