On a hot evening, temperatures in the countryside often begin to fall after sunset. In a nearby city, however, the heat may remain. This pattern is called an urban heat island because the city becomes an island of higher temperature than its surrounding areas.
One major cause is the difference between surfaces. In forests and fields, trees provide shade, while evaporation from plants and soil cools the environment. Asphalt, concrete, and dark roofs absorb more of the Sun’s energy. These surfaces store heat during the day and release it slowly after sunset.
The shape of a city matters as well. Tall buildings along narrow streets can reduce airflow and make it harder for heat to escape. Cars, factories, buildings, and air-conditioning systems also release heat into the environment. A heat island therefore does not have a single cause. It develops through a combination of land cover, architecture, and energy use.
Heat is not distributed equally within a city. Neighborhoods with few trees and large areas of roads or parking lots tend to be hotter than greener areas. High nighttime temperatures increase the electricity needed for cooling and can also raise the health risks associated with hot weather.
Cities have several ways to reduce the effect. Street trees create shade, and plants release moisture. Light-colored roofs and pavements can reflect more sunlight instead of absorbing it. Green roofs can help cool building surfaces. The best combination depends on local climate and city design. No single project can remove all urban heat, but many well-placed changes can make neighborhoods safer and more comfortable, especially after the Sun goes down.