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Cities are hotter than natural areas for several reasons. First, buildings absorb more heat from the sun than plants and soil do. In addition, paved surfaces retain heat, preventing cities from cooling down. Finally, motor vehicles produce heat and exhaust that builds up within cities, further raising the temperature relative to natural areas. Hot temperatures can be uncomfortable and dangerous for city residents. Researchers have proposed several methods for reducing heat in cities.

One popular idea is to install plants on the roofs of buildings. In parks, plants naturally cool the air around them as a result of certain life processes. They also provide shade. The expectation is that when grown on the tops of buildings, plants will cause the roofs to be significantly cooler for the same reasons. Just like parks, where plants provide shade and cool the surroundings, "green roofs"will help lower urban temperatures.

Another approach is to replace the materials ordinarily used to palve roads and sidewalks with materials that absorb water from rainfall. Ordinary paving materials do not soak up rainwater, so water flows away into gutters instead. Water-absorbing pavements, on the other hand, have many tiny holes that hold on to rainwater. When hot air causes this retained water to evaporate, the paved surfaces and the surrounding air grow cooler. So cities with water-absorbing pavements will be significantly cooler.

Finally, some experts believe that cities can be made cooler by creating new, denser urban areas that discourage people from using cars. If new urban areas are built with larger, taller buildings that can house more people and businesses, distances that people have to cover to visit another person or a business will be shorter. As a result, it will be more practical to travel by foot or by bicycle than by Car. With fewer motor vehicles in use in dense urban areas, there will be less heat and exhaust, leading to lower temperatures overall.

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