Urban Climate Flashcards
Urban Areas are warmer than the surrounding rural areas:
1) The phenomenon of urban areas being warmer than rural areas is called the Urban Heat Island effect.
2) Urban areas with higher temperatures than the surrounding rural areas are called urban heat islands. For example, London has a clearly defined UHI.
3) The highest temperatures can be found in industrial areas and in the mostly densely built-up areas.
What are the 4 main causes of the UHI Effect?
Absorption of heat by urban surfaces:
> Concrete, brick and tarmac surfaces absorb and store heat from the sun during the day. They slowly release the heat as long-wave radiation.
Air Pollution:
> Air pollution from cars and factories increases cloud cover over the city. It also creates a ‘pollution dome’- a layer of pollution over the city.
The heat from human activity:
> Cars, factories, offices, central heating, air conditioning units and people themselves release heat.
Less evapotranspiration:
> When it rains the water is quickly removed by drainage systems, so there’s little surface water to evaporate. Also, there isn’t much vegetation, so there’s little transpiration. Evapotranspiration uses heat energy, so less evapotranspiration means higher temperatures.
How does the urban heat island effect vary seasonally and between day and night?
1) The urban heat island effect is stronger at night. Urban daytime temperatures are on average 0.6 degrees warmer than surrounding rural areas, but nighttime temperatures can be 3-4 degrees warmer.
This is due to rural areas cooling down at night and urban areas not cooling as much due to urban surfaces continuing to release heat.
2) It’s stronger in SUMMER. Average winter temperatures can be 2 degrees warmer, but average summer temperatures can be up to 5 degrees warmer. This is because there is more solar radiation in the summer, so urban areas absorb more heat.
3) It’s stronger when there’s an anticyclone. Anticyclones cause clear skies and low winds. If there are no clouds, more solar radiation reaches and heats the ground. Low winds mean warm air isn’t blown away.
How are winds affected by buildings in Urban areas?
1) Average wind speed is usually lower in cities than in rural areas. This is because tall buildings create friction that slows down the moving air.
2) These are areas where wind speed is zero because some areas are totally sheltered from the wind by buildings.
3) You get turbulence around the buildings. This happens when the wind hits the buildings, some deflected down, upwards or sidewards.
4) You get powerful gusts of wind when the wind is channelled down streets- this is known as the canyon effect.
Why are there more rain, fog and thunderstorms in Urban areas?
The UHI effect means the air is more warm in urban areas, and warm air can hold more water. The warm, moist air rises - this is called conventional uplift. As it rises it cools, the water vapour condenses and it rains. This is called conventional rainfall.
Urban areas generate huge amounts of dust and pollution. Particles of dust and pollution floating about in the air act as condensation nuclei. This encourages clouds to form, rather than allowing the warm, moist air to disperse.
