Cloud cover and rainfall in 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