Urban heat island effect is caused by: urban surfaces (brick & tarmac) absorbing insolation during the day and releasing it at night, heat from combustion and blanket effect of pollution stops heat escaping. (size of urban area not important, density is = leicester 300,000 same urban heat island as london 6M)
It is greatest in summer because: more intense solar radiation, skies are clearer & high pressure is more common which stops heat rising and escaping.
It is greatest at night because: rural areas lose heat quickly but urban materials give off heat all night long.
Higher rainfall in urban areas because there are more condensation nuclei (dust/pollution particles) to allow rain to form. Tall buildings give orographic uplift to cool air. Urban surfaces create convectional uplift. There is lower humidity however.
Winds are generally less but are locally faster, funnelled winds caused by grid iron pattern (e.g. USA housing estates) and also by accelerated winds due to wind speeding up as it hits urban building (Croydon, St. Georges Walk)
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