It is well established that urban areas often have higher temperatures than those of surrounding rural areas. This is known as the urban heat island effect. The intensity of the urban-rural temperature difference varies from city to city, and is largely a function of the size and density of the urban area.
Several factors contribute to the urban heat island phenomenon.
Urban materials, such as concrete, glass, and asphalt, have high heat storage capacities and so they store heat during the day and release it at night. Additional anthropogenic heat, used to heat buildings, can escape from the sides of buildings, especially in the winter at higher latitudes where energy consumption is greatest. The irregular geometry of cities reduces wind speeds, and provides more surface area for the absorption of solar radiation. Furthermore, pollutants in the urban canopy trap heat emitted from the urban area and reradiate it back toward the surface. All of these factors contribute to a warming of the city environment.