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.

"Essentials Of Meteorology", by C. Donald Ahrens

"Comptons Encyclopedia", Prodigy On-line Edition