Many natural factors control the climate in urban areas, for example, the latitude, whether the city is in a mountain region or on a flat plain, whether it's close to the sea and what the surrounding land is used for. As a city grows, new factors (e.g. heat from human activities and air pollution) modify the local climate and contribute to the formation of distinct urban climates.
Large numbers of people and heat from human activities, along with the fact that cities are built mainly of concrete, asphalt, bricks and stones, makes the temperature in a city higher than in a non-urban area. High density building alters the wind speed and its direction and a local air circulation, called the urban breeze, developes.
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Urban climates are the result of the interaction of many natural and anthropogenic factors. Air pollution, building materials, emission of heat from human activity, together with natural factors, cause climatic
differences between cities and non-urban areas.
The climate of a particular city is controlled by many natural factors, both at the macro-scale (e.g. the latitude ) and at the meso-scale (e.g. the topography , the presence of water bodies ). As a city grows and develops, new factors modify the local climate of a city and contribute to the formation of distinct urban climates.
Note: Colours used in the text correspond to the colours used in the figure below!
1. Factors controlling urban climate. Author: Sebastian Wypych.
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Large numbers of buildings and streets mean that a large proportion of the ground surface in cities is covered by impermeable materials such as concrete and asphalt. The original, natural land cover may be preserved in lawns or parks, but usually occupies only a small part of the city area. The city surface often, therefore, has a very complex character consisting of a mosaic of different surface materials.
Each surface material has a different
albedo, a measure of the amount of solar radiation reflected back into space or absorbed by the surface. For a city as a whole, the albedo can be as low as
10-15% (the albedo for fresh snow is greater than 80%) which means that a lot of the incoming solar energy is absorbed by the city. Additionally, most of the building materials used in the construction of cities are characterised by a high heat capacity and high heat conductivity.
2. The albedo in the urban environment.
Source: U.S. Environmental Protection Agency, http://yosemite.epa.gov/oar/globalwarming.nsf/ content/ActionsLocalHeatIslandEffect.html
In addition, the shape of a city tends to trap radiation near the surface.
This means that a lot of energy is stored in the city during the day-time and this is then gradually lost during the night. This slows down the night-time cooling of a city compared to non-urban areas.
3. Sky view factor. The sky view factor (SVF) is reduced by the urban built-up. The maximum value of the SVF is 1 which occurs for open areas, without any trees, houses, etc. Author: Sebastian Wypych (after Oke,
1987).
Another important factor modifying urban climate is air pollution . This changes the composition of the urban atmosphere and, as a result, reduces the amount of solar radiation reaching the ground surface. In other words, pollutants make the air less transparent to sunlight. Urban air pollution consists of gases and particles emitted by industry, vehicles, heating systems etc. The city centre is usually more polluted than the suburbs but this depends on where industry and busy roads are located. During the day-time, the highest air pollution concentrations tend to be seen during rush hours. Over the year, highest concentrations are generally seen in winter because of the increased combustion of fuels for heating and because atmospheric conditions are such that polluted air is less likely to mix with clean air and dilute the pollutant concentration. The exception to this is photochemical smog which needs sunlight to form and so is seen in the summer (find out more about this in the sections on ozone smog and negative effects)
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4. Daily changes in air pollution on a typical sunny summer day.
Cracow, Poland, 22 Aug.,
2003.
In the summer, vehicles are the main source of air pollution. In the morning, intensive traffic causes high emissions of nitrogen oxides and carbon monoxide (Fig. 4a and 4b). Around noon and in the afternoon, as the temperature rises and wind speed is low
(Fig. 4c), chemical reactions in the presence of sunlight cause a decrease in nitrogen oxides levels and an increase of tropospheric ozone (Fig.
4a).
These measurements were made in the middle of a busy two-way street
(Krasinskiego Avenue).
The air pollutant concentrations were measured at 4 m above the surface, wind speed was measured 10 m above the surface and air temperature was measured 6 m above the surface. Authors: Anita
Bokwa, Sebastian
Wypych. Source of data: Voivodship
Inspectorate of
Environmental Protection in Cracow.
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5. Daily changes in air pollution on a typical winter day. Cracow,
Poland, 26-27 Dec.,
2002.
In the winter, emissions from energy production are the main source of air pollution.
