For more than a hundred years, it has been known that cities are generally warmer than surrounding rural areas. This region of city warmth, known as the urban heat island, can influence the concentration ofair pollution. However, before we look at its influence, let’s see how the heat island actually forms.

The urban heat island is due to industrial and urban development. In rural areas, a large part of the incoming solar energy is used in evaporating water from vegetation and soil. In cities, where less vegetation and exposed soil exist, the majority of the Sun’s energy is absorbed by urban structures and asphalt. Hence, during warm daylight hours, less evaporative cooling in cities allows surface temperatures to rise higher than in rural areas. The cause of the urban heat island is quite involved. Depending on the location, time of year, and time of day, any or all of the following differences between cities and their surroundings can be important: albedo (reflectivity of the surface), surface roughness, emissions of heat, emissions of moisture, and emissions of particles that affect net radiation and the growth of cloud droplets.

At night, the solar energy (stored as vast quantities of heat in city buildings and roads) is slowly released into the city air. Additional city heat is given off at night (and during the day) by vehicles and factories, as well as by industrial and domestic heating and cooling units. The release of heat energy is retarded by the tall vertical city walls that do not allow infrared radiation to escape as readily as does the relatively level surface of the surrounding countryside. The slow release of heat tends to keep nighttime city temperatures higher than those of the faster-cooling rural areas. Overall, the heat island is strongest (1) at night when compensating sunlight is absent; (2) during the winter, when nights are longer and there is more heat generated in the city; and (3) when the region is dominated by a high-pressure area with light winds, clear skies, and less humid air. Over time, increasing urban heat islands affect climatological temperature records, producing artificial warming in climatic records taken in cities. This warming, therefore, must be accounted for in interpreting climate change over the past century.

The constant outpouring of pollutants into the environment may influence the climate of the city. Certain particles reflect solar radiation, thereby reducing the sunlight that reaches the surface. Some particles serve as nuclei upon which water and ice form. Water vapor condenses onto these particles when the relative humidity is as low as 70 percent, forming haze that greatly reduces visibility. Moreover, the added nuclei increase the frequency of city fog.

Studies suggest that precipitation may be greater in cities than in the surrounding countryside; this phenomenon may be due in part to the increased roughness of city terrain, brought on by large structures that cause surface air to slow and gradually converge. This piling up of air over the city then slowly rises, much like toothpaste does when its tube is squeezed. At the same time, city heat warms the surface air, making it more unstable, which enhances risings air motions, which, in turn, aids in forming clouds and thunderstorms. This process helps explain why both tend to be more frequent over cities.

On clear still nights when the heat island is pronounced, a small thermal low-pressure area forms over the city. Sometimes a light breeze—called a country breeze—blows from the countryside into the city. If there are major industrial areas along the outskirts, pollutants are carried into the heat of town, where they tend to concentrate. Such an event is especially probable if vertical mixing and dispersion of pollutants are inhibited. Pollutants from urban areas may even affect the weather downwind from them.

一百多年来，人们都知道城市一般比周围的农村地区温度高些。这种地区性的温度升高，被称为城市热岛，可以影响空气污染的浓度。然而，在我们考虑它的影响之前，让我们先看看热岛实际上是如何形成的。

城市热岛的成因是工业和城市发展。在农村地区，很大一部分的太阳能被用于从植被和土壤中蒸发水分。城市中植被稀少、土壤裸露，大部分的太阳能被城市建筑和沥青路面吸收。因此，在温暖的白天，城市的蒸发散热较少，地表温度就比农村地区高。城市热岛的成因很复杂。位置、季节、一天当中的时间不同，以及城市和周围的环境之间的任何差异都可能是重要影响因素：比如反照率（地表反射率）、地表粗糙度、散热量、湿度，影响净辐射的微粒量和云滴的增长。

在夜间，太阳能（储存在城市建筑和道路的巨大热量）被慢慢释放到城市空气中。另外，城市的车辆、工厂、以及工业用和家用的制热和制冷系统也在夜间（和白天）释放出热量。释放出来的热能被高耸的城市墙所阻碍，使得红外线无法像周围的乡村相对较为平坦的地面那样容易散去。由于热量释放缓慢，使得城市夜间的温度比容易散热的农村地区要高。总体而言，热岛效应在三种情况下最强：（1）没有太阳光照射补给的夜间；（2）夜晚较长、城市产生更多热量的冬季；（3）微风、晴朗、干燥的高气压地区。随着时间的推移，气候温度记录中出现越来越多的城市热岛现象，城市出现了人工变暖的气候现象。因此，在解读过去的一个世纪的气候变化时必须考虑到这种温度升高。

不断向环境排放污染气体可能影响城市气候。某些粒子会反射太阳辐射，从而减少到达地面的阳光。有些粒子是形成水和冰的凝结核。当相对湿度低至70%时，水蒸气凝结在这些粒子上，就会形成阴霾，大大降低能见度。此外，凝结核的增加会增加城市出现雾的频率。

研究表明，城市的降水可能要比周边农村地区要多；出现这种现象的部分原因可能是大型建筑物使得地表空气缓慢、逐渐地汇聚在一起，使得城市地形粗糙度增加。城市上空堆积的空气慢慢地上升，就像挤牙膏一样。同时，城市的热量使空气受热，变得更加不稳定，加强了气流的上升运动，反过来有助于形成云或雷雨。这个过程帮助解释了为什么城市的云和雷雨更频繁。

在晴朗平静的夜晚，热岛效应明显，城市会形成一个小的热低压区。有时微风——乡下风会从农村吹进城市。如果郊区沿线有大工业区，污染物被带入较热的城市，就会更加聚集。如果污染物在垂直方向上不能混合或者扩散的话，就更容易聚集了。从城市地区吹来的污染物甚至可以影响到它们的顺风方向的天气。