城市热岛的热岛效应尺度与人口的大都市…更大的城市热岛效应越大。热岛不是由于更多的热量生成通过人为活动(如空调),而是这主要是土地利用变化的问题(例如从营养到城市)。植被能够调节热量和水分比混凝土和建筑表面,在晴朗的天空天然后陷阱更多的热量辐射热量在整个夜晚。城市热岛效应是最强的晚上,当白天有显著的直接加热表面(如晴空条件下),当大气边界层温度反演是足够接近地面,防止通风/混合与上方的空气。从http://en.wikipedia.org/wiki/Urban_heat_island:“*有几个城市热岛的原因(热岛)。夜间变暖的主要原因是短波辐射仍在混凝土、沥青、白天吸收和建筑物,与郊区和农村地区。这种能量然后慢慢释放在夜间是长波辐射,使冷却一个缓慢的过程。另外两个原因是表面材料的热性能的变化和缺乏蒸散(例如通过缺乏植被)在城市地区。植被量减少,城市也失去了树木的阴影和冷却效果,低反射率的叶子,和二氧化碳的去除。 Materials commonly used in urban areas for pavement and roofs, such as concrete and asphalt, have significantly different thermal bulk properties (including heat capacity and thermal conductivity) and surface radiative properties (albedo and emissivity) than the surrounding rural areas. 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).*"