Precipitation in Urban Microclimates

Precipitation in Urban Microclimates

1. Distribution of rainfall over a town or city is very much influenced by topography

   • Largest values occurring over the more hilly regions
   • Lowest values in more low— lying areas.

2. Higher levels of rainfall in urban areas than surrounding counrtyside

   • Rain is more intense
     • Why?
       • The Urban Heat Island effect means that the air in urban areas is warm, which can hold more water. The warm moist air rises - convectional uplift
         • As it rises it cools, and water vapour condenses and it rains - convectional rainfall
       • Urban areas generate large amounts of dust and pollution. These particles floating in the air act as condensation nuclei.
         • Condensation nuclei cause water to condense around them
         • This encourage clouds to form, rather than allowing the warm, moist air to disperse.
       • A mixture of building heights causes air turbulence, which promotes increased vertical motion
   • Higher number of thunderstorms
     • Likely in late afternoon and evenings in the summer months