Big duststorms and lightning are not unusual at this time. Why do they occur, why did so many die?
Over 100 people were killed in duststorms, thunderstorms, and lightning at many places in northern, central and eastern India on Wednesday. The National Disaster Management Authority (NDMA) said “lightning and thunderstorm” killed 62 people in Uttar Pradesh and 32 in Rajasthan; the UP government issued a districtwise break-up of deaths that added up to 70.

Expected, unexpected
These casualty figures seem abnormally high for weather events on a single day. However, there was nothing unusual in the occurrence of the weather events themselves. Rainstorms, duststorms, and even tornadoes are expected at this time of the year, and the Meteorological Department routinely issues alerts and warnings. The weather events of Wednesday, too, had been predicted, and warnings were issued.

How the storms built up
Rainstorms and dust storms arise from similar meteorological conditions. They are almost always preceded — and caused — by a spell of intense heat. Thunderstorms or hail occur when the atmosphere has moisture; when it doesn’t, dust storms take place.

Many parts of India witness a build-up of surface heat during this time of the year. The places that were hit by the storms had seen heat-wave like conditions last week.

Dr Mrutunjay Mohapatra of India Meteorological Department said such events take place due to a local “instability” arising out of a deviation from the normal temperature difference between the upper and lower atmosphere. In this case, for example, moist easterly winds from the Bay of Bengal reached up to Himachal Pradesh, which was also receiving dry winds from the north-westerly direction. The two systems destabilized the equilibrium temperature difference between the upper and lower layers of atmosphere, leading to conducive conditions for a thunderstorm.

The final trigger, however, is the development of a largescale air-circulation system. In this particular case, the trigger came from the circulation system that developed over Rajasthan a couple of days ago.

Why so many deaths, then?
It does seem odd. However, a large number of deaths over a few days have been reported earlier. In June 2016, more than 300 people were reported killed by lightning over three days. Lightning is the biggest killer in India among natural calamities. In 2014 and 2015, it killed 2,582 and 2,641, show National Crime Records Bureau data.

What was unusual about Wednesday’s events was that they occurred over a large geographical area within a very short time. Dr Sunil Pawar of the Indian Institute of Tropical Meteorology, Pune, explained that severe thunderstorm clouds can sometimes arrange themselves within a few hours in long “squall lines” of 150-250 km length.

What use the predictions?
In most cases, storms do not kill by themselves. Even lightning rarely strikes people directly. But they trigger incidents that result in deaths. Walls or homes collapse, and people are electrocuted after power lines snap, or after they are caught in fields filled with water. People in the poorest, most densely populated areas are the most vulnerable.

Also, meteorological predictions are for broad geographical areas and timeframes. The events themselves are, however, very localised, both in time and space. It is not yet possible to predict a thunderstorm or lightning at a precise location — say a village or a part of a city.

The exact times these events will hit, too, cannot be predicted. Alerts and warnings are in the nature of a general advisory, telling the people to expect these events, and to take precautions.

(Adapted from The Indian Express)