Degree-day is a quantitative index demonstrated to reflect the demand for energy to heat or cool houses and businesses. The heating degree days (HDD) index is defined as the accumulated Celsius degrees between the daily mean temperature and a threshold temperature when the mean daily temperature is lower than the threshold value. The cooling degree days (CDD) index is defined analogically as the accumulated Celsius degrees between a threshold temperature and the daily mean temperature when the mean daily temperature is higher than the threshold value. The 18 ℃ threshold temperature is chosen in Beijing. The HDD is a good estimation of an accumulated cold during the cold season as well as an index for heating energy consumption within the heating season. The CDD estimates an accumulated warmth during the warm season as well as the cooling energy consumption within warm season. The mean daily temperature during 1951—2004 from Beijing Weather Observatory is collected to calculate HDD and CDD. The mean monthly maximum HDD is 687.9 degree-day in January and zero in July. The annual mean value of HDD equals 2922.6 degree-day. There is a decreasing trend of-99.5 degree-day per decade.The mean monthly maximum CDD is 259.2 degree-day in July.All CDD in January, February, March, November and December are zero. The annual mean value of CDD in warm season is 826.7 degree-day. There is a increasing trend of 39.0 degree-day for CDD per decade. The mean maximum difference of HDD between Beijing station and Miyun station in 1971—2004 is-73.8 degree-day in December. The mean maximum difference of CDD between Beijing station and Miyun station in 1971—2004 is 34.0 degree-day in August.The annual HDD, CDD are highly correlated with the annual temperatures.With the temperature warming in Beijing, the smaller the HDD, the less the energy consumption in winter; the larger the CDD, the more the energy consumption in summer in the future.
Zalom F G, Goodell P B, Wilson L T.Degree-days:The Calculation and Use of heat Units in Pest Management.Division of Agriculture and Natural Resources, University of California, 1972:1-11. http://cesantabarbara.ucanr.edu/files/75290.pdf