2.12. Urban Climate Part 2

Question 1

What happens with radiation or ground fog?

Answer 1

1. Cold, clear night with light winds (less than 5mph). Heat radiates away, cooling the ground and air next to it.
2. Heavier cold air flows downwards
3. Fog forms as air cools to its dew point. Usually less than 200ft deep.
4. Rising sun raises the temp to above the dew point and the fog “burns off”
5. Fog won’t form when it’s windy as cold air near the ground and warm air aloft get mixed up.

Question 2

What happens with precipitation?

Answer 2

• Higher urban temps encourage development of low pressure
• Convection rainfall tends to be heavier and more frequent
• The urban heat island generates convection -> as ground surfaces are heated, rapid evapotranspiration takes place and can result in cumulus cloud and convectional weather patterns
• Presence of high rise buildings and a mixture of building heights induces air turbulence.
• City pollution can increase cloud formation and rainfall.
• Cities may produce large amounts of water vapour from industrial sources and power stations.

Question 3

What is the key figure for precipitation?

Answer 3

There is 5-15% more rainfall in urban areas than surrounding rural areas

Question 4

What happens with wind?

Answer 4

• In rural areas wind speed doesn’t change much with height as there is less friction at the surface and interference of high structures.
• When wind is blown towards a building, it might be forced down or up and over. In the lee of the building, it can create the spiralling down and also gusting.
• Heights, shapes, orientation and layout of buildings and connecting roads and transport infrastructure all have microclimate impacts.
• Urban structures interfere with wind by slowing, redirecting and generally disturbing the overall air flow -> buildings create friction and act as windbreaks

Question 5

What are urban canyons (wind)?

Answer 5

Relatively narrow streets bordered by high-rise buildings funnelling and so concentrating winds

Question 6

What is the Venturi effect?

Answer 6

A particularly violent form of gusting caused in particularly narrow gaps by air rushing to replace low pressure vortices beyond structures

Question 7

Key fact for wind

Answer 7

Wind speeds in urban areas are 30% lower than in rural areas

Question 8

What happens with thunder?

Answer 8

Rapid urban convection:
1. Updraught of warm air (rises high and quickly)
2. Cools and condenses rapidly
3. Rapid cooling creates hail and ice
4. An electrical charge builds that is released and lightning occurs

Question 9

Key fact for thunder

Answer 9

Major urban areas experience 25% more thunderstorms than non-urban areas and hailstorms are 400% more likely