The sum, counted in degrees, by which the actual air temperature rises above or falls below a datum level over a prolonged period. The datum level is usually set at a value relevant to an ecological study or to crop production (e.g. the critical temperature for sustained plant growth is 6°C). If the mean temperature on a particular day is m° above or below (−m°) the datum level and remains there for n hours (= n/24 days), then the accumulated temperature for that day is mn/24 degree-days. Degree-days can be added to give the accumulated temperature over a week, month, season, or year.