Precipitation

• What is precipitation?

  Any product of the condensation of atmospheric water vapour that falls under gravitational pull from clouds. Forms of precipitation include:

  • Rain
  • Sleet
  • Hail
  • Snow

  Urban area have 10% more precipitation that rural areas > more heavy downpours but more dry days, why?

  • Higher urban temp encouraged development of lower air pressure over cities
• Reasons behind higher urban precipitation compared to rural:

  1. UHI
  2. High-rise buildings
  3. City pollution
  4. Water vapour
• 1. UHI:


  UHI > convection > ground surfaces heated > rapid evapotranspiration takes place > results in conventional rainfall (often develop into thunderstorms in summer)
• 2. High-rise buildings:


  High-rise buildings and mixture of heights > turbulence > promotes increased vertical motion > increases convection
• 3. City pollution: 


  City pollution > increases cloud formation and rainfall > pollutants act as hygroscopic (water-attracting) nuclei and assist in rainfall formation
• 4. Water vapour


  Cities produce lots of water vapour from industrial sources and power stations
• What are cloud condensation nuclei?

  Urban pollution particles (e.g. dust) + heat = convection + condensation + rain

  CCN's > rain develop > hygroscopic (stick)

  CCN's are inorganic and synthetic