Finding 1:
- Pollution in Northern and Western Indian may slow down the monsoon.
- How?
- The Indian summer monsoon season begins when the land surface becomes hot enough to drive a powerful rising motion of air in the atmosphere, producing heavy precipitation.
- Cooler, humid air over the Arabian Sea flows inland to compensate for the rising air.
- Air in this compensating circulation encounters the surface heating and also rises, perpetuating the cycle.
- At the smallest scales, an increase in tiny particles in the atmosphere can shade the land surface while absorbing sunlight aloft, causing a reduction in the heat that reaches the surface.
- Clouds that do form in these polluted environments are less likely to rain and more likely to persist because the droplets are smaller. These longer-lived clouds further cool the surface and weaken the circulation.
- In this way more air pollution can mean weakening of monsoonal systems.
Finding 2:
- Erratic behaviour of monsoon rainfall, including phenomenon of concentrated heavy rainfall on small number of days in localized area can be attributed to the rising air pollution, especially the increase in suspended particles in the atmosphere.
- How?
- Excess aerosols, suspended solid particles like dust, smoke and industrial effluents in atmosphere is changing cloud patterns, its shape, size and other properties like temperature, which in turn is resulting in variability in rainfall over Indian sub-continent during monsoon season.
- Aerosols are extremely important for cloud formation.
- In absence of aerosols, no clouds can be formed and consequently no rainfall will take place.
- But due to increase in aerosol content in atmosphere, there is direct consequence of rising air pollution interfering with stable cloud formation system and influencing rainfall patterns.
- In short term, these changes in cloud structure and cloud dynamics lead to sharp variability in rainfall, which is similar to rainfall patterns witnessed very often in India in last few years.
- In the long term, it is likely to lead to overall suppression of rainfall during the monsoon season.