Urban heat island
An urban heat island (UHI) is a metropolitan area that is significantly warmer than its surrounding rural areas due to human activities. The phenomenon was first investigated and described by Luke Howard in the 1810s, although he was not the one to name the phenomenon. The temperature difference usually is larger at night than during the day, and is most apparent when winds are weak. UHI is most noticeable during the summer and winter. The main cause of the urban heat island effect is from the modification of land surfaces, which use materials that effectively store short-wave radiation. Waste heat generated by energy usage is a secondary contributor. As a population center grows, it tends to expand its area and increase its average temperature. The less-used term heat island refers to any area, populated or not, which is consistently hotter than the surrounding area.
Monthly rainfall is greater downwind of cities, partially due to the UHI. Increases in heat within urban centers increases the length of growing seasons, and decreases the occurrence of weak tornadoes. The UHI decreases air quality by increasing the production of pollutants such as ozone, and decreases water quality as warmer waters flow into area streams and put stress on their ecosystems.
Not all cities have a distinct urban heat island. Mitigation of the urban heat island effect can be accomplished through the use of green roofs and the use of lighter-colored surfaces in urban areas, which reflect more sunlight and absorb less heat.
Despite concerns raised about its possible contribution to global warming, comparisons between urban and rural areas show that the urban heat island effects have little influence on global mean temperature trends.
There are several causes of an urban heat island (UHI). The principal reason for the nighttime warming is that the short-wave radiation is still within the concrete, asphalt, and buildings that was absorbed during the day, unlike suburban and rural areas. This energy is then slowly released during the night as long-wave radiation, making cooling a slow process. Two other reasons are changes in the thermal properties of surface materials and lack of evapotranspiration (for example through lack of vegetation) in urban areas. With a decreased amount of vegetation, cities also lose the shade and cooling effect of trees, the low albedo of their leaves, and the removal of carbon dioxide. Materials commonly used in urban areas for pavement and roofs, such as concrete and asphalt, have significantly different thermal bulk properties (including heat capacity and thermal conductivity) and surface radiative properties (albedo and emissivity) than the surrounding rural areas. This causes a change in the energy balance of the urban area, often leading to higher temperatures than surrounding rural areas.
Other causes of a UHI are due to geometric effects. The tall buildings within many urban areas provide multiple surfaces for the reflection and absorption of sunlight, increasing the efficiency with which urban areas are heated. This is called the "urban canyon effect". Another effect of buildings is the blocking of wind, which also inhibits cooling by convection and pollution from dissipating. Waste heat from automobiles, air conditioning, industry, and other sources also contributes to the UHI. High levels of pollution in urban areas can also increase the UHI, as many forms of pollution change the radiative properties of the atmosphere. As UHI raises the temperature of cities, it will also increase the concentration of ozone in the air, which is a greenhouse gas. Ozone concentrations will increase because it is a secondary gas, aided by an increase in temperature and sunlight.
Some cities exhibit a heat island effect, largest at night. Seasonally, UHI shows up both in summer and winter. The typical temperature difference is several degrees between the center of the city and surrounding fields. The difference in temperature between an inner city and its surrounding suburbs is frequently mentioned in weather reports, as in "68 °F (20 °C) downtown, 64 °F (18 °C) in the suburbs". Black surfaces absorb significantly more electromagnetic radiation, and causes the surfaces of asphalt roads and highways to heat. "The annual mean air temperature of a city with 1 million people or more can be 1.8–5.4°F (1–3°C) warmer than its surroundings. In the evening, the difference can be as high as 22°F (12°C)."
The IPCC stated that "it is well-known that compared to non-urban areas urban heat islands raise night-time temperatures more than daytime temperatures." For example, Barcelona, Spain is 0.2 °C (0.4 °F) cooler for daily maxima and 2.9 °C (5.2 °F) warmer for minima than a nearby rural station. A description of the very first report of the UHI by Luke Howard in the late 1810s said that the urban center of London was warmer at night than the surrounding countryside by 3.7 °F (2.1 °C). Though the warmer air temperature within the UHI is generally most apparent at night, urban heat islands exhibit significant and somewhat paradoxical diurnal behavior. The air temperature difference between the UHI and the surrounding environment is large at night and small during the day. The opposite is true for skin temperatures of the urban landscape within the UHI.
