城市热岛是影响城市及其周边地区天气气候和空气质量的重要因素。利用2000-2005年MODIS（Moderate Resolution Imaging Spectroradiometer,中分辨率成像光谱仪）分裂窗算法反演得到的1km分辨率地表温度产品分析了北京的城市热岛效应,发现白天城市热岛强度具有明显的季节变化,夏季最大值可以达到10℃以上,冬季变为冷岛,即城市地表温度低于乡村,最低值可以达到-5℃;模拟结果表明冬季城市冷岛的存在主要是城市和乡村地表热特性（热惯量）的差异引起的。夜间热岛强度的季节变化较小,全年稳定在5℃左右。选择北京周边地区比较典型的乡村耕地、山区森林以及永定河流域来研究乡村的选择对热岛强度的影响。发现选择不同的邻近区域作为乡村时,不仅城市热岛（冷岛）强度有较大变化,而且热岛强度的季节变化情况也有较大差异。冬季白天北京城市冷岛增加了近地层的大气稳定度,可能会降低城市空气污染物的扩散能力,加剧了北京冬季的空气污染。

Urban heat island (UHI) is one of the main factors influencing the weather, clmi ate, and\nenvironmentover the urban and its surrounding area. In the present study, MODIS land surface temperature\nretrievals at1 km resolution produced by the split-window algorithm during the yearof2000—2005 are used to\ninvestigate the UHI over Beijing Metropolitan. It was found that the daytmi e UHI demonstrates distinctive\nseasonal variation, with themaxmi um during summer being larger than 10℃and theminmi um during winter\nbeing negative, which it is called urban cool island here. Numerical smi ulation shows that the urban cool island\nmainly results from the difference of the surface thermal characteristics between urban and rural area. The\nseasonal variation ofnighttmi eUHI ismuch less, being about5℃throughout a year. Beijing is surrounded by\nmountainous area in thewest, the north, and the northeast, whose height reach up to 2000m(See Figure 1). In\nthe east and south ofBeijing, the elevation is close to sea levelwith the complexityofthe underlying surface. The\ndifferent ruralbaseline notonly influences the value ofUHI but its seasonal variation. Three typical areaswith\ndifferent land surface types are selected to study the influence onUHI. The results show that the cropland area nearBeijing city hasuniform vegetation and is suitable to be studied as rural area. The cool island duringwinter\n(spring) maybe increase the near surface air stability and aggravate the airpollution overBeijing urban area.