An urban heat island case study for geography a level

1. In urban areas cloud cover is changed and influenced by human activity. This causes
there to be more cloud cover over urban areas, and they receive thicker and up to
ten per cent more frequent cloud cover than that compared to rural areas. The
reason for this is because the higher temperatures of the urban microclimates
generate convection currents and so there is a larger number of condensation nuclei
which will therefore form more clouds.
The hydroscopic nuclei are also larger and this is all a result from the greater
amounts of pollution in the urban area, with more dust from cars fuel, industry and
quarrying which contribute to the hydroscopic nuclei making them larger. Cloud
cover may also often be the result of smog, a mixture of fog and smoke, as this
causes smoke, which will appear as low lying clouds. This only happens in these
particular humid conditions. An example of this is in London, which has a very dense
population in the town centre with lots of pollution and smoke from cars and
industries surrounding the town centre. These clouds, which are formed, will cause
more heat to be trapped so the temperatures will be raised even more, particularly
during the day and at night they prevent solar radiation from entering the city so in
extreme winters the urban area may be slightly colder. An example of an urban area
that has a greater cloud cover than surrounding areas is Chicago in the USA, as on
satellite imagery maps the area over Chicago is darker than surrounding areas, which
shows more cloud cover. The increase or decrease in amount of cloud cover can
directly impact the precipitation levels in urban areas.
Humidity is another part of the climate, which is affected by the temperature of the
air. Relative humidity can be up to 6% lower in an urban area as the air is warmer
meaning it can hold more moisture, however the lack of vegetation and surfaces
limit evapotranspiration. The humidity is also affected seasonally with the humidity
being about 2% lower in the winter as the air is generally cooler so the air can hold
less moisture and in the summer it can be up to 10% lower as the air is much
warmer, which is increased by the land surfaces within the urban area that absorb
heat due to the dark colour of the buildings and road surfaces. For example
in London the temperature in the city centre is warmer than the outer edge. This
means that the air in the city centre has the ability to hold more moisture; however
the lack of open water in the centre of London means there is little
evapotranspiration so the humidity is lower.