CUE: Urban Precipitation And Thunderstorms

Question 1

What is the precipitation like in urban areas?

Answer 1

Urban areas have between 5 - 15% more precipitation than rural areas in both quantity and in the number of days of rainfall

Question 2

Why is precipitation so much more in urban areas?

Answer 2

• Urban air pollution results in more condensation nuclei – the tiny particles essential for cloud droplets to form
• Furthermore, air is warmer in cities leading to more air rising (low pressure), condensation, cloud formation and convectional rainfall.
• Turbulence amongst buildings and heat island convection also encourage warm air to rise
• Dust and pollution from industry and vehicles also increases the density of condensation nuclei.

Question 3

What about snow in urban areas?

Answer 3

Snow doesn’t lie for as long on the ground in urban areas
- higher urban temps and darker road surfaces which retain more heat encourage melting

Question 4

How are thunderstorms formed?

Answer 4

1. Rising heat can trigger thunderstorms.
2. The more intense the heating, the quicker the updraft, the more violent the storm.
3. Updrafts of hot, humid air rise higher in the atmosphere and rise quickly (rapid convectional uplift)
4. Air cools, condenses rapidly, forming rain.

Question 5

What’s an example answer for how a thunderstorm is formed?

Answer 5

• as hot air rises, it cools and condenses forming small droplets of water. If there is enough instability in the air, the updraft of warm air is rapid and the water vapour will quickly form a cumulonimbus cloud. Typically these clouds can form in under an hour.
• as warm air continues to rise, the water droplets combine to create larger droplets, which freeze to form ice crystals. As a result of circulating air in the clouds, more water freezes on the surface of the droplet or crystal. Eventually the droplets become to heavy to be supported by the up-draughts of air and they fall as hail.
• as hail moves within the cloud, it picks up a negative charge by rubbing against smaller positively charged ice crystals. A negative charge forms at the base of the cloud, where the hail collects, while the light crystals remain near the top of the cloud and creates a positive charge.
• the - charge is attracted to the earths surface and other clouds and objects. When the attraction becomes too strong, the + and - charges come together, or discharge, to balance the difference in a flash of lightening (strike or bolt). The rapid expansion and heating of air caused by lightening produces the accompanying loud clap of thunder.