Air is a great leveller which does not distinguish between
people. We all breath the same air; and wind disperses it to every nook and
corner. A great tragedy took place in Bhopal in December 1984 with the
accidental escape of 45 tonnes of methyl isocyanate (MIC) gas from the Union
Carbide factory. The cool, night-time north wind, spread this poisonous cloud
over nearly all of Bhopal. But the concentration was densest in the 1.5km
radius of the factory, resulting in the deaths of about 3,000 people, and life
long debilitation and untold suffering for 60,000 more.
In general, the dispersion of gaseous and particulate matter
depends on the type of emission and the wind and temperature structure of the
atmosphere. Though the latter two are related, it is still possible to make a
separate assessment of the, role of wind.
Consider a 100 feet tall chimney stack. From its top,
pollutant gases and particles emerge at a certain rate, and across the top
there is a horizontal wind at a varying rate. The stronger the wind, the larger
the amount of air that moves past the chimney. Therefore, the quicker and more
rapid is the dispersal of the pollutants. Thus, strong surface winds produce a
smoke plume with smaller concentrations of emitted materials than light winds.
Fig. 10.6 explains how temperature and wind affect the behaviour of a plume of
smoke from a chimney.
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