Environmental Engineering Reference
In-Depth Information
URBAN CLIMATES
The climate modifications found in woodland are small compared with what happens
when cities are built. Instead of a mixture of soil and vegetation, Earth is covered with a
mosaic of concrete, glass, brick, bitumen and stone surfaces reaching to heights of several
hundred metres. Amongst this, grass and water surfaces and trees may be scattered to
variegate the 'concrete jungle'. The building materials have vastly different physical
properties from soil and plants. For example, the warmth of concrete and brick on a
summer's evening is due to their high heat capacity. This means that the large quantities
of heat added to the material while the sun is shining are slowly released during the night,
adding warmth to the urban atmosphere. In this way city temperatures are kept relatively
high. We notice the effect most in the evening when we travel from the cool of the
countryside to the heat of the city (Figure 8.8). It is an effect called the
urban heat island
.
Early blooming of flowers and decreased snowfall and frost are both indicators of this
effect.
THE URBAN HEAT ISLAND
We can illustrate the different responses of the city and rural areas by comparing their
heat budgets as shown in Figure 8.9. It is the change of the heat budget by the urban
surface which helps to produce the distinctive urban climate, so let us look in more detail
at the way changes are produced. By day, both rural and urban surfaces experience a
radiation surplus. Smoky urban atmospheres may reduce the size of this surplus slightly,
but as 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 allow a certain
amount of radiation to pass through them to 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° the
heat input will reach its maximum. This effect is likely to occur much more frequently in
an urban area, with its vertical walls, than in a rural area. Reflection from light-coloured
buildings and glass can also add to the heat input of the urban canyon.
