The increased heat of our cities increases discomfort for everyone, requires an increase in the amount of energy used for cooling purposes, and increases pollution. Each city's urban heat island varies based on the city structure and thus the range of temperatures within the island vary as well. Parks and greenbelts reduce temperatures while the Central Business District (CBD), commercial areas, and even suburban housing tracts are areas of warmer temperatures. Every house, building, and road changes the microclimate around it, contributing to the urban heat islands of our cities.
Los Angeles has been very much affected by its urban heat island. The city has seen its average temperature rise approximately 1°F every decade since the beginning of its super-urban growth since the World War II era. Other cities have seen increases of 0.2°-0.8°F each decade.
Various environmental and governmental agencies are working to decrease the temperatures of urban heat islands. This can be accomplished in several ways; most prominent are switching dark surfaces to light reflective surfaces and by planting trees. Dark surfaces, such as black roofs on buildings, absorb much more heat than light surfaces, which reflect sunlight. Black surfaces can be up to 70°F (21°C) hotter than light surfaces and that excess heat is transferred to the building itself, creating an increased need for cooling. By switching to light colored roofs, buildings can use 40% less energy.
Planting trees not only helps to shade cities from incoming solar radiation, they also increase evapotranspiration, which decreases the air temperature. Trees can reduce energy costs by 10-20%. The concrete and asphalt of our cities increases runoff, which decreases the evaporation rate and thus also increases temperature.
Increased heat enhances photochemical reactions, which increases the particles in the air and thus contributes to the formation of smog and clouds. London receives approximately 270 fewer hours of sunlight than the surrounding countryside due to clouds and smog. Urban heat islands also increase precipitation in cities and areas downwind of cities.
Our stone-like cities only slowly loose heat at night, thus causing the greatest temperature differences between city and countryside to take place at night.
Some suggest that urban heat islands are the true culprit for global warming. Most of our temperature gauges have been located near cities so the cities which grew up around the thermometers have recorded an increase in average temperatures worldwide. However, such data is corrected by atmospheric scientists studying global warming.