From AMS Glossary
(Abbreviated GDH.) A heat index that relates the development of plants, insects, and disease organisms to environmental air temperature.
Analogous to growing degree-day (GDD), GDH is calculated by subtracting a base temperature from an hourly environmental temperature measurement and values less than zero are set to zero. A summation of hourly GDH is related to development. The temperature threshold (base temperature) below which development either slows or stops is species dependent.