As the air temperatures are significantly below zero (Fig. 4c) heating is essential but the result of all this energy production is high atmospheric concentrations of particulate matter (so called PM10, i.e. particles with a diameter of 10 micrometers or less), carbon monoxide and sulphur dioxide (Fig. 4a and 4b). As the wind speed is low and the air temperatures below zero, layers form in the atmosphere which trap the air pollutants close to the surface. This means that pollutants are not lost from the city and concentrations remain high. The measurements of air pollutants were made in the middle of the city's central square
(The Main Market
Square), 12 m above the surface level (the instruments are installed at the wall of the Town
Hall Tower). The wind speed was measured 10 m above the surface, and the air temperature was measured 6 m above the surface. Authors: Anita
Bokwa, Sebstian
Wypych. Source of data:
Voivodship Inspectorate of Environmental
Protection in Cracow.
6. Domestic heating is one source of anthropogenic heat.
Another important factor controlling urban climate is anthropogenic heat . This is heat released as a by-product of heating systems in winter
(and air conditioning in summer), or from other activities (combustion of fossil fuels, industrial production and from vehicles). The amount of heat emitted depends upon the energy use by individuals, the population density, the amount of industry
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Source: www.freefoto.com and the city's location.
In cities the amount of water lost by evaporation is low because artificial surfaces do not absorb water in the same way as natural surfaces do. When it rains, water quickly runs off into urban sewer systems and buildings and roads dry out rapidly. This means that excess heat is not used to evaporate water (as there is little lying on the ground) but rather warms the air. The presence of large amounts of vegetation in many cities does, to some extent, counteract this effect.
The impact humans on urban climate depends on the city's size and its spatial structure , on the number of inhabitants, and on the concentration of industry. Small towns with relatively low buildings spread among green areas and without any factories or industrial plants, tend to modify the climate less than cities with tall buildings.
How much influence anthrogogenic factors have on local climate depends on the natural setting of a city. For example, a city located in a deep valley may experience frequent fogs and gentle winds. This means that any air pollutants are trapped at the surface and air quality is generally poor.
The urban climate can be improved by planning the urban structure in such a way as to decrease the negative impact of both anthropogenic and natural factors. For example, through the strategic location of parks and water bodies (e.g. ponds and lakes) and by building factories downwind of the city so that air pollution is taken away by the wind and not brought into the urban environment.
7. Fog reduces air quality in a city. It can react with air pollutants to form acid fogs. Source of image: www.freefoto.com
A city is built mainly of concrete, asphalt, bricks and stones. As the air temperature in a certain place depends, to a large extent, on its surface characteristics, the temperature in a city is higher than in a non-urban area. Large numbers of people and emission of heat further enhances
this effect.
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In many cities the air temperature is, on average, 0.5 to 0.8 o C higher than the surrounding non-urban areas. In winter the average temperature difference is even greater, between 1.1 and 1.6 °C. This phenomenon is called the urban heat island (UHI). Lines on a map connecting points of the same temperature (isotherms) generally show a circular pattern with the temperature decreasing towards the city suburbs.
1. Heat Island map of the New York. Source:
System for World Surveillance, Inc.
The number of inhabitants is a major factor controlling the development of the UHI. In cities with populations between 500,000 and 1,000,000 people, air temperatures are usually 1.1 to 1.2
°C higher than surrounding nonurban areas. For cities with more than a million people, the difference between urban and nonurban average temperatures increases to between 1.2 and 1.5 °C. Maximum observed differences can be much higher and an example is shown in Figure 2.
2. Dependence of the maximum intensity of the UHI on the number of inhabitants of a city. Authors: Anita
Bokwa, Pawel Jezioro.
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3. The Urban Heat-Island Profile. Source: Heat Island Group.
The size and spatial structure of a city also govern the extent of the UHI.
Urban areas with low rise buildings spread among green areas do not form typical urban heat islands. The UHI phenomenon is also closely related to the factors already discussed in the chapter "Factors controlling...", i.e. anthropogenic heat emission, air pollution and changes in the natural surface coverage. All these contribute towards the temperature rise in urban areas.
The intensity of the UHI (i.e. the temperature difference between the city and the surrounding non-urban areas) also depends on meteorological factors like the wind speed, how cloudy it is and how much evapotranspiration occurs.
Increases in wind speed and cloudiness may weaken the intensity of the UHI.
