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)。校长>夜间变暖的原因是短波辐射>仍然在混凝土、沥青、和建筑,白天吸收>,与郊区和农村地区。这种能量然后慢慢>在夜间释放是长波辐射,使>冷却一个缓慢的过程。另外两个原因是热的变化>表面材料的性质和缺乏蒸散(通过缺乏植被>例子)在城市地区。>的植被减少,城市也失去>树的阴影和冷却效果,低反射率的树叶和碳>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).