2. Urban Climate

The urban areas across the world experience a climate distinctive from the regional pattern. The process of urbanization changes the physical surroundings and induces alterations in the energy, moisture, and motion regime near the surface. The agglomeration of buildings interferes with the wind and atmospheric characteristics to a degree at least equal to that of a large forest. An urban area changes the air’s composition, temperature and precipitation graphs etc.

Wind speed is lower in cities than in open areas due to obstructive nature of structure of cities. Actual effect varies with the street design, the season and the time of day. The wind tends to channel down streets parallel to the general direction of flow, especially in a city with canyon like streets (high rise buildings). While if street pattern is at right angle to the wind, strong lee effects may be experienced. During the day, city wind speeds are considerably less than surrounding areas, but at night, turbulence over the city makes contrasts less apparent. Rural- urban contrasts are most marked with strong winds, and the effects are therefore more evident in winter4 than in summers.

Cities absorb much less water per area than rural areas, as much of city area is paved or built on. In some areas this creates a need for specific measures to reduce the risk of localised flooding during periods of heavy rainfall. Heavy construction activities in flood plains of rivers flowing through cities increase the period and intensity of flooding.

Cities tend to have lower humidity in contrast to rural or forested areas. Due to concrete surface, rapid surface run-off removes water. The lower density of vegetation and general absence of water bodies etc. also contribute for lower humidity and evaporation. On the other hand, it seems likely that under certain conditions, thermal and turbulences over cities may trigger off precipitation or thunderstorms. Many cities encounter more light rain and thunder than surrounding areas, resulting in a slight increase in total precipitation.