Environmental Engineering Reference
In-Depth Information
conditions of light winds and clear skies, we find the greatest temperature differences
between urban and rural areas (Figure 8.10). 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.
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 HEAT ISLAND MODELLING
new developments
It has long been known that cities generate their own distinctive climates as a result of the
nature of their fabric. Concrete, brick, asphalt and glass respond differently to radiative
exchanges than the vegetative surfaces of the rural surrounds. Studies have proliferated,
demonstrating how much warmer cities can be under ideal conditions of clear skies and
light winds when the radiative properties become the dominant control of temperatures. It
has been shown that the maximum urban heat island intensity is closely related to
building density and how much sky is visible in the city centre (the sky view factor).
For a better understanding of what determines the urban heat island, we need to know
in a quantitative manner just how the nature of the surface reacts with radiative and
energy exchanges to bring about this state of higher temperature. On occasions the city
may be cooler than its surrounds, so what may cause this? To help determine the relative
influence of factors we need to model the urban system in a physically realistic manner.
In this way we can find the relative significance of the factors which give rise to the heat
island and how their importance may vary during the year or from one city to another.
One relatively simple approach that has been taken is to simulate the radiative exchanges,
heat conduction into and out of the urban surfaces, and the thermal status of the surfaces
themselves. To reduce complexity, it is assumed there is no air movement and that only
long-wave exchanges need to be considered for a night-time simulation. Artificial heat
generation can be allowed for by the use of specified building temperature. As models get
more sophisticated it should be possible to incorporate daytime energy exchanges and the
effects of advection across the city. Even with this simple model we need to have a lot of
initial information about the state of the city environment. The initial temperatures of the
surface, soil and internal building temperatures at sunset are needed, together with some
physical properties of the building materials such as their thermal admittance and
emissivity and the sky view factor of the buildings where the horizon is obstructed. The
model has been found to give realistic results under these relatively ideal conditions,
though further work is required to incorporate such features as advection, a better
representation of surface characteristics and the complications of evapotranspiration. The
model confirmed that the effects of street geometry on long-wave radiation exchanges
and the difference in thermal admittance between rural and urban conditions are together
capable of producing urban heat islands of the magnitude we experience.
Other urban models have been developed to predict the nature of air flow across a city,
where air pollution can be a problem. In this case, the surface energy budget of the city is
less important than its aerodynamic responses to air movement or rugosity. Unfortunately
our knowledge of these aspects of the city is very limited and urban modellers still need
more information about the nature of the wind speed profile above cities.
