Heat Islands, the city s impact on local weather

While most folks know that it s always hotter in the city, few have heard of a term called Urban Heat Islands. It is a term now associated with Atlanta and may soon be associated with other large cities. Recent studies have shown that not only are cities hotter, but they affect the weather around them.This urban localized weather is known as heat islands.

Urban areas generate more heat than other areas because of population, manufacturing, transportation and other causes. Urban areas also are a heat trap which compounds the problem. Asphalt roads and tar roofs absorb and hold nearly all of the heat around them. There is little material in urban areas which reflect heat. Altanta is 5 to 8 degrees F hotter than outlying areas and this excess heat produces increased rainfall and thunderstorms.

This finding was presented at the annual meeting of the Association of American Geographers in Honolulu, Hawaii on March 24 by meteorologists Robert Bornstein and Qing Lu Lin from San Jose State University in California. Dale Quattrochi and Jeffrey Luvall of NASA's Global Hydrology Center lead this NASA-sponsored study . The Atlanta Land-use Analysis: Temperature and Air-quality (ATLANTA) project began in 1996 in order to study the impact of urban heat islands on the environment.

As the heat in a city builds, it causes the hot air to rise. Colder air from outside the city then rushes into the vacuum, creating winds. The warmer rising air begins to cool, forming convective clouds which typically produce localized thunderstorms and rain.

It is quite common for cities in the souther United States to experience afternoon thunderstorms.. Atlanta s storms however, occur more frequently. The Project ATLANTA team found that storms around Atlanta were generated during heat island periods. Rather than only experiencing afternoon storms, Atlanta would also experience pre-dawn or early morning rain showers that would continue until noon.
 

Heat islands are created through the process of urbanization. As a city grows, trees, which normally reduce the amount of heat and smog, are cut down to make room for commercial development, roads, and suburban growth.. Plants and soil absorb heat during the day, and then
carry the heat away through evaporation. In Atlanta,urban development has so drastically increased between 1973 and 1992, that almost 380,000 acres of forest were cleared to accommodate that growth.

Material in cities which absorb heat, hold in the heat long after the sun sets, keeping the cities hotter for longer periods of time. Atlanta experiences early morning rain showers because urban heat islands retain their temperature long after nightfall. This causes an even greater difference in temperature between urban and outlying areas.

The increase in urban temperature contributes to an increase in ozone, a particularly destructive type of smog. Ozone interferes with photosynthesis, the process by which plants make food. Ozone also damages the lungs of people and animals (although, high up in the atmosphere, ozone protects us from the ultraviolet rays of the Sun!)

The thunderstorms may be nature s way of fighting back. The storms lower the temperature and the rains help cleanse the air of pollutants. But on the downside, frequent storms can cause flooding because paved ground does not allow water to soak into the soil.

Scientists used both satellite data as well as sensors on board a Leer jet to study urban growth trends and effects on weather.Data showed that temperatures in parking lots could exceed 120 degrees F, while small tree islands in the same lot had temperatures of only 89 degrees F.

The studies have clearly shown that there is a problem with urban heat islands and now the attention turns to what can be done about it. Since it is highly unlikely that urban growth will cease, we must learn to live with the cities but find a way to make them a healthier place.

There are two key areas in which urban heat islands can be affected. The first is the planting of trees. Trees both shade the city and help to dissipate heat. The second key area is to install highly reflective roofing materials.  The Georgia government seems to agree and has eased
 requirements for buildings constructed with reflective roofing. Likewise, the manufacturers of such roofing have used the scientist s data to promote their products and illustrate how such materials can keep a building cooler and lower energy costs.

The scientists next plan to study Houston in much the same fashion as Atlanta. Houston has a different climate and vegetation but the scientists are expecting to find similar data. They believe that if a city is large enough it will be subject to these heat islands and all growing cities should prepare to change the way they look at urban growth.