Download SODAR MSc Thesis - Roger Carter

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temperatures have diurnal and seasonal effects on the thickness and profile of the boundary
layer.
The air quality in heat islands is often poor. This is due to the increased amount of pollution
emitted in an urban environment, coupled with the aforementioned occurrence of back eddies
in the urban canopy layer, which effectively ‘trap’ air within a city. This recirculation of
trapped air leads to the build-up of pollutants and a reduced air quality.
If the isotherms around a city are plotted on a surface weather map, then the resulting effect
would resemble the topographic contours of an island12, giving rise to the term ‘heat island’.
This effect is highlighted in an example from St. Louis in Figure 6.
Figure 6 - Surface map of isotherms over St. Louis metropolitan area11
This microclimatic state is formed due to three main factors. The climatic zone, the
surrounding topographic features and the specific layout of the urban features present within
a city. The urban features would take into account the size and height of buildings, along with
their positioning and thermal composition. The expansion of urban areas has altered the urban
climate and ultimately reduced the quality of the urban environment. In general an urban
climate is warmer and less windy than a rural climate, thus leading to stagnation and a lack of
recirculation. There can be increased discomfort due to a poorly designed urban environment,
including high temperatures, wind tunnelling effects and buffeting turbulence.
Renewable Energy Systems and the Environment MSc Thesis
Roger Carter