Environmental Engineering Reference
In-Depth Information
Figure 8.9
Two-dimensional schematic diagram of the heat
balances of urban and rural surfaces (a) by day and (b) by
night. Q
F
is artifical heat release and Q
Ph
is energy use in
photosynthesis The other symbols are as in Figure 8.5.
the potential for loss of long-wave radiation; much of the emission from the ground is
absorbed by them and reemitted 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. 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 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. Under
