November 2004
Are cities getting hotter? 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 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 highlight the importance of urbanization to environmental change.
What is an urban heat island?
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 (nonurbanized) surroundings. The heat island is an example of unintentional climate modification when urbanization changes the characteristics of the Earth’s surface and atmosphere.
Are there different types of urban heat islands?
There are three types of heat islands:
- canopy layer heat island (CLHI)
- boundary layer heat island (BLHI)
- 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 (Figure 1). 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 (Figure 1).1 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. 2,3
What are the characteristics of heat islands?
Overall spatial form (shape) of the heat island
The isotherms, or lines of equal temperature, form a pattern that resembles an “island” loosely following the shape of the urbanized region, surrounded by cooler areas (Figure 2). There is often a sharp rise in the canopy-layer air temperature at the boundary of rural—suburban areas, followed by a slow and often variable increase towards the downtown core of the urban area where the warmest temperatures occur. The boundary layer heat island shows much less variability than the other heat island types and a cross-section shows its shape resembles a simple dome or plume with warmer air transported downwind of the city.
Heat island intensity
Heat island intensity is a measure of the strength or magnitude of the heat island. At night, the intensity of the canopy layer heat island is typically in the range of 1° to 3°C, but under optimum conditions intensities of up to 12°C have been recorded.4 The BLHI tends to maintain a more constant heat island intensity both day and night (~1.5° to 2°C). The SHI is usually most distinct during the day when strong solar heating can lead to larger temperature differences between dry surfaces and wet, shaded, or vegetated surfaces.
Surface characteristics and the heat island
The nature of the surface is a strong factor on the spatial patterns of surface and canopy layer air temperature in the city. The temperatures are higher in more densely built up areas, and lower near parks or more open areas (Figure 2). Surface temperatures are particularly sensitive to surface conditions: during daytime, dry, dark surfaces that strongly absorb sunlight become very hot, while lighter and/or moist surfaces are much cooler.2,3 Shading of the surface also helps control the temperature. (For visual examples of the surface heat island, see the “learn more” link, EPA Heat Island Pilot Project, at the end of the article.)
Temporal form of the heat island
All heat islands form because of the differences in the rates of warming and cooling of cities relative to their surroundings.
- CLHI: the heat island intensity increases with time from sunset to a maximum somewhere between a few hours after sunset to the predawn hours. During the day the CLHI intensity is typically fairly weak or sometimes negative (a cool island) in some parts of the city where there is extensive shading by tall buildings or other structures and a lag in warming due to storage of heat by building materials.
- SHI: is strongly positive both day and night due to warmer urban surfaces. Daytime SHI is usually largest because solar radiation affects surface temperatures.
- BLHI: is generally positive both day and night but much smaller in magnitude than CLHI or SHI.
How do heat islands form and how are they controlled?
A number of factors contribute to the occurrence and intensity of heat islands; these include
- weather
- geographic location
- time of day and season
- city form
- 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.5
Time of day/season: Daytime impacts were discussed in the section called “Temporal form of the heat island.” Seasons play a role, too. Heat islands of cities located in the mid latitudes usually are strongest in the summer or winter seasons. In tropical climates, the dry season may favor large heat island magnitudes.
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
- relatively dense building materials that are slow to warm and cool and store a lot of energy
- 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
- 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.6 Anthropogenic heating usually has the largest impact during the winter season of cold climates in the downtown core of the city.7 In select cases, very densely developed cities may have significant summertime anthropogenic heating that results from high energy use for building cooling.7
How do heat islands impact cities?
Heat islands have a range of impacts for city dwellers,4 including
- human comfort: positive (winter), negative (summer)
- energy use: positive (winter), negative (summer)
- air pollution: negative
- water use: negative
- biological activity (e.g., growing season length): positive
- ice and snow: positive
Summer heat islands can increase the demand for energy for air conditioning, which releases more heat into the air as well as greenhouse gas emissions, degrading local air quality.8 Higher urban temperatures in the daytime BLHI may increase the formation of urban smog, because both emissions of precursor pollutants and the atmospheric photochemical reaction rates increase.9,10 Heat islands may also directly impact human health by exacerbating heat stress during heat waves, especially in temperate areas, and by providing conditions suitable for the spread of vector-borne diseases.11,12
Biological solutions for alleviating urban heat islands?
