As urban areas develop, changes occur in their landscape. Buildings, roads and other infrastructure replace open land and vegetation. Surfaces that were once permeable and moist become impermeable and dry¹. These changes cause urban regions to become warmer than their rural surroundings, forming an "island" of higher temperatures in the landscape.

Heat Islands occur on the surface and in the atmosphere. On a hot, sunny summer day, the sun can heat dry, exposed urban surfaces, such as roofs and pavement, to temperatures 50-90°F (27-50°C) hotter than the air², while shaded or moist surfaces - often in more rural surroundings - remain close to air temperatures. Surface urban heat islands are typically present day and night, but tend to be strongest during the day when the sun is shining.

In contrast, atmospheric urban heat islands are often weak during the late morning and throughout the day and become more pronounced after sunset due to the slow release of heat from urban infrastructure. The annual mean air temperature of a city with 1 million people or more can be 1.8-5.4°F (1-3°C) warmer than it's surroundings.³ On a clear, calm night, however, the temperature difference can be as much as 22°F (12°C).⁴

Source: EPA's Reducing Urban Heat Islands: Compendium of Strategies