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An urban heat island (UHI) is a metropolitan area which is significantly warmer than its surrounding rural areas. The phenomenon was first investigated and described, though not by name, by Luke Howard in the 1810s. The temperature difference usually is larger at night than during the day, and is most apparent when winds are weak. Seasonally, UHI is seen during both summer and winter. The main cause of the urban heat island is modification of the land surface by urban development which uses materials which effectively retain heat. Waste heat generated by energy usage is a secondary contributor. As population centers grow they tend to modify a greater and greater area of land and have a corresponding increase in average temperature. The lesser-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. Increases in the death rate during heat waves has been shown to increase by latitude due to the urban heat island effect. 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, which stresses 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, any impact of the urban heat island on global warming is uncertain, its impact on climate change has not been proved observationally or by any quantitative modelling, though recent qualitative speculations indicate that urban thermal plumes may contribute to variation in wind patterns that may itself influence the melting of arctic ice packs and thereby the cycle of ocean current.

Causes



There are several causes of an urban heat island (UHI). The principal reason for the nighttime warming is that buildings block surface heat from radiating into the relatively cold night sky. Two other reasons are changes in the thermal properties of surface materials and lack of evapotranspiration in urban areas. Materials commonly used in urban areas, 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. The energy balance is also affected by the lack of vegetation in urban areas, which inhibits cooling by evapotranspiration.

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. 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.

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: e.g., " downtown, in the suburbs". The color black absorbs significantly more electromagnetic radiation, and causes the surfaces of roads and highways to heat up substantially.

Diurnal behavior

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, Spainmarker is 0.2 °C cooler for daily maxima and 2.9 °C warmer for minima than a nearby rural station. A description of the very first report of the UHI by Luke Howard in the late 1810's said that the urban center of London was warmer at night than the surrounding countryside by 3.7 °F . 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.

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. 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 to 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.

Health effects

UHIs have the potential to directly influence the health and welfare of urban residents. Within the United Statesmarker 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 Chicagomarker and New Yorkmarker 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 Miamimarker, 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. 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. Rapid temperature changes can be stressful to aquatic ecosystems.

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 Angelesmarker about US$100 million per year in energy. Conversely, those that are in cold climates such as Moscow, Russiamarker would have less demand for heating.

Mitigation

The heat island effect can be counteracted slightly by using white or reflective materials to build houses, pavements, and roads, thus increasing the overall albedo of the city. 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 and contributes to the warming of cities. By paving with light colored concrete, in addition to replacing asphalt with light-colored concrete, communities can lower their average temperature. This is a long established practice in many countries.

A second option is to increase the amount of well-watered vegetation. These two options can be combined with the implementation of green roofs. 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 Angelesmarker 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.

Relation to global warming

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.

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."

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.

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. While the "heat island" warming is an important local effect, there is no evidence that it biases trends in 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 citing 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.

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, including Gavin A. Schmidt, have said the results can be explained away as an artifact of spatial autocorrelation. McKitrick & Nicolas Nierenberg have submitted a rebuttal defending their results.

Climate Change 2007, the Fourth Assessment Report from the IPCC states the following.
Studies that have looked at hemispheric and global scalesconclude that any urban-related trend is an order of magnitudesmaller than decadal and longer time-scale trends evidentin the series (e.g., Jones et al., 1990; Peterson et al., 1999).This result could partly be attributed to the omission from thegridded data set of a small number of sites (<1%) with="" clear=""></1%)>urban-related warming trends. In a worldwide set of about 270stations, Parker (2004, 2006) noted that warming trends in nightminimum temperatures over the period 1950 to 2000 were notenhanced on calm nights, which would be the time most likelyto be affected by urban warming. Thus, the global land warmingtrend discussed is very unlikely to be influenced significantly byincreasing urbanisation (Parker, 2006). ...Accordingly, this assessment adds the same level of urbanwarming uncertainty as in the TAR: 0.006°C per decade since 1900for land, and 0.002°C per decade since1900 for blended land with ocean, as ocean UHI is zero.

As the Fourth assessment hints, oceanic data is in hand from a wide variety of different data collection methods, taken by both civil and national defense groups, as well as multiple subsurface readings, in addition to lower-, middle-, upper-, and ultrahigh-atmosphere datasets.

See also



References

  1. Luke Howard, The climate of London, deduced from Meteorological observations, made at different places in the neighbourhood of the metropolis, 2 vol., London, 1818-20
  2. Anthony Nicholl Rail; Urban thermal plumes, their possible impact on climate change; Sudbury, Suffolk; July 2007.
  3. McKitrick, R.R. and P.J. Michaels (2007), Quantifying the influence of anthropogenic surface processes and inhomogeneities on gridded global climate data, J. Geophys. Res., 112, D24S09, doi:10.1029/2007JD008465, full text
  4. Non-technical summary of M&M 2007 by McKitrick
  5. Gavin A. Schmidt, 2009, "Spurious correlations between recent warming and indices of local economic activity." International Journal of Climatology, http://dx.doi.org/10.1002/joc.1831, full text
  6. preprint, submitted to International Journal of Climatology, 2009


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