Western disturbance too weak to bring respite, light rain could worsen air quality
The hopes of Delhiites and residents of surrounding areas for a cleaner Diwali will likely be dashed this year. A western disturbance (WD) currently affecting northwest India could have helped reduce the heightened air pollution levels, which are at over 400 parts per million in most of the city as of November 9, 2023. However, the WD appears too weak to provide relief.
The concentration of pollutants like particulate matter 10 (particles with size less than 10 micrometres, also called PM10) and PM 2.5 (with sizes less than 2.5 micrometres), will further increase as the festival of Diwali approaches. Smoke from firecrackers will add to it, creating a gas chamber, as has happened during the festival celebrations in most years since 2016.
Western disturbances are extratropical storms that travel from the Mediterranean region and bring most of the winter rainfall to the northwest and parts of central India and snowfall to the Himalayan region.
They also increase the wind speeds in the regions that they affect and cause thunderstorms. The wind and rainfall are the best natural agents that can bring down particulate matter from the air and deposit it on the ground.
A WD started affecting northwest India from the night of November 7 onwards, according to the India Meteorological Department (IMD). On November 8, the IMD predicted isolated light rainfall over Haryana, Chandigarh, Punjab and west Rajasthan for November 9 and over Haryana, Chandigarh and Delhi on November 10.
On November 9, the weather agency revised their prediction slightly by saying that Punjab could also receive isolated light rainfall.
“The approaching western disturbance doesn’t appear to be a strong one, so it is not expected to dump widespread and heavy rain in the plains of northwest India”, said Akshay Deoras, a research scientist at the National Centre for Atmospheric Science and Department of Meteorology, University of Reading, United Kingdom, told Down To Earth.
“At the most, Delhi-NCR may witness a light shower on November 10. The wind speed will increase slightly on November 10 and 11, so we can expect some improvement in the air quality, which might not be even noticeable in some parts,” he added.
“The wind speed will decrease from November 13 and no fresh WD is expected in the next week, so this will favour an increase in air pollution,” Deoras explained.
Further, if there is no heavy rainfall or significant winds, there could be increased moisture levels in the atmosphere. This could make the situation far worse, almost creating a gas chamber over Delhi. This is because the particulate matter will settle down on the water vapour droplets and stay there much longer, with no wind or rain to dispel them.
“Since the WD is weak, we can't expect a significant increase in near-surface moisture levels,” Deoras added.
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