The intensity of the UHI changes on both a daily and on a yearly cycle. In winter, it may be twice as large as in the summer due to emissions of anthropogenic heat caused by heating the buildings. The intensity of the UHI is also higher in the night than in the day as intensive radiation from the surface into the atmosphere takes place during the night. In some cities, for example, Tokyo, the intensity of the UHI decreases during weekends and holidays.
4. Emission of anthropogenic heat - cars. Photo:
Sebastian Wypych.
Apart from its horizontal range, the
UHI also has some vertical structure.
It usually reaches up to between 200 and 300 m into the air, about 3 to 5 times the height of the buildings. In a cloudless sky it may reach up to 500 m into the atmosphere. Two distinct layer are seen:
5. Emission of anthropogenic heat - cooling towers
Source: www.freefoto.com
1. the urban canopy layer occurs nearest the ground and results from heat emitted by low level emitters such as house chimneys, from the buildings themselves (as they absorb lots of solar radiation and emit it back as heat) and also from vehicles.
2. the chimney layer occurs above the urban canopy layer. Here the heat is emitted into the air from the high level emittors, e.g. the chimneys of power plants.
The UHI causes changes in the urban climate compared to non-urban areas.
There are more hot days, less days with ground frost, the growing season is longer, the amounts of rain are higher and cumulus clouds are noted more often.
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The presence of an UHI has a negative impact on humans, particularly in summer, as it can cause overheating. To counteract this, the number of parks and lakes should be increased in our cities.
The presence of tall densely packed buildings in cities changes the prevailing wind speed and direction. They also allow local air circulation patterns, for example the urban breeze, to be set up. Wind may improve the air quality in a city by clearing the air of pollutants but
it also may cause too much heat to be lost from the buildings.
Air circulation in a city is controlled by natural and anthropogenic factors, for example, the air temperature, the roughness of the surface and the presence of various barriers (hills, forests, high buildings).
Urban areas warm up much faster during the day-time than non-urban area.
This results in an atmospheric pressure difference between the two areas, low pressure over the city and higher pressure over the surrounding countryside.
This pressure difference generates winds which blow into the centre of the city.
These local winds occur over much shorter distances than those caused by atmospheric circulation patterns and air pressure differences over continents.
When the wind in the city drops, the urban breeze can develop. As the air in the city warms up, it becomes less dense and it rises. As it rises, it spreads out and cools. As it cools, it becomes heavier and it sinks down over the suburbs.
It then returns to the city and this cycle is known as the urban breeze.
1. The Urban breeze. Author: Mateusz Kaminski.
Wind reaching the city changes direction. Streets with high buildings on each side of the road create tunnels for the wind to travel through and buildings perpendicular to the original wind direction change both the direction and the speed of the wind. Main roads leading into the city act as the main corridors by which the wind enters the city. In wide streets, the wind simply follows the direction of the street. In narrow streets the wind speed is significantly increased at street corners and local eddies are generated at squares and street junctions where different air currents meet.
Buildings act as barriers to the wind and, in the centre of a city, average wind speeds are about 20% lower than in the suburbs. Weak winds (wind speeds less than 3 m s -1 countryside.
) are seen more often in cities than in the surrounding
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When the wind blows perpendicular to a row of buildings, the windward side is exposed to strong gusts of wind, while leeward side is in a so-called aerodynamic shadow. The strong gusts of wind often mean too much air gets blown into the building and this can have a negative impact on health and the comfort of the inhabitants. Local eddies develop on the leeward side of rows of flats and the size of the eddy increases with the height of the building.
Decreasing the distances between the blocks of flats lowers the wind speed by up to 50%.
When the wind hits a high building, the air stream divides. A part of it moves upwards and the rest goes around the building. This causes an increase in the wind speed by upto 30% at the corners of the building. Lower buildings in the same area often suffer as a result of this modification in the wind direction. Air streams generated by the high buildings may, for example, cause the low buildings to vibrate.
2. Idealised air stream around a building; Animation: Mateusz
Kaminski
3. Idealized flow in the vicinity of buildings. The circling air produces eddies (which are like whirlpools in water). Author: Mateusz Kaminski.
Wind speeds greater than 3 m s -1 generally have a positive impact on the air quality in a city by improving the ventilation and increasing evaporation.
However, these winds also disperse air pollutants to other regions and increase heat loss from buildings in winter.
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