Urban Climate Flashcards
What is the phenomenon of urban areas being warmer than rural areas called?
The Urban Heat Island Effect
What is an urban heat island?
Urban areas with higher air temperatures than the surrounding rural areas are called urban heat islands (UHI’s).
What are temperature sinks?
There are pockets of cool air above parks and bodies of water (e.g. rivers or ponds). These are called temperature ‘sinks’.
What are temperature plateaus?
Areas within the city with the same land use (e.g. industry) generally have the same temperature.
What are temperature cliffs?
Temperature can change rapidly when land use changes (form inner city housing to CBD high rise buildings). Rapid changes are referred to as temperature ‘cliffs’.
How does absorption of heat by urban surfaces cause the UHI?
Concrete, brick and tarmac surfaces absorb and store heat from the sun during the day (urban surfaces have a low albedo - they absorb lots of energy instead of reflecting it). 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 UHI?
Air pollution from cars and factories increase cloud cover over the city. It also create a ‘pollution dome’ - a layer of pollution one the city. Both these things trap outgoing heat radiation and reflect it back to the surfaces.
How does heat from human activity cause the UHI?
Cars, factories, offices, central heating, air conditioning units and people themselves all release heat.
How does less evapotranspiration cause the UHI effect?
When it rains the water’s 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.
Why is the UHI effect stronger at night?
The UHI effect is stronger at night. Urban daytime temperatures are on average 0.6 degrees warmers than surrounding rural areas, but night time temperatures can be 3-4 degrees warmer. This is because rural areas cool down at night, but urban areas don’t cool as much because urban surfaces continue to release heat that they’ve absorbed during the day.
Why is the UHI effect stronger in the winter?
This is because there’s more solar radiation in summer, so urban areas absorb more heat.
Why is the UHI effect stronger when there’s an anticyclone?
It 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.
Why is average wind speed usually lower in cities rather then rural areas?
This is because tall buildings create friction that slows down the moving air.
How do you get turbulence?
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.
What are vortices?
When these winds hit other buildings or the ground they cause vortices (bodies of swirling air).