Throughout the daytime, particularly when the skies are free of clouds, urban surfaces are warmed by the absorption of solar radiation. Surfaces in the urban areas tend to warm faster than those of the surrounding rural areas. By virtue of their high heat capacities, urban surfaces act as a giant reservoir of heat energy. For example, concrete can hold roughly 2,000 times as much heat as an equivalent volume of air. As a result, the large daytime surface temperature within the UHI is easily seen via thermal remote sensing. As is often the case with daytime heating, this warming also has the effect of generating convective winds within the urban boundary layer. It is theorized that, due to the atmospheric mixing that results, the air temperature perturbation within the UHI is generally minimal or nonexistent during the day, though the surface temperatures can reach extremely high levels.
At night, the situation reverses. The absence of solar heating causes the atmospheric convection to decrease, and the urban boundary layer begins to stabilize. If enough stabilization occurs, an inversion layer is formed. This traps urban air near the surface, and keeping surface air warm from the still-warm urban surfaces, forming the nighttime warmer air temperatures within the UHI. Other than the heat retention properties of urban areas, the nighttime maximum in urban canyons could also be due to the blocking of "sky view" during cooling: surfaces lose heat at night principally by radiation to the comparatively cool sky, and this is blocked by the buildings in an urban area. Radiative cooling is more dominant when wind speed is low and the sky is cloudless, and indeed the UHI is found to be largest at night in these conditions.
Impact on animals
Ant colonies in urban heat islands have an increased heat tolerance at no cost to cold tolerance.
Other impacts on weather and climate
Aside from the effect on temperature, UHIs can produce secondary effects on local meteorology, including the altering of local wind patterns, the development of clouds and fog, the humidity, and the rates of precipitation. The extra heat provided by the UHI leads to greater upward motion, which can induce additional shower and thunderstorm activity. In addition, the UHI creates during the day a local low pressure area where relatively moist air from its rural surroundings converges, possibly leading to more favorable conditions for cloud formation. Rainfall rates downwind of cities are increased between 48% and 116%. Partly as a result of this warming, monthly rainfall is about 28% greater between 20 miles (32 km) to 40 miles (64 km) downwind of cities, compared with upwind. Some cities show a total precipitation increase of 51%.
Research has been done in a few areas suggesting that metropolitan areas are less susceptible to weak tornadoes due to the turbulent mixing caused by the warmth of the urban heat island. Using satellite images, researchers discovered that city climates have a noticeable influence on plant growing seasons up to 10 kilometers (6 mi) away from a city's edges. Growing seasons in 70 cities in eastern North America were about 15 days longer in urban areas compared to rural areas outside of a city's influence.
UHIs have the potential to directly influence the health and welfare of urban residents. Within the United States alone, an average of 1,000 people die each year due to extreme heat. As UHIs are characterized by increased temperature, they can potentially increase the magnitude and duration of heat waves within cities. Research has found that the mortality rate during a heat wave increases exponentially with the maximum temperature, an effect that is exacerbated by the UHI. The nighttime effect of UHIs can be particularly harmful during a heat wave, as it deprives urban residents of the cool relief found in rural areas during the night.
Research in the United States suggests that the relationship between extreme temperature and mortality varies by location. Heat is more likely to increase the risk of mortality in cities at mid-latitudes and high latitudes with significant annual temperature variation. For example, when Chicago and New York experience unusually hot summertime temperatures, elevated levels of illness and death are predicted. In contrast, parts of the country that are mild to hot year-round have a lower public health risk from excessive heat. Research shows that residents of southern cities, such as Miami, tend to be acclimated to hot weather conditions and therefore less vulnerable to heat related deaths.
Increased temperatures and sunny days help lead to the formation of low-level ozone from volatile organic compounds and nitrous oxides which already exist in the air. As urban heat islands lead to increased temperatures within cities, they contribute to worsened air quality.
Impact on nearby water bodies
UHIs also impair water quality. Hot pavement and rooftop surfaces transfer their excess heat to stormwater, which then drains into storm sewers and raises water temperatures as it is released into streams, rivers, ponds, and lakes. Additionally, increased urban water body temperatures lead to a decrease in diversity in the water. In August 2001, rains over Cedar Rapids, Iowa led to a 10.5C (18.9F) rise in the nearby stream within one hour, which led to a fish kill. Since the temperature of the rain was comparatively cool, it could be attributed to the hot pavement of the city. Similar events have been documented across the American Midwest, as well as Oregon and California. Rapid temperature changes can be stressful to aquatic ecosystems. Permeable pavements may mitigate these effects by percolating water through the pavement into subsurface storage areas where it can be dissipate through absorption and evaporation.
