Cloud cover and rainfall in urban areas
Name ………………………………………..Date………
General
Rain is formed when water vapour in the air condenses to form water droplets that fall to earth.
Rain is more likely to form in urban areas than in the surrounding countryside because 
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The pollution from cars and factories produces a large number of minute particulates which act as
hygroscopic nuclei which attract water vapour to condense on them and produce water droplets 
cloudsrain. This means that more rain is produced.
The tall buildings found in the centre of towns causes the air to rise above it. As the air rises it cools
and the water vapour condenses to form droplets, forming clouds and rain. This means that there is
more rain in urban areas.
In the summer the urban heat island effect creates very high temperatures over the city. The hot air
rises and as it rises it cools, condenses and forms clouds and rain. The very high temperatures
cause the air to rise quickly, producing very large cumulonimbus clouds which produce very heavy
rain. This means that urban areas can have more intense rain storms than the surrounding rural
areas.
When a band of rain associated with the front in a depression passes overhead it slows down over
a town because of frictional drag. This means that more rain is deposited in the urban areas.
Urban areas have
 5%-10% more cloud cover – thicker and more frequent
 5%-30% more rainfall
 10%-40% more thunderstorms
 70%-400% more hailstones
The increase in cloud cover and rainfall is linked to 2 main factors
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Pollution provides minute particulates. These particulates act as condensation (hygroscopic) nuclei,
which provides a surface for water vapour to condense on and form water droplets.
The urban heat island. The build up of heat over the city causes instability in the lower layers and
causes air to rise by convection. As the air rises it cools and water vapour condenses to form clouds
Other Factors
• As a depression passes overhead it is slowed by surface friction caused by the irregular urban
surface. This means the fonts take longer to clear the city and deposit more rain
• The tall buildings can cause uplift of air creating orographic rainfall/clouds
• Industrial processes and traffic create water vapour
• The storage of heat in bricks etc. causes instability in the lower layers and makes rapid convective
ascent likely- summer thunderstorms
Summer Thunderstorms
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UHI creates a localised are of low pressure in the centre
Air is drawn in from surrounding areas
As the air approaches the urban area it warms up
The warm air is subject to convectional uplift
The uplift accelerates as the rate of cooling of the rising air is less than the surrounding air making
the temperature difference greater
Large cumulonimbus clouds develop storms
The presence of hygroscopic nuclei from pollution aids the formation of water droplets  clouds rain
Effects
• More rain and heavy thunderstorms in summer causes flash floods
• Traffic problems …
• Unpleasant
• Fewer hours of sunshine  SAD; impact on vegetation
• Less UV reaches surface  less vitamin D produced in people-> rickets/less skin cancer