That's the explanation found in the WeatherLink doc :
I hope it will help, because it begins to sound Japanese for meData: Heating & Cooling Degree-Days
Although degree-days are most commonly used in agriculture, they are also useful in building design and construction, and in fuel use evaluation. The construction industry uses heating degree-days to calculate the amount of heat necessary to keep a building, be it a house or a skyscraper, comfortable for occupation. Likewise, cooling degree-days are used to estimate the amount of heat that must be removed (through air-conditioning) to keep a structure comfortable. Heating and cooling degree-days are based on departures from a base temperature, typically 65ºF (18ºC).
One heating degree–day is the amount of heat required to keep a structure at 65ºF when the outside temperature remains one degree below the 65ºF threshold for 24 hours. One heating degree–day is also the amount of heat required to keep that structure at 65ºF when the temperature remains 24ºF below that 65º threshold for 1 hour.
Likewise, one cooling degree–day is the amount of cooling required to keep a structure at 65ºF when the outside temperature remains one degree above the 65ºF threshold for 24 hours. One cooling degree–day is also the amount of cooling required to keep that structure at 65ºF when the temperature remains 24ºF above that 65º threshold for 1 hour.
Depending on the calculation method, both heating and cooling degree-days can accumulate in the same day. Also, note that there are no negative degree-days. If the temperature remains below the threshold, there is no degree-day accumulation.
Heating and Cooling degree-days may be calculated by either the High/Low method or the Integration method.
Data: Degree-Day Calculation Methods
High / Low method
If you select the high/low method, the software uses the highest temperature and the lowest temperature for a given day to calculate the average temperature for that day. The difference between the average temperature and the base threshold are assumed to be the number of degree-days accumulated on that day. For example, if the average of the highest and lowest temperatures is 24º above the base threshold, the software assumes 24 degree–days for the entire day.
Note: Unless 15 hours worth of records exist in the database for that day (from midnight to 3pm, for example), the software will not calculate degree-days for that day.
Integration method
If you select the integration method, the software calculates degree–days using the average temperature for an interval and the interval time. For example, if the average temperature during a 15 minute interval was 24º above the base threshold, the software would calculate 0.25 degree-days during that interval (24 * 15 minutes in interval/1440 minutes per day). The number of degree-days during each interval are added together to arrive at a degree-day total. This method calculates degree-day totals more accurately than the high/low method.
