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, Spain
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 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. 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 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.
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 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.
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
- 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
- Anthony Nicholl Rail; Urban thermal
plumes, their possible impact on climate change; Sudbury, Suffolk;
July 2007.
- 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
- Non-technical summary of M&M 2007 by
McKitrick
- 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
- preprint, submitted to International Journal of
Climatology, 2009
Further reading
External links