Urban climate refers to climatic conditions in an urban area that differ from neighboring rural areas, and are attributable to urban development. Urbanization tremendously changes the form of the landscape, and also produces changes in an area's air.
Temperatures are higher in cities than the surrounding rural areas and this is called the urban heat island. There are a number of causes of the urban heat island: building materials have a lower specific heat capacity than grass and trees. Specific heat capacity is the amount of heat required to raise one unit of the material one degree Celsius. The specific heat capacity of concrete is 800 Joules/kg per degree C whereas for soil it can be 2000 Joules/kg per degree C. So concrete heats up more quickly in the day, warming the air around it.
Rural areas have more wet surfaces (in towns, rivers are often covered and water runs off into drains). Evaporation from these wet surfaces produces cooling because heat is absorbed by the process of evaporation. Buildings are heated, and vehicles and air conditioning systems generate heat.
Buildings act as a barrier to winds. As winds can convey lower temperatures from the sea this has the effect of keeping the city warmer.
Buildings with dark surfaces such a roof tiles have a lower albedo – they reflect less solar radiation, absorb more and become hotter. Dark coloured asbestos, often used as a road surface, has a low albedo.
The denser the built-up area the hotter it tends to be, so the central business district is the warmest part of the city.
Temperature contrasts are greatest when wind speeds are low because wind will carry in cool air from outside the city.
The urban heat island effect tends to be greater in winter because more heat is being generated to keep buildings warm. The effect is also greater at night because buildings store heat generated during the day and slowly emit it at night. A field in a rural area is pointing towards the sky and will reradiate long wave radiation back into space if the sky is clear. A canyon of buildings does not point towards the sky and this lack of a 'sky view' impedes reradiation.
Because cities are warmer, the hot air is more likely to rise and if it has a high humidity it will cause convectional rainfall – short intense bursts of rain and thunderstorms. Urban areas produce particles of dust (notably soot) and these act as hygroscopic nuclei which encourages rain production. Because of the warmer temperatures there is less snow in the city than surrounding areas.
Wind speeds are often lower in cities than the countryside because the buildings act as barriers (wind breaks). On the other hand, long streets with tall buildings can act as wind tunnels – winds funnelled down the street – and can be gusty as winds are channelled round buildings (eddying).
Cities are often less humid that the surrounding rural areas because the city is warmer, and higher temperatures lowers the relative humidity. Precipitation is quickly removed from the surface into drains. Surfaces in towns are impermeable and water runs off quickly into drains. In rural areas water is held in the soil and this raises the humidity of the air above. Finally,vegetation in rural areas emits water vapour by the process of transpiration.