Insufficient data over gray shaded areas. Links Certain of the following material originate with organizations not subject to the Official Languages Act and are available on this site in the language in which it was written. An index used to determine the duration and intensity of winter cold. Once the outdoor temperature falls below 18 o C, space heating is required in a building to maintain a comfortable indoor temperature. HDD's are calculated simply by subtracting the mean daily temperature from 18 o C. HDD's are also used comparatively to measure the relative severity of winter.
Warmer weather implies accumulation of fewer HDDs.
Although HDD figures do provide a reasonably accurate estimation of heating requirements, the actual amount of fuel consumed may differ slightly from the expected due to variations in windspeed and direction, humidity, radiation and individual home heating efficiency.