ZIMSEC O Level Geography Notes: Urban Heat Islands.
- Due to a number of reasons the climate and weather of urban areas is different from the adjacent rural areas.
- Although due to the presence of tall buildings light does not reach some areas especially alleyways and narrow streets,
- Normal buildings absorb heat during the day.
- Dark-coloured roofs, concrete blocks and bricks all have a high thermal capacity which means they are capable of absorbing heat energy during the day and releasing it slowly at night.
- Additional heat is yielded by car fumes, factories, power stations and the high population concert rations of people.
- All things being equal urban build up areas experience higher temperatures which is highest in the CBD and decreases gradually as one goes towards the edges.
- Daylight temperatures are on average higher than surrounding areas generally by about 0.6°C.
- Night temperatures are higher than surrounding areas due to the smoke and dust clouds over the city/town creating a blanket/insular effect.
- The mean winter temperature is also significantly higher as well as the summer temperature which might be as much as 5°C higher.
- Even though their temparature is higher cities and towns receive less sunshine and more cloud cover than their adjacent rural areas.
- Dust and other particles over the build up area absorb and reflect much of the sunlight preventing it from reaching the city/town below.
- Wind velocity is reduced by the buildings which create friction as well as act as windbreaks.
- Urban wind velocities are typically lower than that of surrounding rural areas.
- Sometimes however high rise building create wind tunnels which have streams of very high velocity winds which trouble pedestrians and can sometimes knock them over.
- There is reduced wind chill factor.
- Small scale turbulence and eddies can occur as a result in temperature differences with adjacent rural areas.
- Relative humidity is lower in urban areas where the warmer air can hold more moisture and there is lack of vegetation reducing the amount of evapotranspiration.
- Urban areas appear to receive thicker and up to 10% more frequent clouds.
- Towns and cities receive more rains and thunderstorms.
- This is a result of thermal currents within urban areas.
- Also greater chances of rain, snow and sleet in urban areas.
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