Causes of the UHI: The Urban Energy Budget
The UHI arises from the fact that urban areas modify their surface and atmospheric characteristics relative to the surrounding rural regions. These alterations result in a modified surface energy budget in cities. The energy budget describes the exchanges of energy at the Earth’s surface: how the radiation energy arriving at the city surface from the Sun and atmosphere is absorbed and partitioned into heat energy that is used to warm the air through mixing (convective sensible heat), heat energy used to evaporate water from the surface or to transpire water from vegetation (convective latent heat), and heat that is conducted and stored in the substrate materials. As a general rule, urban areas tend to increase both the relative amount of energy used to warm the air and that which is stored in the urban substrate materials, and to decrease the relative amount of energy directed toward evapotranspiration (the combined evaporation and transpiration of water) compared to the energy budget of surrounding nonurban areas. In addition, cities also directly add heat to their atmosphere through the use of energy. Building heating and cooling, electricity use, and transportation are important contributors to this anthropogenic energy component.171 Therefore, we can say that altered energy budgets underlie UHI formation. To better understand the reasons for urban energy budget changes, we turn to an examination of the surface characteristics of urban areas.
Causes of the UHI: Characteristics of the Urban Surface
The surface characteristics of urban areas provide important controls on the surface energy budget and therefore on the UHI. Three groups of urban surface characteristics are important. First, the surface coverage and relative fractions of buildings, vegetation, and impervious surfaces are important. Buildings and other impervious surfaces such as roads are designed to shed water so that energy absorbed by these surfaces is directed only toward heating the material and the air above it. As the relative fraction of buildings and impervious surfaces increases and vegetation decreases, greater fractions of energy will be directed toward warming the air and substrate because evapotranspiration from vegetation becomes more limited.
Second, the structure or form of the surface as determined by the dimensions and spacing of buildings and trees is important to UHI formation. By day, the urban surface is better at absorbing solar radiation than most rural surfaces. Absorption is enhanced by several factors: the reflectivity of the surface to solar radiation (known as the albedo)—which may be less (darker) for some surfaces (such as asphalt roads and dark-colored roofs) compared to the rural surroundings—and the three-dimensional structure of the urban surface, which serves to increase the effective area of the surface that can absorb sunlight and which also serves to “trap” some radiation that is initially reflected off of individual surfaces. At night, the three-dimensional structure of the surface serves to effectively block the view of the sky for many parts of the urban surface, especially within the UCL (e.g., the roadway separating adjacent tall buildings), and thereby reduces the loss of radiative heat energy. The sky view factor that describes the relative obstruction of the sky to the surface is strongly related to the UHI magnitude of the UCL air.1'1 The structure of the urban surface is also responsible for altering winds near the surface; this provides both sheltering effects and increased turbulence (mixing). During the day, the effective mixing of the atmosphere assisted by the rough surface helps to warm the urban boundary layer with heat from the surface. At night, the sheltering of air between buildings contributes to the increased warmth of the nighttime UCL heat island.
Third, the properties of materials that affect temperature, such as radiative, thermal, and moisture characteristics, differ in urban areas from their rural surroundings. Many urban materials are drier, have higher heat capacities, and may be darker (lower albedo). This allows them to more readily absorb solar energy, to store it, and to have higher temperatures because no energy is used to evaporate water. Urban materials thus tend to act as a sink for heat during daytime and a source of heat at night. The material properties contribute to a temporal lag in the warming of urban areas in the morning and a similar lag in their cooling at night.
The widely used Land Cover Zone classification provides a climatically relevant land-cover classification scheme for cities and their surroundings.181 It explicitly recognizes the importance of surface characteristics to variations in urban-scale climate and is a highly useful tool for planning and assessing UHI studies, especially those related to the UCL.