Theory

• Rainfall can be higher over urban areas than rural areas. This is partly because higher urban temperatures encourage the development of lower pressure over cities in relation to the surrounding area.
• Convection rainfall tends to be heavier and more frequent, as does the incidence of thunder and lightning:

  • urban heat island generates convection - ground surfaces heat, causing rapid evapotranspiration
  • high-rise buildings promotes increased vertical motion
  • surface winds are drawn in from the surrounding rural area, due to low pressure caused by rising air
  • city pollution increases cloud formation and rainfall (pollutants act as hydroscopic nuclei)
  • cities produce large amounts of water vapour from industrial sources