For given off by vehicles andfactories, as well

For given off by vehicles andfactories, as well

For more than 100 years, it has been known that two adjacent cities aregenerally warmer than the surrounding areas. This region of city warmth, knownas an urban heat island, can influence the concentration of air pollution. Theurban heat island is formed when industrial and urban areas are developed andheat becomes more abundant. In rural areas, a large part of the incoming solarenergy is used to evaporate water from vegetation and soil. In cities, whereless vegetation and exposed soil exists, the majority of the sun’s energy isabsorbed by urban structures and asphalt.

Hence, during warm daylight hours,less evaporative cooling in cities allows surface temperatures to rise higherthan in rural areas. Additional city heat is given off by vehicles andfactories, 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 citybuildings and roads, is released slowly into the city. The dissipation of heatenergy is slowed and even stopped by the tall building walls that do not allowinfrared radiation to escape as readily as do the relative level surfaces of thesurrounding countryside. The slow release of heat tends to keep citytemperatures higher than those of the unpaved faster cooling areas.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 countrybreeze which blows from the countryside into the city.

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If there are majorindustrial areas along the city’s outskirts, pollutants are carried into theheart of town, where they tend to concentrate.At night, the extra warmth of the city occasionally produces a shallow unstablelayer near the surface. Pollutants emitted from low-level sources, such as homeheating units, tend to concentrate in this shallow layer, often making the airunhealthy to breathe.

The constant outpouring of pollutants into the environment may actuallyinfluence the climate of a city. For an example, certain pollutants reflectsolar energy, thereby reducing the sunlight that reaches the surface. Someparticles serve as nuclei upon which water and ice form. Water vapor condensesonto these particles, forming haze that greatly reduces visibility. Moreover,the added nuclei increases the frequency of city fog.Pollutants from urban areas may even affect the weather downwind from them. Just such a situation is described in a controversial study conducted at LaPorte, Indiana, a city located about thirty miles downwind of the industries ofsouth Chicago.

Scientists suggested that La Porte had experienced a notableincrease in annual precipitation since 1925. Because this rise closely followedthe increase in steel production, it was proposed that the phenomenon was due tothe additional emission of particles. As industrial output increases pollutionparticles increase available condensation nuclei, thus increasing rainfall.

This process of increasingly wet climate is the result of industries to thewest of La Porte.Bibliography :”Disasters”, by Charles H. V. Ebert”Physical Geography Of The Global Environment”, by H. J. de Blij & Peter O.

Muller”Essentials Of Meteorology”, by C. Donald Ahrens”Comptons Encyclopedia”, Prodigy On-line Edition

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