The Hindu Business Line : Aerosols to slow down winds, impact rainfall

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Thiruvananthapuram Jan.31 Latest research findings by NASA and Stanford University indicate that aerosol pollution will slow down winds, impacting normal rainfall pattern in tropical countries.

The unique combination of meteorology, landscape (relatively flat plains framed by the Himalayas to the north and open ocean to the south), and the large population maximise the effects of aerosol pollution in India.

The skies over North India are seasonally filled with a thick soup of aerosol particles all along the southern edge of the Himalayas, streaming southward over Bangladesh and the Bay of Bengal. Most of this air pollution comes from human activities.

COOLER GROUND

Aerosol particles floating in the atmosphere absorb or scatter solar radiation, and prevent it from getting to the ground. This cools the Earth's surface and reduces daytime vertical convection that mixes the slower winds found near the ground with the faster winds at higher altitudes.

The overall effect is a reduction in the speed of near-surface winds, which help evaporate water and cause rain clouds to build. The slowing winds may have accounted for a weakening Asian monsoon already.

Slower winds evaporate less water from oceans, rivers and lakes. Furthermore, the cooling of the ground provoked by the aerosol particles reduces the evaporation of soil water.

LESSER RAIN

Accumulation of aerosol particles in the atmosphere also makes clouds last longer without releasing rain. This is because atmospheric water forms deposits on naturally occurring particles, like dust, to form clouds. But if there is pollution in the atmosphere, the water has to deposit on more particles.

Spread thin, the water forms smaller droplets. Smaller droplets in turn take longer to coalesce and form raindrops. In fact, rain may not ever happen, because if the clouds last longer they may move to drier air zones and evaporate.

Earlier last year, Prof V. Ramanathan of the Scripps Institution of Oceanography had observed in a paper that aerosols could exert a regional cooling influence on Earth's surface that is about three times greater than the warming effect of greenhouse gases.

They also increase the solar energy absorbed in the atmosphere by 50 per cent — thus making it possible to both cool the surface and warm the atmosphere. This heating perturbs atmospheric circulation and rainfall patterns.

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