Unit 3 Case Study: London's Urban Microclimates

Question 1

Temperature

Answer 1

Urban microclimates are the most complex of microclimates. The general pattern in London shows the highest temperatures in the city centre (10-11C) compared with the rural fringe of 5C. Temperatures fall more rapidly along the River Thames to the east of the city. Temperature gradient is more gentle in the west of the city due to the density of urban buildings.

Question 2

Pollution

Answer 2

Recent research on Londons heat island has shown that the pollution domes can also filter insolation, reducing the build-up of heat during the day. At night the dome may trap some of the heat accrued during the day so the domes may be reducing the sharp differences between urban and rural areas

Question 3

Wind

Answer 3

There is an absence of strong winds to disperse the heat and bring in cooler air from rural and suburban areas. Urban heat isalnds are often most clearly defined on calm summer evenings under blocking anticyclones

Question 4

Rainfall and humidity

Answer 4

The distribution of rainfall is mainly influenced by topography with the largest values occurring over the more hilly regions and lowest values in more low-lying areas. Kenley on the North Dawns at an altitude of 170m above mean sea level has an average annual rainfall of nearly 800mm whereas London Weather Centre at 43m has 550mm. Humidity is lower in London than surrounding areas partly due to higher temperatures (warm air can hold more moisture so relative humidity may be lower) but water is removed from large urban areas due to the combination of drains and sewers, the large amount of impermeable surfaces and reduced vegetation cover

Question 5

Urban boundary

Answer 5

The urban heat island creates the urban boundary layer which is a dome of rising warm air and low pressure. As ground surfaces are heated, rapid EVT occurs. This EVT, although lower compared to rural areas, occurs quicker and can result in cumulus clouds and convectional weather patterns. Due to the low pressure caused by rising air, surface winds are drawn in from the surrounding rural fringe. This air converges as it is forced to rise over the high urban canopy. The urban boundary creates an orographic process similar to a mountain barrier. The movement of winds contribute to increased rainfall patterns over the city that are most pronounced to the leeward side of the city core. As air passes over the urban boundary layer it begins to sink leading to lower precipitation at the leeward rural area. These differences are more pronounced in summer compared to winter

Question 6

Weekly rain

Answer 6

Some studies have demonstrated a pattern of increased rain through the week and Saturday rain to be a result of a build-up of pollutants due to 5 consecutive commutes. By Monday, pollutants have fallen and rainfall is less likely to form

Question 7

The london plane tree

Answer 7

With an extensive and healthy urban forest, air quality can be greatly improved. Trees help lower air temperatures through increasing EVT. This reduction of temperature not only lowers energy use but improves air quality as the formation of ozone is dependent on temperature. Large shade trees can reduce local ambient temperature by 3-5C. Maximum and midday temperature reduction due to trees range from 0.04C to 0.2C per 1% canopy cover increase. Living roofs offer greater cooling per unit area than light surfaces but less cooling per unit area than curb side planting. Although street trees provide the greatest cooling potential per unit area, light surfaces provide the greatest overall cooling potential when available area is taken into account since there is more available area to implement this strategy compared to others

Question 8

Characteristics of the London plane tree

Answer 8

Aesthetic value
Does it make a mess?
Pollution tolerance
Pests and diseases
Soil conditions
Space
Safety hazards
Microclimate
Biodiversity