Impact on energy usage
Another consequence of urban heat islands is the increased energy required for air conditioning and refrigeration in cities that are in comparatively hot climates. The Heat Island Group estimates that the heat island effect costs Los Angeles about US$100 million per year in energy. Conversely, those that are in cold climates such as Moscow, Russia would have less demand for heating. However, through the implementation of heat island reduction strategies, significant annual net energy savings have been calculated for northern locations such as Chicago, Salt Lake City, and Toronto.
The temperature difference between urban areas and the surrounding suburban or rural areas can be as much as 5°C (9°F). Nearly 40 percent of that increase is due to the prevalence of dark roofs, with the remainder coming from dark-colored pavement and the declining presence of vegetation. The heat island effect can be counteracted slightly by using white or reflective materials to build houses, roofs, pavements, and roads, thus increasing the overall albedo of the city. Relative to remedying the other sources of the problem, replacing dark roofing requires the least amount of investment for the most immediate return. A cool roof made from a reflective material such as vinyl reflects at least 75 percent of the sun’s rays, and emit at least 70 percent of the solar radiation absorbed by the building envelope. Asphalt built-up roofs (BUR), by comparison, reflect 6 percent to 26 percent of solar radiation.
Using light-colored concrete has proven effective in reflecting up to 50% more light than asphalt and reducing ambient temperature. A low albedo value, characteristic of black asphalt, absorbs a large percentage of solar heat creating warmer near-surface temperatures. Paving with light-colored concrete, in addition to replacing asphalt with light-colored concrete, communities may be able to lower average temperatures. However, research into the interaction between reflective pavements and buildings has found that, unless the nearby buildings are fitted with reflective glass, solar radiation reflected off light-colored pavements can increase building temperatures, increasing air conditioning demands.
A second option is to increase the amount of well-watered vegetation. These two options can be combined with the implementation of green roofs. Green roofs are excellent insulators during the warm weather months and the plants cool the surrounding environment. Air quality is improved as the plants absorb and convert carbon dioxide to oxygen. The city of New York determined that the cooling potential per area was highest for street trees, followed by living roofs, light covered surface, and open space planting. From the standpoint of cost effectiveness, light surfaces, light roofs, and curbside planting have lower costs per temperature reduction.
A hypothetical "cool communities" program in Los Angeles has projected that urban temperatures could be reduced by approximately 3 °C (5 °F) after planting ten million trees, reroofing five million homes, and painting one-quarter of the roads at an estimated cost of US$1 billion, giving estimated annual benefits of US$170 million from reduced air-conditioning costs and US$360 million in smog related health savings.
Green building programs
Voluntary green building programs have been promoting the mitigation of the heat island effect for years. For example, one of the ways for a site to earn points under the US Green Building Council’s (USGBC) Leadership in Energy and Environmental Design (LEED) Green Building Rating System is to take action that reduces heat islands, minimizing impacts on microclimates and human and wildlife habitats. Credits associated with reflective roofing or planted roofs can help a building achieve LEED certification. Buildings also receive credits by providing shade. Similarly, The Green Building Initiative (GBI)’s Green Globes program awards points to sites that take measures to decrease a building’s energy consumption and reduce the heat island effect. As many as 10 points may be awarded to sites with roof coverage from vegetation, highly reflective materials, or a combination of the two.
Because some parts of some cities may be hotter than their surroundings, concerns have been raised that the effects of urban sprawl might be misinterpreted as an increase in global temperature. Such effects are removed by homogenization from the raw climate record by comparing urban stations with surrounding stations. While the "heat island" warming is an important local effect, there is no evidence that it biases trends in the homogenized historical temperature record. For example, urban and rural trends are very similar.
The Third Assessment Report from the IPCC says:
- However, over the Northern Hemisphere land areas where urban heat islands are most apparent, both the trends of lower-tropospheric temperature and surface air temperature show no significant differences. In fact, the lower-tropospheric temperatures warm at a slightly greater rate over North America (about 0.28°C/decade using satellite data) than do the surface temperatures (0.27°C/decade), although again the difference is not statistically significant.
