Microclimates and local climates - Fundamentals of the Physical Environment - page 163

experience a radiation surplus. Smoky urban atmospheres
may reduce the size of this surplus slightly, but as this
aspect of the quality of urban air has improved because
of pollution controls, the differences in inputs have
become slight.
At a smaller scale the differences are more significant.
Trees and crops absorb and reflect a certain amount of
radiation, preventing it reaching the ground surface. They
transpire moisture and have a low heat capacity. As we saw
earlier, this results in cooler temperatures beneath the
canopy. In the city, the building materials of concrete,
brick and stone all have high heat capacities, enabling
them to store large amounts of heat. Shadowing can be
important but there are still numerous surfaces exposing
large, dry areas to the sun's rays. When the angle between
the receptive surface and the sun's rays approaches 90

have absorbed and stored during the day ( Q G in Figure
8.8 ) and, coupled with the heat from combustion ( Q F in
Figure 8.8 ), this reduces the rate of cooling. The physical
presence of the buildings also reduces the potential for loss
of long-wave radiation; much of the emission from the
ground is absorbed by them and re-emitted to the ground.
This is known as the sky view factor and can be significant
in reducing cooling from the urban core. The relative
warmth in the city prevents the development of an
inversion, so heat transfer and evaporation still take place.
Dewfall or condensation is much less frequent than in
rural areas. If we view the spatial extent of the area of
warmth it appears like an island with a sharp 'cliff edge'
at the boundary of the urban area, a plateau of warmth
through suburbia, then a peak of heat in the area of
greatest building density or with the lowest sky view factor
( Figure 8.9 ). It is this urban heat, especially in the tropics
and subtropics, which many city dwellers find so
uncomfortable in the summer; it is why they long for the
coolness of the countryside; and why, irritated by the
conditions, they may tend to react violently.

the
heat input will reach its maximum. This effect is likely to
occur much more frequently in an urban area, with its
vertical walls and sloping roofs, than in a rural area.
Reflection from light-coloured buildings and glass can
also add to the heat input of the urban canyon.
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 8.8 ). 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 more energy than it
receives, 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 with the sultry warmth of the city.
There the buildings continue to give off heat which they

Effect of winds

If winds were strong, all this surplus heat would be rapidly
removed from the city, to be mixed with the cooler air
around, and the urban climate would be less distinct. It
is under conditions of light winds and clear skies that we
find the greatest temperature differences between urban
and rural areas. The pattern of night-time minimum
temperatures usually shows highest values near the high-
rise city centre, fairly uniform levels in the low-density
suburbs and then a sharp boundary into the cooler rural
areas ( Figure 8.9 ). This is seen most clearly in cities, where
light winds and clear skies predominate and where relief
features are few. Valleys, hills and parkland within the
urban area can produce major changes. The parkland,
especially if irrigated, has different heat capacities, albedos,
moisture levels and emission temperatures from the
surrounding buildings, giving slightly lower day and
night-time temperatures. The advantages of these 'urban
lungs' extend well beyond their aesthetic appeal, especially
during hot summer weather.
Even when winds are not light, the presence of the
urban structure tends to slow down air movement. Wind
records from city-centre sites show lower average speeds
than suburban or rural locations near by, although the
degree of gustiness may be higher, especially in summer.
As the air flows over the very irregular surface of a city,
friction with the buildings retards the wind in the lowest
layers ( Figure 8.10 ). The presence of skyscrapers, however,
produces eddies which can cause strong local winds. At