The understanding of the physical mechanisms underlying heat island formation provides a basis to develop controls that may promote or alleviate heat islands, but in some cases the application of these controls is difficult. For example, widespread change of the urban surface geometry by spacing buildings is usually not feasible. However, other strategies are possible— for example, using white or other light-colored roofs and pavement.
A biologically related solution is to use vegetation to reduce urban heat. Vegetation provides important shading effects as well as cooling through evaporation. Some examples include:
- Planting trees around individual buildings to shade urban surfaces to reduce their temperature, especially roofs and south-, east-, and west-facing walls. The reduction in surface temperature also leads to substantial reductions in energy use for air conditioning.
Trees can also be used to shade roads and parking lots, which would otherwise become very hot during the day and which store heat for later release at night. Shading of vehicles in parking lots can reduce evaporative emissions from gasoline, which contribute to increased levels of urban ozone.
“Green roofs” use living vegetation on roofs in order to help reduce heat accumulation of buildings. For example, the city of Chicago has more than 80 municipal and private green roofs as of June 2004, including the first municipal green roof in the country, the City Hall rooftop garden. A green roof is much cooler than a traditional roof because a significant fraction of the absorbed energy is used to evaporate water rather than to heat the roof and the overlying air.
Creation of greenspace such as parks can be used to assist in cooling of neighborhoods,13,14 and an overall greening of the city can lead to a cooler urban atmosphere.15
These strategies can provide cost benefits. A building owner benefits from reduced energy consumption costs. Residents downwind of the urban area benefit from air quality improvements because:
- pollutants are deposited on trees
- greenhouse gas and pollutant emissions from air conditioning use are reduced
- emissions of volatile organic compounds that contribute to urban smog are lessened
- the rate of ozone formation is potentially reduced
The US Environmental Protection Agency has undertaken the Urban Heat Island Pilot Project as part of the Heat Island Reduction Initiative. Pilot cities include Baton Rouge, Chicago, Houston, Sacramento, and Salt Lake City.
Do urban heat islands affect global climate?
Urban heat islands themselves are not responsible for global warming because they are small-scale phenomena and cover only a tiny fraction of the Earth’s surface area. However, there are some urban to global scale connections that are worth noting:
Approximately half of the world’s population currently lives in cities, and this value is expected to increase to 61% by 2030.16 The high rate of urbanization, particularly in the tropics, means that increasing numbers of people will be exposed to impacts resulting from heat islands in the future.
Urban areas have historically been the site of some of the earliest established observation stations that are used to help construct the global surface temperature record used to document large scale climate changes. The effects of urbanization, and consequently urban heat islands, on these stations over time can lead to some “contamination” of the temperature record. The ability to fully remove these influences remains the subject of some debate since changes can occur independently of population17 and current techniques used to remove urban effects may be inadequate.17-19
Most greenhouse gas emissions that contribute to global climate change come from urban areas. These emissions therefore contribute to both local and global scale weather and climate modification.20 Further urbanization will increase emissions originating from cities. Investigation of the larger scale impacts of urban emissions is seen as an important area of future research.20
The climate modifications that have occurred in large cities over the past century show similarities in terms of the rates and magnitude expected with projected future climate changes. Therefore cities may serve as a model for assessing the impacts of, and adaptation strategies to, climate change on both local and global scales.4
These factors underscore the importance of urban climates not only to the local environment but also to the state of the environment for the planet as a whole.
© 2004, American Institute of Biological Sciences. Educators have permission to reprint articles for classroom use; other users, please contact editor@actionbioscience.org for reprint permission. See reprint policy.
