Annual and monthly heating and cooling degree days

Documentation

The Heating Degree Days and Cooling Degree Days maps are based on daily temperature observations, with each month having at least 25 records and no less than 15 years of data between 1961-1990.

Heating and Cooling Degree Days (CDDs & HDDs), which indicate the level of comfort, are based on the average daily temperature. The average daily temperature is calculated as follows: [maximum daily temperature + minimum daily temperature] / 2.

If the average daily temperature falls below comfort levels, heating is required and if it is above comfort levels, cooling is required. The HDDs or CDDs are determined by the difference between the average daily temperature and the BASE (comfort level) temperature. The BASE values used are 12 and 18 degrees Celsius for heating and 18 and 24 degrees Celsius for cooling.

For example:

If heating is being considered to a temperature BASE of 18 degrees, and the average daily temperature for a particular location was 14 degrees, then heating equivalent to 4 degrees or 4 HDDs would be required to maintain a temperature of 18 degrees for that day. However if the average daily temperature was 20 degrees then no heating would be required, so the number of HDDs for that day would be zero.
If cooling is being considered to a temperature BASE of 24 degrees, and if the average temperature for a day was 27 degrees, then cooling equivalent to 3 degrees or 3 CDDs would be required to maintain a temperature of 24 degrees for that day. However if the average temperature was 21 degrees, then no cooling would be required, so the number of CDDs for that day would be zero.

The daily HDD and CDD values shown on the maps are based on daily values, which are summed for each month/year and then averaged over the period (years) of record.