The departure of the average daily temperature from a defined base, e.g. 4 °C, the minimum recognized temperature for the growth of many plant species, or 18 °C, which determines the need for heating in buildings. It is calculated by multiplying the number of days on which the mean temperature is above or below the datum level over a specified period (usually one month) by the sum of the number of degrees by which it deviates during that period. The number of day degrees may be totalled to assess the accumulated warmth of a particular year’s growing season for crops. See also accumulated temperature; agrometeorology; month degrees.