Thursday 20 November 1997
The smog-weary inhabitants of Singapore will soon be able to breathe again. For the past two weeks, the Pollutant Standards Index of air quality has been hovering near the limit of the healthy range, well down from its life-threatening peak of a couple of months ago. Now the monsoon, delayed by El Nino, is on its way, and the resulting rains and wind will wash away the remains of the smog.
Monsoons are usually among the most reliable - and spectacular - products of the weather machine. They begin in the Intertropical Convergence Zone (ITCZ), a narrow zone around the Equator where low-level north-easterly winds from the northern hemisphere meet south-easterlies from below the Equator, forcing the air to rise, resulting in an area of low pressure.
In summer, land in the tropics is much hotter than the sea, leading to even lower pressure over the land. Warm, moist sea air from the ITCZ is drawn over land, where uplift from hills and surface heating combine to produce torrential rains. In the winter the temperature differential is less between land and sea, and winds change direction. With El Nino warming the waters, however, the change was delayed this year.
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