🧍3.2.3.4 - Urban Climate Flashcards
What aspects of the climate can urban areas modify?
Wind speeds
Temperature
Clouds and precipitation
Pollution
In what ways can winds be modified by urban areas?
Lower speeds
Greater variability
Large-scale convection
Why are wind speeds generally lower in urban areas?
The roughness of the land surface, consisting of buildings at a variety of heights, creates greater surface friction
What is an urban canyon?
A street with tall buildings either side but a space in between
What is the Venturi effect?
The effect of ‘squeezing’ the airflow, increased as buildings get taller towards the city centre
Where is the highest air pressure?
In the upper part of the building with air flowing down the front and over the top
How does convection occur over urban areas?
Convection and uplift over the area means that air pressure is lowered and air is drawn in from surrounding rural areas
What is the urban heat island?
Urban areas are on average 1-2 degrees warmer than their surrounding rural areas
What is albedo?
The degree of warming of a surface depends on the amount of insolation absorbed by the surface, which in turn depends on how much is reflected. The reflection is albedo.
What is sky view factor?
The amount of sky you can see without your view being impeded by tall buildings
What is specific heat capacity?
The amount of energy required to raise the temperature of 1kg of a material by 1 Kelvin
What is the intensity of an urban heat island?
The maximum difference between the temperature of the rural area and the peak urban temperature
How can weather conditions impact the intensity of the urban heat island?
High pressure systems with clear nights and sunny days increase the intensity
What are the five main factors causing urban heat islands?
Anthropogenic heat Height and arrangement of buildings Nature of building materials Presence of water Pollutants
What is sensible heat?
Heat energy which can be felt, as opposed to latent heat which must be measured
How does cloud cover differ in urban areas?
It is greater
Why is cloud cover greater in urban areas?
There are more condensation nuclei, so cloud formation is greater
Why are there more thunder storms in cities?
Enhanced convectional uplift from the heat island can lead to instability in thermals, creating more storms
How do precipitation levels differ in urban areas?
They are higher
What is the speed effect?
Wind is slowed by friction from the building, friction with obstacles
What is the downdraught effect?
Turbulence is created around the top of the building, downdraught forms at the front
What is a downwind eddy?
A circular motion of wind behind the building and turbulence near the top of the building occurs as the wind is forced upwards
What can photochemical pollution lead to?
Smog - consisiting of ozone and peroxyacetyl nitrate (PAN)
When were London’s ‘Pea Soupers’?
December 1952
