Trapped in a hot house?

THE SCORCHING summer sun combined with high humidity make life unbearable for people in the metro. Worse, intense heat wave conditions during the summer months take a toll of life. But the reason why the big cities have become 'urban heat islands' is being increasingly attributed to 'human factors.' These are being studied extensively in the west and more recently in a few Indian cities.

An 'urban heat island' is nothing but increased surface temperature in some pockets of a city, caused by an ever-changing micro-climate, which in turn, is induced by human activities.

The rapid growth of Chennai is causing permanent changes to the landscape at the expense of natural resources. The conversion of the metro into a'heat island' calls for a proper scientific study.

An 'urban heat island' study was carried out in Pune, Mumbai, Calcutta, Delhi, Vishakapatnam and Bhopal and in 1987, in Chennai by a team led by N. Jayanti of the Regional Meteorological Centre, Nungambakkam. The temperature in a 'heat island' was found to vary up to a maximum of 10 degrees C in Mumbai, 0.6 degrees C in Vishakapatnam and Chennai, and 4 degrees C in Calcutta.

The daily weather report gives the maximum and minimum temperature at two places in Chennai - Nungambakkam and Meenambakkam (airport). But this temperature does not prevail uniformly throughout the city because Nungambakkam is an urban area while Meenambakkam is a suburb, where human intervention is relatively low. The temperature varies in different parts of the city, with hot and cool pockets at various locations.

Normally, the central business district (CBD) or centre of the city experiences the highest temperature than the other parts. This is because it is entirely built up. Most of the structures comprise concrete and a lot of glass. There is practically no tree cover and the dense vehicular traffic that plies on the roads emits smoke which is a major pollutant. This creates an unhealthy microclimate.

Most man made structures such as concrete, asphalt and the like heat up very fast in response to radiation from the sun. They release the stored heat energy very slowly, sometimes even till the night. This keeps the daily minimum temperature from plummeting too low - a welcome phenomenon in the cold countries during winters, but, makes life unbearable for people in the tropics.

The 'urban heat' study in Chennai was done 12 years ago. But there has since been a tremendous increase in urbanisation, without a proper land use plan, emergence of massive structures, and indiscriminate felling of trees. This warrants an immediate meteorological study to ascertain drastic changes in the climate. As Chennai has a tropical climate, building design should be given priority to allow for sufficient ventilation and spacing between buildings. This would let the accumulated heat escape.

With the present rate of growth, type of materials used for construction, increased human activities and movement of vehicles leading to higher levels of pollution and generation of heat, an integrated scientific study would throw up several interesting findings.

As a supplement to meteorological data, remote sensing satellite imagery gives the latest land use/land cover pattern and with temperature data combined, it is possible using GIS tools (Geographical Information System) to map hot and cool areas across the city.

Such mapping can help determine the direction of unplanned growth (urban sprawl), areas requiring a proper land use plan, future vulnerable areas and places devoid of natural vegetation, parks and water bodies. Such information cannot be obtained using meteorological data alone. It would enable architects, government departments and planners to take better decisions on land use.

Due to the concentration of human activities at a micro level, the city will experience - lower humidity, increased turbidity and reduced wind speed, increased surface run-off, increased flooding, increased cloud cover, increased precipitation and poor visibility.

Actually tall buildings, concrete and asphalt in a city absorb and store greater quantities of solar radiation than do vegetation and soil, typical of rural areas. Further, the road surfaces in the city are impermeable, resulting in rapid run-off of rain water and correspondingly, in the rate of evaporation. Thus, heat that would have been used to convert liquid to gas now goes to increase surface temperature.

Also, tall buildings alter the flow of air. Because of greater surface roughness, wind speeds within an urban area get reduced. The lower wind speed decreases ventilation in a city by inhibiting the movement of cooler outside air which, if allowed to penetrate, would reduce the higher temperatures in the city centre.

According to the 1987 study in Chennai and the suburbs, 'urban heat island' conditions (heat pockets) prevailed in the Mambalam, Vepery and Ennore industrial areas and the cooler areas were in and around Raj Bhavan and Guindy because of the vegetative cover and adequate spacing between the buildings and partially due to suburban characteristics.

A fresh study is essential now integrating satellite imagery and GIS data with meteorological data to ascertain microclimatic changes caused by human activities.

Planners can use the information to improve urban living conditions and public health, besides reducing ecological imbalances.

O. M. MURALI

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