Urban Precipitation Flashcards
What are the 5 main reasons for rainfall patterns in urban areas?
UHI Building height Air movement Pollution Water vapour from industry
How does the UHI affect rainfall patterns?
It generates convection.
Causes rapid evapotranspiration.
Creates cumulus cloud and convectional weather patterns.
How does building height affect rainfall patterns?
High rise buildings and a mixture of building heights can induce air turbulence and promote increased vertical motion.
What is vertical motion?
This is where the wind moves vertically.
What are the two main things to do with air movement that cause urban rainfall patterns?
Rising air causes low pressure.
The movement of prevailing wind over the city.
How does rising air that causes low pressure create urban precipitation?
Rising air causes low pressure. This results in wind being drawn in from the surrounding rural area. Air then converges as it is forced to rise over the higher urban canopy.
How does the movement of the prevailing wind over the city cause urban precipitation?
Friction for the urban boundary creates an orographic process, similar to a mountain barrier. The high rise buildings may cause the air to split. As the air comes back together downwind of the buildings, it is thought to converge and rise upwards, forming rainclouds.
How does pollution influence the urban rainfall pattern?
City pollution increases cloud formation and rainfall because pollutants act as hydroscopic nuclei and assist in rain drop formation. There is some suggestion that city pollution enhances lightening as cloud droplets take on different electrical charges.
Where does the water vapour from industry come from?
It comes from industries and also power stations.
True or false: rainfall downwind of major urban areas can be as much as 20% greater than it is in upwind areas?
This is true.
What causes instability in the atmosphere?
The heating of the surface and overlying air.
What does instability in the atmosphere encourage?
It encourages air to rise.
What happens as the air rises?
It cools and water vapour condenses where it falls as rain downwind of the city.
What has the occurrence of fog increased in line with?
Industrialisation
What fact was discovered in the 1950s concerning the average number of particles in developed cities?
The average number of particles was greater in developed cities than in rural areas.
