Download Unit 2: Urban climate

Climate in cities
Basics
Unit 2:
Urban climate
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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Part: 1 What controls the urban climate?
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
Part 2: Heat Island
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 oC 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.
Part 3: Air circulation
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) are seen more often in cities than in the surrounding
countryside.
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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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