Average annual and monthly heating and cooling degree days

What do the maps show?

These maps show the average annual and average monthly number of heating and cooling degree days across Australia.

The number of cooling degree days increases towards the north of the continent, while the number of heating degree days increases towards the south of the continent, following the pattern of the mean temperature.

These maps are commonly used in the energy industry, and by air conditioning and heating manufacturers. For example, the average annual number of cooling degree days with a base temperature of 18 °C for Melbourne is about 250, whereas in Brisbane it is approximately 1000. For two homes with similar structure in the two cities, on average four times the energy would be required to cool a home in Brisbane than Melbourne. This information can guide building design for more energy efficient homes through the understanding of the local conditions.

The number of heating degree days and cooling degree days are based on temperature data collected between 1951 and 2020.

How are the values calculated?

Heating and cooling degree days are based on the average daily temperature, computed by

$$\mathrm{average daily temperature}=\frac{\mathrm{daily maximum temperature}+\mathrm{daily minimum temperature}}{2}$$

A threshold is chosen for a comfort level. If the average daily temperature falls below the comfort level, heating is required; and if it goes above the comfort level, cooling is required.

The heating degree days or cooling degree days are determined by the difference between the average daily temperature and the comfort level temperature.

Examples

Heating degree days: if the comfort level is 18 °C, and the average daily temperature for a particular location was 14 °C, then heating equivalent to 4 °C (4 heating degree days) would be required to maintain a temperature of 18 °C for that day. If the average daily temperature was 20 °C, then no heating would be required, so the number of heating degree days for that day would be zero.

Cooling degree days: if the comfort level of 24 °C, and if the average temperature for a day was 27 °C, then cooling equivalent to 3 °C (3 cooling degree days) would be required to maintain a temperature of 24 °C for that day. If the average temperature was 21 °C, then no cooling would be required, so the number of cooling degree days for that day would be zero.

The comfort level values used in the plots provided here are 12 °C and 18 °C for heating and 18 °C and 24 °C for cooling.

Average annual heating and cooling degree days are calculated by adding heating and cooling degree days totals over the years specified and dividing the total by the number of years in that period (30 years in this case). Similarly, average monthly heating and cooling degree days are calculated by adding monthly totals and dividing by the number of years in the specified period.

Why 30-year periods?

Climate is often colloquially considered as the "average weather" of a region, which can be more rigorously described as the mean and variability of certain elements (including the temperature, rainfall, and wind) over a period of time. The World Meteorological Organisation identified 30 years as the appropriate length of time to calculate these values (referred to as climate normal) as this window of time sufficiently captures climate variability. The WMO climate normals are updated at the end of every decade (WMO, 2025). These maps show Australia's climate based on current and past climate normals from 1951, or 1971 if they required humidity data unavailable until that period.

Each map is based on combinations of annual and monthly average values, for rainfall and/or temperature. Annual averages are created by examining all values over the years from the relevant period and then dividing by the number of years. Monthly averages are calculated by adding monthly values and dividing by the number of years in the specified period.

The data to create these maps is the AGCD data set (Jones, Wang, & Fawcett, 2009; Evans, Jones, Smalley, & Lellyet, 2020).