好问题-抱歉没有明确回答。 It appears there are some gaps and conflicts in the limited research on this topic.

A grad student at UMN suggests that UHI effect is worse in winter (without really explaining why), while TR Oke suggests it is worse in summer, while James Voogt suggests that UHI effect is generally stronger in summer and winter compared to spring and autumn (in the mid-latitudes).

城市热岛(UHI)效应的规模与大都市地区的人口…城市越大，城市热岛效应越大。城市热岛不是由于人为活动(例如空调)产生了更多的热量，而是主要是土地利用变化问题(例如从植被到城市)。植被比混凝土和建筑表面更能调节热量和水分，混凝土和建筑表面在晴朗的天气会吸收更多的热量，然后在整个晚上散发热量。城市热岛效应在夜间最强，此时白天有显著的地表直接加热(例如晴朗的天空条件)，当大气边界层逆温足够接近地面以防止通风/与上方空气混合时。

From http://en.wikipedia.org/wiki/Urban_heat_island:

城市热岛(UHI)有几种原因。夜间变暖的主要原因是，与郊区和农村地区不同，白天吸收的短波辐射仍然在混凝土、沥青和建筑物内。这些能量在夜间以长波辐射的形式缓慢释放，使冷却成为一个缓慢的过程。另外两个原因是地表材料热性质的变化和城市地区缺乏蒸散作用(例如由于缺乏植被)。随着植被数量的减少，城市也失去了树木的遮荫和冷却作用，树叶的低反照率，以及二氧化碳的去除。城市地区常用的路面和屋顶材料，如混凝土和沥青，与周边农村地区相比，具有显著不同的热容性能(包括热容量和热导率)和表面辐射性能(反照率和发射率)。 This causes a change in the energy balance of the urban area, often leading to higher temperatures than surrounding rural areas. Other causes of a UHI are due to geometric effects. The tall buildings within many urban areas provide multiple surfaces for the reflection and absorption of sunlight, increasing the efficiency with which urban areas are heated. This is called the "urban canyon effect". Another effect of buildings is the blocking of wind, which also inhibits cooling by convection and pollution from dissipating. Waste heat from automobiles, air conditioning, industry, and other sources also contributes to the UHI. High levels of pollution in urban areas can also increase the UHI, as many forms of pollution change the radiative properties of the atmosphere. As UHI raises the temperature of cities, it will also increase the concentration of ozone in the air, which is a greenhouse gas. Ozone concentrations will increase because it is a secondary gas, aided by an increase in temperature and sunlight.

From http://www.epa.gov/heatisland/about/index.htm it says:

As urban areas develop, changes occur in their landscape. Buildings, roads, and other infrastructure replace open land and vegetation. Surfaces that were once permeable and moist become impermeable and dry. These changes cause urban regions to become warmer than their rural surroundings, forming an "island" of higher temperatures in the landscape. Heat islands occur on the surface and in the atmosphere. On a hot, sunny summer day, the sun can heat dry, exposed urban surfaces, such as roofs and pavement, to temperatures 50–90°F (27–50°C) hotter than the air, while shaded or moist surfaces—often in more rural surroundings—remain close to air temperatures. Surface urban heat islands are typically present day and night, but tend to be strongest during the day when the sun is shining. In contrast, atmospheric urban heat islands are often weak during the late morning and throughout the day and become more pronounced after sunset due to the slow release of heat from urban infrastructure. The annual mean air temperature of a city with 1 million people or more can be 1.8–5.4°F (1–3°C) warmer than its surroundings.3 On a clear, calm night, however, the temperature difference can be as much as 22°F (12°C).