Contemporary Urban Environments - Urban Climate Flashcards
Why do cities form microclimates?
Occur as a result of extra sources of heat released from industry, commercial and residential buildings as well as from vehicles.
In addition concrete, glass, bricks and tarmac all act differently from soil and vegetation. They absorb large quantities of heat which they release slowly at night.
The release of pollutants also helps trap radiation in urban areas.
What is an urban microclimate?
The distinctive climate of an urban area compared to its surrounding rural area.
What is the urban heat island effect?
The increasing temperature with distance towards the centre of an area.
Where can the urban heat island extent to?
May extend 250-300m upwards and as much as 10km downward of the urban area.
What are the three types of heat islands?
1) Urban canopy layer.
2) Urban boundary layer.
3) Urban surface heat
What is the urban canopy layer?
The urban canopy layer is the layer of air closet to the surface of cities, extending upwards to approximately the mean building height.
What is the urban boundary layer?
Above the urban canopy layer lies the urban boundary layer. This may be 1km or more in height during daytime, nut as little as a few hundred metres at night. It is the urban boundary layer that forms a dome of warmer air that extends downwind of the city. Wind often changes the dome to a plume shape.
What is the urban surface heat?
The urban surface heat shows the relative warming of the ground surface within an urban area.
What is affected by the urban microclimate?
1) Temperatures
2) Wind and air movements
3) Visibility
4) Moisture
5) Precipitation
Why do urban microclimates form?
Surfaces in urban areas reflect less of the sun insolation. Lower wind speeds allow warmth to accumulate. Heat is given off my factories , vehicles and anthropogenic heat.
How do building/road surfaces affect the microclimate?
Building surfaces are non-reflective and therefore absorb heat. Road surfaces such as tarmac and concrete have a high thermal capacity and therefore also absorb large amounts of heat due to their dark colour.
How are winds and air movement affected by the microclimate?
Lower annual mean wind speeds.
Lower frequency of extreme gusts.
Higher frequency of calms.
How is visibility affected by the microclimate?
In the past smog was a problem caused by industrial pollution. Today photochemical smog is more of a problem.
How does fog differ between winter and summer, in urban areas?
Urban areas may have 100% more fog in winter and 30% more fog in the summer.
How is precipitation affected by the mciroclimate?
Urban areas usually have higher totals compared with rural areas.
On average there are 10% more rainy days and 5-30% more rainfall.
This is caused by micro scale orographic effect of tall buildings. Higher temperatures encourage low pressure and uplift, causing convectional rainfall and a greater number of condensation nuclei, increase precipitation.
What is urban albedo?
The capacity of urban surfaces to reflect solar radiation.
What is particulate air pollution?
Particulate air pollution isanything solid or liquid suspended in the air. It includes smoke, fumes, soot, and other combustion by-products, but also natural particles such as windblown dust, sea salt, pollen, and spores.
What is photochemical pollution?
Air pollution containing ozone and other reactive chemical compounds formed by the action of sunlight on nitrogen oxides and hydrocarbons, especially those in vehicle exhaust.
What is temeprature inversion?
A temperature inversion isa layer in the atmosphere in which air temperature increases with height. As temperature inversions do not allow convection, they trap pollution in the lower layer of the atmosphere.
Why are thunderstorm more likely to form in urban areas?
- Urban convection is strong in the late summer afternoons, when heat energy has had the chance to build up in the atmosphere.
- This creates an updraft of humid, hot air which rises, cools and condenses rapidly.
- However, there are continued updrafts of warm, humid air that is carried upwards repeatedly.
- Resulting in the formation of towering cumulonimbus clouds.
- Air is moved by updrafts and downdrafts, so that water droplets increase in size.
- When large enough to overcome gravity, the fused ice crystal will fall.
- As raindrops split in the updrafts, there is an electrical discharge into the air - this is lightening.
- Thunder is a result of the sudden increase in pressure and temperature from the lightening causing a rapid expansion of are around it.
What are some urban pollution reduction policies?
London - Congestion charge, LEZ, ULEZ, ‘Boris Bikes’. Bejing - number plate segreagtion and car lottery.
What is the diurnal range?
Range between daytime and night time temperatures
What are some impacts of the urban heat island?
1) Excessive heats puts increased strain on the need for cooling and AC
2) Increased water consumption.
3)High rate of heat related illnesses, such as heat stroke and organ damage.
How do cool surfaces manage the UHI?
Building roofs from materials with high solar reflectance or albedo store less energy during the day so therefore emit less heat at night. Pavements can also be made cooler too.
How do green roofs manage the UHI?
Can reduce rooftop temperatures by 20-40 degrees C on a sunny day. There are over 700 green roofs in central London alone, covering an area of over 175,000 square metres
How does urban greening manage the UHI?
Planting more trees and vegetation around the city provides areas of shade and have a natural cooling effect, as seen by lower temperatures in urban parks around the world.
How does the sky view factor manage the UHI?
This is the relative openness between buildings in an urban area. Narrow streets accumulate heat within ‘street canyons’ and conseqently increase air temperatures. In addition, streets that are angled perpendicular to the prevailing wind will limit the chances of ventilation and accumulate heat between the buildings.
What is channelling?
Wind redirected down long, straight, canyon-like streets where there is less friction. These are sometimes referred to as urban canyons.
What is the venturi effect?
The phenomenon in which wind velocities increase when air flows through an increasingly narrow gap (for example, between two buildings)
