Urban Climate Flashcards
What is the urban heat island effect?
The phenomenon of urban areas being warmer than rural areas
What is an urban heat island?
Urban areas with higher air temperatures than. the surrounding rural areas
Where are the highest temperatures found?
Industrial areas and in the most densely built up areas like the CBD
What are temperature sinks?
Pockets of cool air above parks and bodies of water (like rivers and ponds)
What are temperature plateaus?
Areas within the city with the same land use (like industry) generally have the same temperature
When does temperature change rapidly. - what are these called?
When land use changes. Rapid changes are called cliffs
What are the 4 main causes of the UHI effect?
- Absorption of heat by urban surfaces
- Air pollution
- Heat. from human activity
- Less Evapotranspiration
How does absorbing heat from urban surfaces work?
Concrete, brick and tarmac surfaces absorb and store heat from the sun during the day
How does heat get released (AOHUS)
They slowly release heat as long wave radiation - this is more noticeable at night, when it warms the air
How does air pollutiony cause the UHI effect?
Air pollution from cars and factories increases cloud cover over a city creating pollution dome. They trap outgoing heat radiation and reflect it. back to the surface
How does heat from human activity cause the UHI effect?
Cars, fcatories, 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 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 of it means higher temperatures
When is the UHI effect stronger
AT night
What are urban temperatures like compared to rural areas
In the day they’re 0.6 warmer, but at night they’re 3-4 warmer
Why are urban areas hotter than rural areas especially at night?
Rural areas cool down at night, but urban areas don’t cool down as much because urban surfaces continue to release heat that they’ve absorbed in the day
Whys the UHI stronger in the summer?
Average winter temperatures can be 2 warmer, but average summer temperatures can be up to 5 warmer. This is because there’s more solar radiation summer, so urban areas absorb more heat
When is the UHI stronger?
When there’s an anticyclone
What do anticyclones cause?
Clear skies and low wind
What happens if there’s no clouds when there’s anticyclones?
More solar radiation reaches and hits the ground. low winds mean warm air isn’t blown away
Why is average wind speed lower in cities than rural areas?
Because tall buildings create friction that slows down moving air - some areas have a wind speed of 0
Why do you get turbulence around buildings?
When the wind hits the face-off a building - some of its deflected down, some around the sides and some over the top
What happens when winds hit other buildings. or the ground?
They cause vortices
What are vortices?
Bodies of swirling air
WHta is the canyon effect?
You get powerful gusts of wind when wind is channelled down streets
What is rain like in urban areas in comparison to the countryside?
It rains more often and it’s more intense there’s also more thunderstorms
What are the 2 reasons that there’s more rain and thunderstorms
- The UHI effect means that air in urban areas is warm and warm, moist air rises. As it rises it cools, the water vapour condenses and it rains
- Urban areas generate huge amounts of dust. and pollution. Particles of dust and pollution floating about in the air act as condensation nucelli. This encourages clouds to form rather than allowing warm moist air to disperse
What type of rain is in urban areas?
Convectional
What is a convectional uplift
Warm moist air rises
What increases the frequency of fog?
the higher concentration of condensation nucelli
WHy doesn’t it snow as. often in urban areas?
Its warmer because of the UHI effect
