These temperature differences constitute intra-urban heat islands. Some areas are hotter than others due to the uneven distribution of heat-absorbing buildings and pavements, while other spaces remain cooler as a result of trees and greenery. Heat islands are usually measured by the temperature difference between cities relative to the surrounding areas.

For example, nearby mountains can block wind from reaching a city, or create wind patterns that pass through a city. Geographic features can also impact the heat island effect. Conversely, strong winds and cloud cover suppress heat island formation. Calm and clear weather conditions result in more severe heat islands by maximizing the amount of solar energy reaching urban surfaces and minimizing the amount of heat that can be carried away. These sources of human-generated, or anthropogenic, waste heat can contribute to heat island effects. Vehicles, air-conditioning units, buildings, and industrial facilities all emit heat into the urban environment. Cities with many narrow streets and tall buildings become urban canyons, which can block natural wind flow that would bring cooling effects. In heavily developed areas, surfaces and structures obstructed by neighboring buildings become large thermal masses that cannot release their heat readily. The dimensions and spacing of buildings within a city influence wind flow and urban materials’ ability to absorb and release solar energy. Often, heat islands build throughout the day and become more pronounced after sunset due to the slow release of heat from urban materials. Conventional human-made materials used in urban environments such as pavements or roofing tend to reflect less solar energy, and absorb and emit more of the sun’s heat compared to trees, vegetation, and other natural surfaces. Hard, dry surfaces in urban areas – such as roofs, sidewalks, roads, buildings, and parking lots – provide less shade and moisture than natural landscapes and therefore contribute to higher temperatures. Trees, vegetation, and water bodies tend to cool the air by providing shade, transpiring water from plant leaves, and evaporating surface water, respectively. Reduced Natural Landscapes in Urban Areas.Heat islands form as a result of several factors: Research predicts that the heat island effect will strengthen in the future as the structure, spatial extent, and population density of urban areas change and grow. Humid regions (primarily in the eastern United States) and cities with larger and denser populations experience the greatest temperature differences. These pockets of heat are referred to as “heat islands.” Heat islands can form under a variety of conditions, including during the day or night, in small or large cities, in suburban areas, in northern or southern climates, and in any season.Ī review of research studies and data found that in the United States, the heat island effect results in daytime temperatures in urban areas about 1–7☏ higher than temperatures in outlying areas and nighttime temperatures about 2–5☏ higher. Urban areas, where these structures are highly concentrated and greenery is limited, become “islands” of higher temperatures relative to outlying areas. Structures such as buildings, roads, and other infrastructure absorb and re-emit the sun’s heat more than natural landscapes such as forests and water bodies.

Radiation island black cloud