1.4.1 Urban microclimates Flashcards
Why are urban areas warmer than rural areas?
The urban heat island effect
What are urban heat islands?
Urban areas with higher air temperatures than the surrounding rural areas.
Changes in temperature within urban heat islands
- The highest temperatures are found in industrial areas and in the most densely built-up areas, e.g. the CBD.
There are pockets of cool air above park and bodies of water (e.g. rivers or ponds). These are called temperature ‘sinks’. - Areas within the city with the same land use (e.g. industry) generally have the same temperature. These are called temperatures ‘plateaus’.
- Temperatures can change rapidly when land use changes (e.g. from inner city housing to CBD high rise buildings). Rapid changes are referred to as temperature ‘cliffs’.
What are the main causes of the Urban Heat Island effect?
- Absorption of heat by urban surfaces
- Air pollution
- Heat from human activities
- Less evaporation
How does the absorption of heat by surfaces cause the urban heat island?
Concrete, brick and tarmac surfaces absorb and store heat from the sun during the day. They slowly release the heat as long wave radiation – this is most noticeable at night, when it warms the air.
How does air pollution cause the urban heat island?
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. Both these things trap outgoing heat radiation and reflect it back to the surface.
How does heat from human activites cause the urban heat island?
Cars, factories, offices, central heating, air conditioning units and people all release heat.
How does decreased evaporation cause the urban heat island?
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.
Variation in the urban heat island effect
The UHI effect is:
- Stronger at night
- Stronger in summer
- Stronger when there is an anti-cyclone
Case studies for wind and the UHI
- Toronto
- Masdar City, UAE
Case Study: Wind in Toronto
In the 1980s, Toronto introduced climbing ropes to stop people being blown off their feet in the streets. Toronto are now encouraging the use of podiums in high rise buildings as they redirect wind. Hedges also act as barriers to stop wind.
Case study: Masdar City, UAE
A wind tower has been built, which creates a strong wind around the city and cools it down.
What is the Canyon Effect?
You get powerful gusts of wind when wind is channelled down streets – this is known as the canyon effect.
The canyon effect has implications for building design and town planning, e.g. positioning of buildings and the location of doorways on larger buildings.
Wind in urban areas
Average wind speeds is usually lower in cities than in rural areas. This is because tall buildings create friction that slows down the moving air.
There are areas where wind speed is zero, because some areas are totally sheltered from wind by buildings.
You get turbulence around buildings. This happens when wind hits the face of a building – some of its deflected down, some around the sides and some over the top. When these winds hit other buildings or the ground they cause vortices (bodies of swirling air).
Snow in urban areas
It doesn’t snow as often in urban areas, and when it does, the snow melts faster. This is because it’s warmer due to its UHI effect.
