Degree Day
A degree day is a measure of average temperature over a day, and is usually related to a temperature threshold. For instance, an actual average temperature of 5°C on a day is 2 degree days warmer than an expected average temperature of 3°C. Can also be used to measure cumulative cold weather over a period. Thus if the Seasonal Normal temperature for a month is 18°C and the average actual temperature through the month is expected to be 8°C, the month will be 10 x 30 = 300 degree days colder than normal.