Questions and Answers on Urban Heat Islands

Questions and Answers on Urban Heat Islands

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  • Created on: 02-01-11 18:44
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Hugo Clay
Describe and comment on the extent of the urban heat island shown. (7 marks)
The urban heat island is shown by the beige coloured shaded on the diagram. The urban
heat island is hotter than all of its surroundings, for example the diagram shows that the
peak temperature is near the centre of the urban area at 3.5*C. A large majority of the
urban area has temperatures of over 0.0*C whereas outside the urban area temperatures
are below 0.0*C, not one part is above this.
There is an incline in temperatures from outside the urban area to the peak area which is
inside the urban area. Along the boundary of the urban areas temperatures are mainly
-1.0*C but as we move towards the hottest part at 3.5*C the temperature slowly increase.
The incline is a steep one and temperatures increase rapidly from -1.0*C to 3.5*C. This is
illustrated by the lines being very close together in grid squares A4, B4, C4 and D4. In these
areas, temperatures increase from -1.0*C to 2.0*C in less than 2km (about 1.8km). On the
other hand, the lines are quite a distance apart in the urban area showing a small range in
temperatures emphasising the effects of urban heat islands.
The reason for the changes in temperature mentioned above are all down to how urban
areas can affect temperatures. The urban heat island is a warm spot in the "sea" of
surrounding cooler rural air. It can be down to a number of reasons. Building materials
such as concrete, bricks and tarmac act like bare rock surfaces, absorbing large quantities
or heat during the day. Much of this heat is stored and slowly released at night. Heat comes
from industries, buildings and vehicles, which all burn fuel ­ these are obviously very
popular in cities and urban areas. Air pollution from industries and vehicles increases
cloud cover and creates a "pollution dome", which allows in short-wave radiation but
absorbs a large amount of the outgoing radiation as well as reflecting it back to the surface.
Explain how urban structures and layout affect winds. (8 marks)
Wind speed is reduced in cities by as much as 30 per cent because there is increased
frictional drag as the wind is obstructed by buildings. Conversely, very tall buildings can
channel wind in between narrow spaces creating a canyon effect where, locally, wind
speeds are higher. This effect, combined with turbulence, can prove hazardous to people
at street level.
There are three main types of effects which urban structures and layouts have. The
surface area of cities is uneven because of the varying height of the buildings. Buildings
exert a powerful frictional drag on air moving over and around them. This creates
turbulence, giving rapid and abrupt changes in both wind direction and speed. Average
wind speeds are lower in cities than in the surrounding areas and they are also lower in
city centre than in suburbs.
High-rise buildings may slow air movement but they also channel air into the "canyons"
between them. Winds in such places can be so powerful that they make buildings sway and
knock pedestrians off their feet.
On calm and clear nights when the urban heat island effect is at its greatest, there is a
surface inflow from the cooler areas outside the city to the warmer areas in the city centre.

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Hugo Clay
Winds are affected by the size and shape of buildings. A single building can alter an airflow
passing over it. Air is displaced upwards and around the sides of the building and is also
pushed downwards in the lee of the structure.
If two separate buildings allow airflow between them, then the movement may be subject
to the Venturi effect in which the pressure within the gap causes the wind to pick up speed
and reach high velocities.…read more

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Hugo Clay
The introduction of Bio-fuels has been very unsuccessful in reducing emissions as the
production and manufacturing of it causes more pollution that the Bio-fuels actually
reduce.…read more


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