From AMS Glossary
(Or cumulative temperature.) In Great Britain, the accumulated excess of temperature above a given standard temperature.
It is expressed in degree-hours or degree-days. For each day, degree-hours are determined as the product of the length of time, in hours, during which temperatures are above the standard, and the amount, in degrees, by which the mean temperature of the period exceeds the standard. Division of the resulting degree-hour value by 24 gives a value in degree-days. Summation of either over the period of interest creates the accumulated temperature. The concept of accumulated temperature was introduced into plant geography by A. de Candolle (1855); his standard temperature was 6°C (42.8°F), below which he considered that no vegetative growth took place. It was introduced into agricultural meteorology in Britain in 1878, when accumulated temperatures in excess of 42°F were first published regularly in the Weekly Weather Report. These were calculated from the daily maximum and minimum temperatures. In heating calculations, a form of accumulated temperature is calculated as the number of degree-days below the standard, which is taken as 65°F in the United States and 60°F in Great Britain. Since the standard temperatures differ, degree-days are not interchangeable between the two countries.
de Candolle, A. 1855. Géographie Botanique Raisonné. 2 vols., Paris: V. Masson.