3.2.3.4 Urban climate Flashcards
Definition of climate:
The weather conditions in an area longer then a period - normally 30 years
Definition of Microclimate:
Climate within a small area that differs significantly from the climate of the surrounding area
Define Albedo
Proportion of light or radiation that is reflected by a surface
Urban microclimate
Human activity chemically and physically alters air and weather characteristics over and around urban areas, making it different from air and weather over rural areas
What is climatic dome
Where cities create their own climate
How many m does the urban boundary layer extend above roof level?
Greater then 300m
How many degrees higher is climate dome winter temperatures compared to rural areas.
1 degree higher
What other things are affected due to the climate dome with stats?
5-10% more precipitation
2-3 weeks less frost per year
more cloud cover
What does closer lines of isotherms mean?
Steep increase in temperature gradient
What is thermal mass?
A materials capacity to absorb store and release heat
Does asphalt have a high or low thermal mass and why?
High
absorbs solar radiation during day and heat accumulates under the surface
releases heat at night and causes a sweltering night
Give 3 common surfaces and there albedo?
Snow - 80%
Dry concrete - 17%
Black top asphalt - 5%
What happens during the night to the urban canopy layer due to high thermal mass and low albedo?
Common urban materials such as concrete, tarmac and roofing tiles have a high thermal capacity, and a low albedo. Solar radiation is absorbed by these materials during the day, and then at night is released into the surrounding atmosphere. This released heat is often trapped, by tall buildings in the urban canopy layer, and therefore prevented from escaping into the upper atmosphere.
What does buildings with a lack of insulation do?
Separate from thermal mass, buildings release heat through lack of insulation. This escaped heat, is then released into the urban canopy, raising temperatures.
Give two pieces of stats about the loss of heat through house
25% heat loss through roof
35% through uninsulated walls
What large buildings reflect due to steel and glass?
Large expanses of glass and steel reflect solar radiation, and therefore heat, between each other and into the surrounding area
What does hygroscopic mean?
Moisture absorbing
How does pollution affect the UHIF
The emission of hygroscopic pollutants from cars and industry, such as sulfur dioxide, act as condensation nuclei. This leads to the formation of cloud and smog, which can trap radiation and reflect it back to the surface.
What does less evapotranspiration mean for the UHIF
There is less vegetation, and bodies of water, in urban areas so there is less evapotranspiration. Drains and sewers also remove surface water before it can be evaporated. This means that the heat energy that could have been used for evaporation, as it’s endothermic, is not used and therefore remains within the urban canopy
Exhaust heat?
AC units work by removing the heat from a room. The heat is ‘exhausted’ through these fans you see on buildings. They add 1°C to the outside temp.
Car exhaust are around 300°C. They add a further 1°C to the outside temp.
