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The heat island effect, also referred to as the urban heat island effect, is a situation where the average temperature of an area is higher than nearby rural areas. It's mostly caused by the fact that the materials in urban areas, like 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 buildup of heat has an affect not only on the city itself, but also on nearby areas downwind, which generally see between 10% and 30% more annual rainfall than areas upwind.
There are a number of small factors that contribute to the heat island effect, and when combined, they increase average temperatures substantially. The difference in urban temperatures over regional averages is seen most strikingly at night, and especially during the winter. One of the main causes of the increased temperature is that in an urban environment large buildings block out a great deal of the sky, which at night is much cooler than the ground, and so heat does not dissipate as rapidly as it does in rural areas with no large buildings.
Another cause is the introduction of large swaths of new materials that absorb and retain heat energy from the sun. Asphalt, for example, absorbs much more sunlight than normal earth, because of its dark color, and both asphalt and concrete tend to retain heat much better than an average patch of ground. As a result, sunlight is absorbed as heat into parking lots, sidewalks, and streets, and it takes much longer to dissipate than surrounding non-urban areas.
There are significant health risks associated with a heat island, and depending on the region, mortality rates during heat waves can be substantially increased in urban areas as a result. Already high temperatures can soar even higher as heat is absorbed and trapped in cities, and without a ready outlet to bleed off at night, there can be little respite for those afflicted by the heat. Although there are only around 1,000 deaths each year in the United States from extreme heat, a sizable portion of these are thought to be due to the increased temperatures of urban environments.
Perhaps even more dramatic is the financial cost of the heat island effect. Higher temperatures during summertime mean increased electricity costs to power air conditioning to keep interiors cool. The city of Los Angeles estimates that an extra $100 million US Dollars (USD) is spent each year on climate control due to this phenomenon. As a result, a number of innovative cooling programs have been suggested, both to lower annual costs and to increase standard of living.
Planting trees, which reflect light and cool the environment, using lighter-colored surfaces, planting living roofs, and coating roads with a reflective layer are all commonly-used tactics to reduce the heat island effect. In the city of Los Angeles, one proposed program looks at spending $1 billion USD to coat approximately one-quarter of the roads in the city, plant ten million trees, and refit five million houses. The annual savings are estimated at around $170 million USD in energy, and an additional $360 million USD in healthcare, with the project paying for itself in only two years.