Geoscience Reference
In-Depth Information
street level these can become quite unpleasant, raising
dust, perhaps even rubbish, and making walking difficult.
Quite a few shopping precincts were unpopular with
shoppers until architects realized that such winds could
be a problem and took measures to minimize their effects.
spread sources such as road transport and domestic
heating. Initially invisible in a gaseous form, chemical
reactions can take place between these pollutant gases and
the sun's rays to produce a chemical mix known as
photochemical smog
.
In some cities, skies are rarely blue
but instead have a predominantly whitish hue as a result
of scattering by the pollutants. Large subtropical cities
such as São Paulo, Mexico, Los Angeles and Bangkok are
renowned for their poor air quality. This can have a slight
effect on the urban climate through its influence on the
radiation budget, though its effects are less than the other
factors mentioned. See additional case study 'Urban areas
and photochemical smog' on the suport website at
www.
Effects of pollution
Urban areas may also differ from their rural surroundings
in terms of air quality. Dense fogs associated with coal
burning are largely a feature of the nineteenth century, but
air quality can be poor where pollutants are trapped
within the urban boundary layer. Although industry can
be locally important, much pollution comes from wide-
Urban heat island modelling
NEW DEVELOPMENTS
It has long been known that cities generate their own distinct 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 occasion the city may be cooler than its surroundings, 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
surface-atmosphere energy exchanges at a range of scales including the effects of advection across the city. Even
with this simple model we need to have a lot of 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.
