Cities create their own climate and weather conditions (small scale variations). This is also known as climatic dome effect within which the weather is different from that of the surrounding rural areas in terms of temperature, relative humidity, precipitation, visibility and wind speed.
Within the urban climatic dome two levels can be recognized
- Urban canopy- below roof level and spaces between buildings
- Urban boundary layer- above the urban canopy layer.
Factors affecting urban microclimate
- Building structure, design, size, spacing and materials
- Park and open spaces, fringe forest, green belt
- Distance from water bodies
- Volume of traffic and level and type of pollution
- Power stations and location of polluting industries
- Population density and consumption pattern (eg. use of air conditioning, cars, pattern of energy used etc. )
- Prevailing wind and air pressure
Urban Heat Island (UHI) effect
Urban heat island signifies the temperature differences between the urban and rural areas. Urban areas are generally 2 to 4 degree C warmer than those of the surrounding countryside, depending upon the size and character of the city. An urban area can be identified as a warm spot in the sea of the surrounding cooler rural air mass.
Cities tend to be warmer than the surrounding rural areas for the following reasons:
- Building materials such as concrete, bricks and tarmac absorb large amount of heat and affect the urban Albedo. Albedo is the amount of insolation reflected by the earth surface and atmosphere. Highly reflective surfaces absorb very little insolation and reflect it back into the atmosphere, keeping the urban areas cool. Darker surfaces like tarmac tend to absorb insolation more and then reradiate as long wave energy that heats up the urban area.
- Temperature rises steadily to a peak in the CBD where building densities are highest. There could be a second cliff after the cliff at the CBD. This is mainly due to the difference in land use pattern between the city environment (built up surface) and adjoining countryside (farm land or forest)
- There are variations that reflect the distribution of industries, power stations; water bodies and open spaces (generate cooling effect due to dispersion of pollutants). Heat comes from industries, buildings (domestic heating, air conditioning) and vehicles which all burn fuel.
- In winter, rural areas keep snow for much longer period and therefore generate a greater albedo.
Impact of UHI
- Elevated environmental problems- high level of ozone and suspended particles , photochemical smog
- Compromised human health and comfort – Heat cause serious problems for elderly and children, heat stress and illness, high temperature increases the likelihood of vector and waterborne diseases.
- Increased energy consumption- urban heat islands raise demand for electrical energy in summer like air conditioning etc. Companies that supply electricity typically rely on fossil fuel power plants to meet much of this demand, which in turn leads to an increase in air pollutant and greenhouse gas emissions.
Humidity and precipitation
- Overall, relative humidity (amount of moisture available) is low in cities than in rural areas as there are fewer water bodies and sparser vegetation cover cause lower rates of evapotranspiration. In the cities, precipitation is directly channeled into drains and is hardly absorb by the concrete surfaces.
- Urban heat island generates convection and high-rise building promotes vertical motion, creating stronger thunderstorm in summer.
Size and shape of the buildings affect wind pattern in the cities. Air is forced to rise upward and is displaced side wards on the windward side of the buildings. On the other hand, it is pushed downwards at the leeward side of the structure (refer to the diagram). The uneven surface area of the cities due to varying structures and skyscrapers generate frictional drag on air moving over and around them.
- Varying height of buildings creates turbulence and abrupt change in wind direction and speed.
- High rise building channel air into canyons (strong wind) between them. Canyons are created due to the Venturi effect in which the pressure difference between the gap of the high rise buildings cause the wind to pick up speed and reach high velocities. Airflow at the street level is necessary to dilute harmful pollution. Efficient building design with tilts and wind passage can solve this problem to some extent. For example, pollution emitter pipe or chimneys should be constructed high enough to ensure that the pollutants are released into the undisturbed boundary layer above the buildings and not into the eddies.
Air quality- Smog and visibility
Air quality in urban areas is often poorer than in rural areas due to suspended particulates in the air mainly from power stations and vehicle exhausts. This also includes cement, dust, tobacco smoke, carbon monoxide, sulphur dioxide, ash, and pollen. Coastal cities may also have large number of sea salts. All these may produce haze, acid rain, smog (mixture of fog and smoke) etc.
Photochemical smog is a haze like condition that develops when pollutantslike oxides of nitrogen and volatile organic compounds created from fossil fuel combustion together with low level ozone interact under the influence of sunlightto produce a mixture of different hazardous chemicals (also known as secondary pollutants). It decreases visibility due to blue-brown haze
Health hazard created by photochemical smog
- Eye irritation and respiratory trouble
- some secondary pollutants are carcinogenic
- Retards plant growth, damages plastics,breaks down rubber and erodes urban structures like bridges and buildings.
Mexico city and Los Angeles have a serious problem with photochemical smog because oft he high-density vehicle and favourable topography that trap the high concentration of photo oxidant gases at low level.
Mexico City and temperature inversion
Mexico city is located in a basin where cool air remains trapped by a layer of warm air above it. This is called temperature inversion. The air fails to mix and produce intense pollution effect and photochemical smog. Mexico city is one oft he most polluted places in the world