Over and above the global climate and site climate, man-made environments, such as in towns and cities can create micro-climates of their own, which may be termed as the urban climate. The factors which cause deviations in regional climate are the following :
- Changed surface qualities - pavements and buildings increase the absorption of solar radiation and reduce evaporation,
- Buildings - they cast shadows, channelise winds, and store absorbed heat, and slowly releasing it at night,
- Energy seepage - it occurs through walls of heated buildings, out of A. C. plants, as heat out put of internal combustion engines, and due to electrical inefficiencies and as heat loss from industry and factories, and
- Atmospheric pollution - water vapours from boilers, exhaust from domestic and industrial chimneys, exhaust air from motor vehicles, and fumes, vapour and dust in general.
The extent of deviations with respect to the site climate can be substantial. The air temperature can be 8° to ll°C higher then the surrounding countryside. The relative humidity is reduced by 5 to 10% due to quick run-off of rain water from paved areas, absence of vegetation and higher temperature. Wind velocity can be reduced to less than half of that in the adjoining open country.
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