For more than 100 years, it has been known that two adjacent cities are
generally warmer than the surrounding areas. This region of city warmth, known
as an urban heat island, can influence the concentration of air pollution. The
urban heat island is formed when industrial and urban areas are developed and
heat becomes more abundant. In rural areas, a large part of the incoming solar
energy is used to evaporate water from vegetation and soil. In cities, where
less vegetation and exposed soil exists, 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. Additional city heat is given off by vehicles and
factories, as well as by industrial and domestic heating and cooling units.

At night, the solar energy, which is stored as vast quantities of heat in city
buildings and roads, is released slowly into the city. The dissipation of heat
energy is slowed and even stopped by the tall building walls that do not allow
infrared radiation to escape as readily as do the relative level surfaces of the
surrounding countryside. The slow release of heat tends to keep city
temperatures higher than those of the unpaved faster cooling areas.

On clear, still nights when the heat island is pronounced, a small thermal low-<...