Ground temperature measurements, like most weather observations, are logged by location. Their siting predates the massive sprawl, roadbuilding programs, and high- and medium-rise expansions which contribute to the UHI. More importantly, station logs allow sites in question to be filtered easily from data sets. Doing so, the presence of heat islands is visible, but overall trends change in magnitude, not direction. The effects of the urban heat island may be overstated. One study stated, "Contrary to generally accepted wisdom, no statistically significant impact of urbanization could be found in annual temperatures." This was done by using satellite-based night-light detection of urban areas, and more thorough homogenisation of the time series (with corrections, for example, for the tendency of surrounding rural stations to be slightly higher in elevation, and thus cooler, than urban areas). If its conclusion is accepted, then it is necessary to "unravel the mystery of how a global temperature time series created partly from urban in situ stations could show no contamination from urban warming." The main conclusion is that microscale and local-scale impacts dominate the mesoscale impact of the urban heat island. Many sections of towns may be warmer than rural sites, but surface weather observations are likely to be made in park "cool islands."
Not all cities show a warming relative to their rural surroundings. After trends were adjusted in urban weather stations around the world to match rural stations in their regions, in an effort to homogenise the temperature record, in 42 percent of cases, cities were getting cooler relative to their surroundings rather than warmer. One reason is that urban areas are heterogeneous, and weather stations are often sited in "cool islands" – parks, for example – within urban areas.
Studies in 2004 and 2006 attempted to test the urban heat island theory, by comparing temperature readings taken on calm nights with those taken on windy nights. If the urban heat island theory is correct then instruments should have recorded a bigger temperature rise for calm nights than for windy ones, because wind blows excess heat away from cities and away from the measuring instruments. There was no difference between the calm and windy nights, and one study said that we show that, globally, temperatures over land have risen as much on windy nights as on calm nights, indicating that the observed overall warming is not a consequence of urban development.
A view often held by skeptics of global warming is that much of the temperature increase seen in land based thermometers could be due to an increase in urbanization and the siting of measurement stations in urban areas. For example, Ross McKitrick and Patrick J. Michaels conducted a statistical study of surface-temperature data regressed against socioeconomic indicators, and concluded that about half of the observed warming trend (for 1979–2002) could be accounted for by the residual UHI effects in the corrected temperature data set they studied—which had already been processed to remove the (modeled) UHI contribution. Critics of this paper, including Gavin A. Schmidt, have said the results can be explained away as an artifact of spatial autocorrelation. McKittrick and Nicolas Nierenberg stated further that "the evidence for contamination of climatic data is robust across numerous data sets."
The preliminary results of an independent assessment carried out by the Berkeley Earth Surface Temperature group, and made available to the public in October 2011, found that among other scientific concerns raised by skeptics, the urban heat island effect did not bias the results obtained by NOAA, the Hadley Centre and NASA's GISS. The Berkeley Earth group also confirmed that over the past 50 years the land surface warmed by 0.911°C, and their results closely matched those obtained from earlier studies.
Climate Change 2007, the Fourth Assessment Report from the IPCC states the following.
Studies that have looked at hemispheric and global scales conclude that any urban-related trend is an order of magnitude smaller than decadal and longer time-scale trends evident in the series (e.g., Jones et al., 1990; Peterson et al., 1999). This result could partly be attributed to the omission from the gridded data set of a small number of sites (<1%) with clear urban-related warming trends. In a worldwide set of about 270 stations, Parker (2004, 2006) noted that warming trends in night minimum temperatures over the period 1950 to 2000 were not enhanced on calm nights, which would be the time most likely to be affected by urban warming. Thus, the global land warming trend discussed is very unlikely to be influenced significantly by increasing urbanisation (Parker, 2006). ... Accordingly, this assessment adds the same level of urban warming uncertainty as in the TAR: 0.006°C per decade since 1900 for land, and 0.002°C per decade since 1900 for blended land with ocean, as ocean UHI is zero.
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- Lawrence Berkeley National Laboratory Heat Island Group
- The Surface Temperature Record and the Urban Heat Island From RealClimate.org
- Urban Heat Island research group - NSF project, Department of Geography, Indiana State University
- UrbanHeatIslands.com - Urban Heat islands in Canada and the world
- Cool Roof and Pavement Toolkit
- Global Cool Cities Alliance