Urban Climate

Question 1

what’s the urban heat island effect?

Answer 1

• the phenomenom where the characteristics of urban areas and the activity within them cause the areas to produce and retain more heat than surrounding rural areas
• urban areas with higher temperatures than surrounding rural areas are called urban heat islands (UHIs) e.g. London
• the highest temperatures are found in industrial areas and the most densely built up areas e.g. the CBD

Question 2

why do urban heat islands develop?

Answer 2

• wasted heat from urban activities
• urban materials
• lack of evapotranspiration

Question 3

effect of urban materials

Answer 3

• urban surfaces (tarmac, concrete, brick) have a lower albedo so they absorb and store more solar energy during the day
• they emit this absorbed heat as long wave radiation which warms the surroundings, especially at night

Question 4

effect of lack of evapotranspiration

Answer 4

• heat energy is lost through evapotranspiration
• urban areas tend to have less vegetation, meaning less heat energy is lost via transpiration
• urban drainage systems reduce the amount of surface water, meaning less heat energy is lost via evaporation

Question 5

effect of heat from urban activity

Answer 5

• air pollution increases cloud cover and creates a layer of pollution over the city, trapping heat radiation and reflecting it back to the surface
• cars, factories, offices, central heating, air conditioning, and people all release heat

Question 6

what are temperature sinks?

Answer 6

pockets of cool air above parks and bodies of water

Question 7

what are temperature plateaus?

Answer 7

areas within the city with the same land use generally have the same temperature

Question 8

what are temperature cliffs?

Answer 8

temperature can change rapidly when land use changes

Question 9

how does the effect vary by season and day/night?

Answer 9

• the UHI effect is stronger at night (3-4 degrees warmer than surrounding rural areas)
• this is because rural areas cool down at night but urban areas don’t, urban surfaces continue to release heat
• the UHI effect is stronger in summer (5 degrees warmer) because there’s more solar radiation
• the UHI effect is stronger when there’s an anticyclone. Clear skies means less cloud cover, and low winds means warm air isn’t blown away

Question 10

precipitation patterns in urban areas

Answer 10

• rainfall is higher and more intense in urban areas than surrounding rural areas
• convectional uplift
• condensation nuclei
• wind patterns

Question 11

what’s convectional uplift?

Answer 11

• the UHI effect means urban air is warm
• warm air can hold more water, making it moist
• warm, moist air rises and cools
• water vapour condenses and falls as convectional rainfall

Question 12

what are condensation nuclei?

Answer 12

• particles of dust and pollution act as condensation nuclei
• they cause water to condense around them
• this encourages clouds to form

Question 13

effect of altering wind patterns