CUE 5 - Urban climate Flashcards
What is the urban heat island effect
The phenomenon or urban areas being warmer than rural areas
What are urban heat islands (UHI)
Urban areas with higher air temperature than the surrounding rural areas
Where are the highest temperatures found
In industrial areas and in the most densely built-up areas (CBD)
What are temperature sinks
Pockets of cool air above parks and bodies of water (like rivers and ponds)
What are plateaus?
Areas within the city with the same land use (like industry) generally have the same temperature
What are rapid changes in temperature reffered as?
Cliffs
When does temperature change rapidly
When land use changes (eg - from inner-city housing to CBD high rise buildings)
What are the 4 main causes of the UHI effect
- Absorption of heat by urban surfaces
- Air pollution
- Heat from human activity
- Less evaportranspiration
What surfaces absorb and store heat during the day
Concrete, brick and tarmac
What happens to these surfaces
Slowly release heat as long wave radiation - its more noticeable at night when it warms the air
What does air pollution from cars and factories do
Increases cloud cover over the city
WHat does this create
A pollution dome (layer of pollution over the city
What do these things do?
Trap outgoing heat radiation and reflect it back to the surface
Where does heat from human activity come from?
Cars, factories, offices, central heating. Air conditioning units and people themselves
What happens when it rains?
The water is quickly removed by drainage systems so there’s little surface water to evaporate
What happens because theres not much vegetation
Theres little transpiration
When is the UHI effect stronger
At night
What happens if theres higher temperatures
Evapotranspiration uses heat energy, so higher, so higher temp means less evapotranspiration
What are urban night temperatures like in comparison to rural areas
3 - 4 higher
Why does this happen
Rural areas cool at night, but urban areas don’t cool as much because they continue to release heat that they’ve absorbed during the day
Why is it stronger
There’ more solar radiation in summer so urban areas absorb more heat
What are urban daytime temperatures like in comparison to surrounding rural areas
Theyre on average 0.6 higher
What do anticyclones cause
Clear skies and low winds
When is UHE stronger
In summer and when there’s an anticyclone
What do anti cyclones cause (3)
1 Clear skies and low winds
2 If there is no cloud more solar radiation hits the ground
3 Low winds means warm air isn’t blown away
Why is average wind speed usually lower in cities than in rural areas
The tall buildings create friction that slows down the moving air
Why do some areas have wind speed of 0
They are totally sheltered from the wind by buildings
When do you get turbulence around a building
1 when the wind hits the face of the building some of it is deflected down, some around the sides and some over the top
When does a vortices (body of swirling wind occur)
When the wind hits the building or the ground
What is the canyon effect
When powerful gusts of wind is channeled down the streets
Give 2 reasons to explain why it rains more often in urban areas , rain is more intense and there are more thunderstorms than compared to the countryside
1 The UHI effect means the air in urban areas is warm which means it holds more water.
The warm moist air rises (conventional uplift)
As it rises the water vapor condenses and it rains - this is called conventional rainfall
2 urban areas generate lots of dust and pollen which float about in the air acting as a condensation nucelli - n which trigger water to condense around them. This encourages clouds to form and not allowing the warm air to disperse
What is another effect of higher concentration of condensation nucelli in urban areas
Increases frequency of fog
What is the effect of UHI on snow in urban areas
It doesn’t snow often and when it does it melts faster and have fewer days frost.
