Urban areas have higher temperatures than rural areas due to materials such as concrete and asphalt absorbing sunlight and retaining heat. This causes health risks and financial costs, but planting trees and using lighter colored surfaces can reduce the heat island effect. Los Angeles is proposing a program to repaint streets, plant trees, and renovate homes to save energy and healthcare costs.
The heat island effect, also known as the urban heat island effect, is a situation where the average temperature of an area is higher than that of nearby rural areas. It is mainly caused by the fact that materials in urban areas, such as concrete and asphalt, absorb a lot of sunlight and store it in large thermal masses. A secondary cause is waste heat generated by massive uses of energy. The heat buildup has an effect not only on the city itself, but also nearby downwind areas, which generally see between 10% and 30% more annual rainfall than upwind areas.
There are a number of small factors that contribute to the heat island effect, and when combined, they raise average temperatures substantially. The difference in urban temperatures compared to the regional averages is more evident at night, and especially during the winter. One of the main causes of the increase in temperature is that in an urban environment large buildings block out a large part of the sky, which is much cooler than the ground at night, and therefore the heat does not dissipate as quickly as in rural areas without large buildings .
Another cause is the introduction of large quantities of new materials that absorb and retain the sun’s thermal energy. Asphalt, for example, absorbs much more sunlight than regular earth, due to its dark color, and both asphalt and concrete tend to retain heat much better than a regular patch of land. As a result, sunlight is absorbed as heat in parking lots, sidewalks and streets, and takes much longer to dissipate than in surrounding non-urban areas.
There are significant health risks associated with a heat island, and depending on the region, death rates during heat waves can be substantially increased in urban areas. Already high temperatures can soar even higher as heat is absorbed and trapped in cities, and with no outlet ready to bleed out at night, there can be little respite for the heat-plagued. Although there are only about 1,000 deaths each year in the United States from extreme heat, a sizable portion of these are thought to be due to rising temperatures in urban environments.
Perhaps even more dramatic is the financial cost of the heat island effect. Higher temperatures during the summer mean higher electricity costs to power the air conditioning and keep the interior cool. The city of Los Angeles estimates that an additional US$100 million is spent on climate control each year due to this phenomenon. As a result, a number of innovative cooling programs have been suggested, both to lower annual costs and to raise living standards.
Planting trees, which reflect light and refresh the environment, use lighter colored surfaces, plant living roofs and lining the streets with a reflective layer are all commonly used tactics to reduce the heat island effect. In the city of Los Angeles, a proposed program is to spend $1 billion to repaint about a quarter of the city’s streets, plant ten million trees and renovate five million homes. Annual savings are estimated at about $170 million in energy and an additional $360 million in health care, with the project paying for itself in just two years.
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