This story is from July 3, 2017

Bad air to blame for irregular rain

A study, published in the Atmospheric, Chemistry and Physics journal, has highlighted how air pollution could delay rains in some areas, on one hand, and cause intense rain in a relatively short period, on the other.
Bad air to blame for irregular rain
Representative image.
NEW DELHI: It’s not just your lungs that the city’s bad air is hurting. A study, published in the Atmospheric, Chemistry and Physics journal, has highlighted how air pollution could delay rains in some areas, on one hand, and cause intense rain in a relatively short period, on the other.
This — and several other similar studies — drives home how air pollution may already be impacting crop yields and food production.

The study carried out by IIT-Kanpur, Weizmann Institute in Israel and Indian Meteorological Department has assessed long-term data of clouds, aerosols and rainfall from June to September between 2002 and 2013. “Satellite modelling studies showed that aerosols caused formation of a large number of cloud droplets that are relatively smaller in size. These release a lot of energy, or latent heat. Then, the droplets are pushed vertically, crossing the freezing line. This is also an energy-extensive process that fuels cloud growth,” said professor SN Tripathi of department of Centre for Environmental Science and Engineering at IIT-Kanpur.
In simple terms, he explained, “more aerosols can cause intense rain in a relatively short period. It can also delay the onset of rain during monsoon”. Aerosol Optical Depth — the degree to which aerosols (airborne solid and liquid particles) prevent transmission of light in an area — is also considered an indirect proxy for air quality. In this study, scientists found AOD in the summer monsoon region during the monsoon months to be 0.6. There is no safe standard for AOD, but in cleaner areas such as in the US, the AOD is about 0.1.
A study by Princeton University in 2011 had indicated that monsoon rain was decreasing over the years, mainly because of anthropogenic aerosol emissions.
Professor Sagnik Dey, associate professor at Centre for Atmospheric Sciences at IIT-Delhi, said: “The frequency and intensity of rainfall during monsoon is getting affected. When it’s raining, it’s raining very heavily and the monsoon is often not progressing smoothly. The aerosol load is higher over the Indo-Gangetic plains, the heating gradient is higher and the cloud patterns are also changing.”
According to Pallavi Pant, a US-based scientist: “The role of aerosols in rainfall is quite complex. Presence of particles in the atmosphere can influence cloud formation because they can act as nuclei or seeds for clouds to form. In some cases, a higher number of particles in the atmosphere can result in heavier rainfall.”
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