Urban Heat Islands Essay, Research Paper

Urban Heat Islands

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-

pressure area forms over the city. Sometimes a light breeze, called a country

breeze which blows from the countryside into the city. If there are major

industrial areas along the city’s outskirts, pollutants are carried into the

heart of town, where they tend to concentrate.

At night, the extra warmth of the city occasionally produces a shallow unstable

layer near the surface. Pollutants emitted from low-level sources, such as home

heating units, tend to concentrate in this shallow layer, often making the air

unhealthy to breathe.

The constant outpouring of pollutants into the environment may actually

influence the climate of a city. For an example, certain pollutants reflect

solar energy, 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, 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 La

Porte, Indiana, a city located about thirty miles downwind of the industries of

south Chicago. Scientists suggested that La Porte had experienced a notable

increase in annual precipitation since 1925. Because this rise closely followed

the increase in steel production, it was proposed that the phenomenon was due to

the additional emission of particles. As industrial output increases pollution

particles increase available condensation nuclei, thus increasing rainfall.

This process of increasingly wet climate is the result of industries to the

west 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