What causes the "heat island" effect?
The urbanization of cities and metropolitan areas with less vegetation or evaporation of water causes dense urban areas to have more warmth than in nearby rural areas.
When humans cover green space and land with roads, buildings, and other structures, humans inadvertently change the local climate, land, and atmosphere. When the land is covered by buildings, and other structures the thermal properties of the surrounding atmosphere is modified. Additionally, buildings and structures release heat through various processes (i.e. greenhouse gas releases, machines, etc.). Additionally buildings and structures in dense urban areas have different specific heat capacities as opposed to rural areas that may have grass and more vegetation.
Tall buildings also contribute the heat island effect since the way the sun's energy is absorbed is altered or changed in various areas of an urban landscape. Another contributor to the heat island effect is the lack of evaporation in a city. Many cities have storm drain systems that take up water. In a natural environment, the evaporation process helps act as natural cooling process with moisture being absorbed into the atmosphere.
Other factors come into play such as pollutants from industrial processes, traditional internal combustion engine cars, air conditioning units etc. contribute to the heat island effect in a more direct and obvious way to the heat island effect.