Abstract: Based on the data of 20 meteorological stations from 1990 to 2004, the characteristics of Beijing urban heat island in the last 15 years were analyzed. The results show that the UHI in the evening is stronger than that in the daytime. The UHI in the evening tends to get stronger year by year, but which is not obvious in the daytime. In the evening, higher the average monthly surface temperature is, stronger the average monthly UHI is. But in the daytime the correlativity is not so obvious. Furthermore, with the additional data got by two automatic meteorological stations and the 325m meteorological tower of LAPC, the characteristics and influencing factors of a strong UHI and a weak UHI in winter were analyzed contrastively. And the results show that the cooling rate of the suburban surface atmosphere is bigger than that of the urban surface atmosphere after sunset, which directly leads to the formation of the strong UHI. And the heating rate of the suburb surface atmosphere is bigger than that of the urban surface atmosphere after sunrise, which directly causes the weakening and disappearance of the strong UHI. In the clear night of the sunny day, the occurrence and maintenance of the weak wind (≤2.0m/s) below 100m and the inversion temperature below 320m have important effects on the formation and maintenance of the strong UHI. And in the daytime the solar radiation has most important effects on the weakening and disappearance of the strong UHI. It makes the suburban surface atmosphere temperature rise faster than the urban atmosphere temperature, and makes the inversion temperature disappear, and further makes the strong UHI weaken and disappear at about 2:00 p.m. in the end even if the wind maintains weak in the daytime.

Key words: urban heat island (UHI), strong UHI, sunny day, weak wind, inversion temperature