Growing Degree Days
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 GDD's 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 GDD's 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°C and a maximum temperature of 30°C. So if one day the maximum temperature is 21°C and the minimum temperature is 15°C, the average temperature of that day was 18°C. To determine how many GDD's were in that day, simply subtract the base temperature from the average temperature (18°C-10°C) remembering that this all happened on one day. Thus for this example, there were 8 GDD accumulated by the corn during that day. Had the base temperature been 5°C, there would have been 13 GDD accumulated.
Annual GDD's is just a measure of the average number of GDD's accumulated in a particular area under normal climatic conditions. In this dataset, a base temperature of 5°C was used to compute monthly GDD's and those monthly values were then totaled to yield an annual number.
Climate Research Unit, Univ. of East Anglia. Available at this address.
New, M.G., M. Hulme and P.D. Jones, 1999: Representing 20th century space-time climate variability. I: Development of a 1961-1990 mean monthly terrestrial climatology. J. Climate. 12, 829-856.
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