An urban heat island is the name given to describe the characteristic warmth of both the atmosphere and surfaces in cities (urban areas) compared to their non-urbanized surroundings. The heat island is an example of unintentional climate modification when urbanization changes the characteristics of the Earth’s surface and atmosphere.
As cities add roads, buildings, industry, and people, temperatures in the city rise relative to their rural surroundings, creating a heat island. These urban heat islands may be up to 10-15°F warmer under optimum conditions. With increasing urban development, heat islands may increase in frequency and magnitude. Los Angeles, California, for example, has been 1˚F hotter every decade for the past 60 years. These heat islands have impacts that range from local to global scales and so represent a significant impact on environmental change.
There are three types of heat islands: 1). canopy layer heat island (CLHI), 2). boundary layer heat island (BLHI), and, 3). surface heat island (SHI).
The first two refer to a warming of the urban atmosphere; the last refers to the relative warmth of urban surfaces. The urban canopy layer (UCL) is the layer of air closest to the surface in cities, extending upwards to approximately the mean building height. Above the urban canopy layer lies the urban boundary layer, which may be 1 kilometer (km) or more in thickness by day, shrinking to hundreds of meters or less at night. It is the BLHI that forms a dome of warmer air that extends downwind of the city. Wind often changes the dome to a plume shape.
Heat island types vary in their spatial form (shape), temporal (related to time) characteristics, and some of the underlying physical processes that contribute to their development. Scientists measure air temperatures for CLHI or BLHI directly using thermometers, whereas the SHI is measured by remote sensors mounted on satellites or aircraft.
A number of factors contribute to the occurrence and intensity of heat islands; these include: 1). weather, 2). geographic location, 3). time of day and season, 4). city form, and 5). city functions.
Weather, particularly wind and cloud, influences formation of heat islands. Heat island magnitudes are largest under calm and clear weather conditions. Increasing winds mix the air and reduce the heat island. Increasing clouds reduce radiative cooling at night and also reduce the heat island. Seasonal variations in weather patterns affect heat island frequency and magnitude.
Geographic location influences the climate and topography of the area as well as the characteristics of the rural surroundings of the city. Regional or local weather influences, such as local wind systems, may impact heat islands; for example, coastal cities may experience cooling of urban temperatures in the summer when sea surface temperatures are cooler than the land and winds blow onshore. Where cities are surrounded by wet rural surfaces, slower cooling by these surfaces can reduce heat island magnitudes, especially in warm humid climates.
City form comprises the materials used in construction, the surface characteristics of the city such as the building dimensions and spacing, thermal properties, and amount of greenspace.
Heat island formation is favored by: 1). relatively dense building materials that are slow to warm and cool and store a lot of energy, 2). replacement of natural surfaces by impervious or waterproofed surfaces, leading to a drier urban area, where less water is available for evaporation, which offsets heating of the air, and 3). lower surface reflectivity to solar radiation — dark surfaces such as asphalt roads absorb more sunlight and become much warmer than light-colored surfaces.
City functions govern the output of pollutants into the urban atmosphere, heat from energy usage, and the use of water in irrigation. Anthropogenic heat, or heat generated from human activities, primarily fossil fuel combustion, can be important to heat island formation. Anthropogenic heating usually has the largest impact during the winter season of cold climates in the downtown core of the city. In select cases, very densely developed cities may have significant summertime anthropogenic heating that results from high energy use for building cooling.