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Metro Plus
The `cool' factor
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Being situated on the coast, Chennai enjoys the benefit of sea breeze in summer. It not only cools the land but also helps keep air pollution in check.
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WHEN IT comes to keeping air pollution under check, Chennai is better compared to other cities in the country.
Though the air is sullied by the release of obnoxious gases, the sky is clear each morning and there is rarely any trace of this pollution.
The atmospheric conditions, water and to an extent, soil also contribute to cleaning the air.
Thanks to the sea breeze, Chennai is relatively safer as far as air borne diseases are concerned, as otherwise their incidence would increase alarmingly among children, pregnant women, the elderly and those who are already suffering from respiratory ailments.
Sea breeze cleanses the air. Its velocity is stronger during the summer months and is felt for many kilometres inland and is often the harbinger of thunderstorms.
It occurs as a result of the differences in temperatures of sea and land. When the sun rises, it heats the coast where the land more readily absorbs and emits radiation than a water surface.
So, the air over the land becomes more buoyant and rises as it is warmed by radiation.
This column of air expands. The air over the water is cooler during the daytime since water retains heat for a longer time than land.
When the sun rises, it heats the land, which absorbs and emits radiation faster.
The pressure decreases over the land as this process continues.
A pressure gradient develops on the surface from the sea to the land. This phenomenon is called `sea breeze.'
The velocity at which sea breeze blows depends on the contrasting temperature of the land.
Concrete, asphalt, compact and densely populated areas tend to absorb more heat and so get heated up faster than vegetated or open spaces.
As the sea breeze becomes more powerful in such areas, it has a cleaning effect.
Depending on the strength of the sea breeze, the polluted air of the city gets cleaned up faster.
The type of coastline, manmade structures also affects the strength and direction of the sea breeze over a given area. Its onset varies from place to place even within the city.
The extent of air pollution depends on several other factors too such as the volume of traffic, the road condition, slope, width of the road, the number of intersections, existing building structures and their height, orientation of roads, season and the meteorological conditions.
But the severity of such pollution gets reduced when there is a factor like sea breeze.
In respect of roads in the city, areas like George Town, Royapuram, Anna Nagar, K.K. Nagar, Besant Nagar, Adyar etc., have near perpendicular roads. Hence, sea breeze is more effective in dispersing polluted air.
The sea breeze from a south-easterly direction blows inland at an average speed of about15km/hr. But during the northeast monsoon, its direction changes.
This change is not healthy because it drives the polluted air from north Chennai that is highly industrialised to other densely populated areas of the city.
Unlike Mumbai, with its many high-rise buildings, Chennai is less built-up and sea breeze, which can normally penetrate to a height of 700 m helps clean up the air faster.
But in areas that are far removed from the coast and obstructed by tall buildings and where roads are in a north-south direction, the effect of sea breeze will not be felt.
The alternative to sea breeze therefore is the developing of a green belt, which will not only reduce pollution but also the urban heat island effect.
The Pollution Control authorities in the city could consider such meteorological factors as sea breeze when tackling the problem of air pollution.
O.M. MURALI
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