me parece ser o número de dias em que a temperatura se manterá subindo ou descendo

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Note added at 2003-02-10 16:12:53 (GMT)
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Em função da definição abaixo reformulo minha sugestão, acho melhor deixar em inglês e colocar nota de tradutor, pois o conceito não é fácil de explicar

Understanding heating, cooling degree days

Heating engineers who wanted a way to relate each day\'s temperatures to the demand for fuel to heat buildings developed the concept of heating degree days.

To calculate the heating degree days for a particular day, find the day\'s average temperature by adding the day\'s high and low temperatures and dividing by two. If the number is above 65, there are no heating degree days that day. If the number is less than 65, subtract it from 65 to find the number of heating degree days.

For example, if the day\'s high temperature is 60 and the low is 40, the average temperature is 50 degrees. 65 minus 50 is 15 heating degree days.

Cooling degree days are also based on the day\'s average minus 65. They relate the day\'s temperature to the energy demands of air conditioning. For example, if the day\'s high is 90 and the day\'s low is 70, the day\'s average is 80. 80 minus 65 is 15 cooling degree days.

Heating and cooling degree days can be used to relate how much more or less you might spend on heating or air conditioning if you move from one part of the country to another. Of course you\'d have to take into account how well insulated your new home will be in comparison to your old one and the different costs of electricity, gas or heating oil. You could also use records of past heating degree days to see if the money you\'ve spent on insulation, or a newer furnace or air conditioner is paying off. To do this, you\'d also need records of past energy use.

In general, the easiest way to obtain daily, monthly, or yearly heating degree days or cooling degree days is from the Web site of the National Weather Service office that covers the place you are interested in.

Here are examples of annual average heating degree days (HDD) and cooling degree days (CDD) at some places around the USA. City HDD CDD
Barrow, Alaska 20,370 0
Bismarck, N.D. 8,932 499
Hilo, Hawaii 0 3,134
Kansas City, Mo. 5,326 1,388
Key West, Fla. 68 4,820
Yuma, Ariz. 983 4,244


Source: The USA TODAY Weather Almanac by Jack Williams

http://www.usatoday.com/weather/resources/askjack/waskdays.h...