Although it may seem tricky at first, the concept of the growing degree day (GDD) is based simply on the fact that temperature directly affects how fast plants grow. If the temperature is too hot or cold, the plants growth will be hampered. Thus, growing degree days are calculated for a range, and in many cases they are calculated for a range specific to the type of plant the person is interested in.
Farmers often use growing degree days to figure out how far along their crops are, and eventually when to harvest them.
Taking corn as an example, it grows with a minimum temperature of 10 degrees Celsius and a maximum temperature of 30 degrees Celsius. So if one day the maximum temperature is 21 degrees Celsius and the minimum temperature is 15 degrees Celsius, the average temperature of that day was 18 degrees Celsius.
To determine how many growing degree days were in that day, simply subtract the base temperature from the average temperature remembering that this all happened on one day. Thus for this example, there were eight growing degree days accumulated by the corn during that day. Had the base temperature been 5 degrees Celsius, there would have been 13 GDD accumulated.
Annual growing degree days are just a measure of the average number of growing degree days accumulated in a particular area under normal climatic conditions. In this dataset, a base temperature of 5 degrees Celsius was used to compute monthly growing degree days and those monthly values were then totaled to yield an annual number.
Full Citation
Climate Research Unit, Univ. of East Anglia.
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