Environmental Engineering Reference
In-Depth Information
Figure 8.8
Temperature cross-section of the urban heat
island of Chester in relation to built-up area.
Source: Nelder (1985).
Of the energy which is available as net radiation, some is used to heat the air, some is
used in evaporation and the remainder is absorbed by the soil or buildings and other
artificial surfaces. This is where the main contrasts arise. In a city, sewers and drainage
systems lead to the rapid removal of water, and actively growing vegetation is infrequent.
Surfaces soon become dry once rain has stopped, so the use of energy for evaporation and
transpiration is small. This means that more is available for heating the air and the
buildings than is being used for evaporation, which is 'non-productive' in terms of
heating. A final factor can be significant in the city. Large amounts of fuel are used in
industrial processes, to heat or cool buildings depending on the time of year and for
transport. Even human activity generates appreciable amounts of heat where population
density is high, and all this heat is eventually released into the urban atmosphere (
Q
F
in
Figure 8.9). On Manhattan Island, New York, research has shown that, during the
average January, the amount of heat produced from combustion alone is greater than the
amount of energy from the sun by a factor of 2·5. In summer that ratio is only about 0·15.
At night the ground surface loses energy, resulting in cooling. In rural areas the
ground becomes cooler than the air above, giving an inversion of temperature. There is
then a weak transfer of heat to the surface from the soil and from the atmosphere, but
these additions do not compensate for the radiational losses and so temperatures fall. In a
hot summer this may feel refreshing compared to the sultry warmth of the city. There the
buildings continue to give off heat which they have absorbed and stored during the day
(
Q
G
in Figure 8.9) and, coupled with the heat of combustion (
Q
F
in Figure 8.9), this
reduces the rate of cooling. The physical presence of the buildings also reduces
