Urban Heat Island Causes Of UHI Maximum UHI Effect At Night

There is no controversy about cities generally tending to be warmer than their surroundings. What is controversial about these heat islands is whether, and if so how much, this additional warmth affects the (global) temperature record. The current state of the science is that the effect on the global temperature record is small to negligible - see below.

Scientists compiling the historical temperature record are aware of the UHI effect, but they vary as to how significant they think it is. Some scientists (see Peterson, below) have published peer reviewed papers indicating that the effect of the UHI has been overestimated, and that it does not affect the record at all. Other scientists have used various methods to compensate for it. Some advocates charge that temperature data from heat islands has been mistakenly used as evidence for the global warming theory.

1 Causes of UHI

There are several causes of UHI. The principle reason for the night-time warming is (comparatively warm) buildings blocking the view to the (radiatively cold) night sky. Replacement of high- albedo natural ground with low-albedo asphalt and buildings also has an effect, though natural forest has a low albedo already. Also the cooling effect of evaporation is absent from built-over areas.

Different climatic regions may have very different experiences of UHI's. In an already warm area they will be unwelcome; in a cold area they might be beneficial.

The lesser-used term heat island refers to any area, populated or not, which is consistently hotter than the surrounding area.

Some cities exhibit a heat island effect, largest at night (see below), and particularly in summer [2], or perhaps in winter [3], with 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., "68 degrees downtown, 64 in the suburbs".

One consequence of urban heat islands is the increased energy required for air conditioning and refrigeration, but only for cities that are already in comparatively hot climates. Those that are in cold climates would presumably need somewhat less in the way of heating. The Heat Island Group estimates that the heat island effect costs Los Angeles about $100 million per year in energy.

The heat island effect can be counteracted slightly by using white or reflective materials to build houses, pavements, and roads. This is a long established practice in many countries. Another option is to increase the amount of well watered vegetation.

2 Maximum UHI effect at night

The IPCC states that "it is well-known that compared to non-urban areas urban heat islands raise night-time temperatures more than daytime temperatures." [4]. For example, Moreno-Garcia (Int. J. Climatology, 1994) found that Barcelona was 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 in 1820 says:,

The explanation for the night-time maximum is that the principal cause of UHI is blocking of "sky view" during cooling: surfaces lose heat at night principally by radiation to the (comparitavely cold) sky, and this is blocked by the building in an urban area. Radiative cooling is more dominant when wind speed is low and the air is cloudless, and indeed the UHI is found to be largest at night in these conditions [6] [7].

3 Relation to Global Warming

Because some parts of some cities may be several degrees hotter than their surroundings, a difference double or triple the warming observed over the historical temperature record, there is a risk that the effects of urban sprawl might be misinterpreted as an increase in global temperature. However, the fact that the UHI is so large is, paradoxically, evidence that it largely absent from the record, otherwise warming would be shown as much larger in the record. The 'heat island' warming does unquestionably affect cities and the people who live in them, but it is not at all clear that it biases the historical temperature record: for example, urban and rural trends are very similar.

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. [8]

Note that not all cities show a warming relative to their rural surroundings. For example, Hansen et al. (JGR, 2001) adjusted trends in urban stations around the world to match rural stations in their regions, in an effort to homogenise the temperature record. Of these adjustments, 42% warmed the urban trends: which is to say that in 42% of cases, the 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 Intergovernmental Panel on Climate Change, which has issued several influential reports on climate trends, says that the effects of urban heat islands on the recorded temperature "do not exceed about 0.05°C over the period 1900 to 1990". This rests on various sources, contributing reasons being [9]:

land, sea and borehole records are in reasonable agreement over the last century. [10]
the trends in urban stations for 1951 to 1989 (0.10°C/decade) are not greatly more than those for all land stations (0.09°C/decade)
simlarly the rural trend is 0.70°C/century from 1880 to 1998, which is actually larger than the full station trend (0.65°C/century) (Peterson et al., GRL, 1999)
the differences in trend between rural and all stations are also virtually unaffected by elimination of areas of largest temperature change, like Siberia, because such areas are well represented in both sets of stations.
Over the Northern Hemisphere land areas where urban heat islands are most apparent 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). [11]

A 2003 paper ("Assessment of urban versus rural in situ surface temperatures in the contiguous United States: No difference found"; J climate; Peterson; 2003) indicates that the effects of the urban heat island may have been overstated, finding that "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, and thus cooler, than urban areas). As the paper says, 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 micro- and local-scale impacts dominate the meso-scale impact of the urban heat island: many sections of towns may be warmer than rural sites, but meteoroloical observations are likely to be made in park "cool islands".

A study by David Parker in Nature in November 2004 attempts to test the urban heat island theory, by comparing tempature 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 the author says: 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 [12] [13]

Another view, often held by global warming skeptics, is that the heat island effect accounts for much or nearly all warming recorded by land-based thermometers. However, these views are only aired in the "popular literature" and there are no known scientific peer-reviewed papers holding this view.

4 External links

Urban heat island features of southeast Australian towns. - Australian Meteorological Magazine 50: 1-13.
Learning about Heat Islands - from the Heat Island Group
Land-surface air temperature - from the IPCC
Urban Heat Islands and Climate Change - from the University of Melbourne, Australia

Climate change

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