Several of India’s northern states – particularly Uttar Pradesh, Rajasthan and Delhi – have experienced a spate of severe thunderstorms and dust storms with strong winds and lightning in the past month. The storms have killed over 124 people and left a trail of devastation.

According to the Indian Meteorological Department, the thunderstorms in North West India this year are peculiar. “[The] peculiarity of thunderstorms this season is the heating in North West India that has coincided with the movement of western disturbances,” said KJ Ramesh, director general of meteorology at the India Meteorological Department.

A western disturbance is a storm that originates in the Mediterranean region, carries moisture from the Mediterranean Sea, Atlantic Ocean, Caspian Sea and the Arabian Sea and travels from west to east. Also described as an area of reduced air pressure, it is commonly associated with precipitation (rain, snow, hail) in North India during the winter months. Ramesh said the frequency of western disturbances was low this winter but that they are now “happening back to back”.

And it is these western disturbances that have reportedly triggered the thunderstorms in North West India, with some remnant impact in eastern India. “These thunderstorms move from west to east and there is more moisture incursion from the Bay of Bengal that sustains these storms,” Ramesh said.

North East India and East India are no strangers to thunderstorms in the April to June period, which is referred to as Kal Baisakh, where kal refers to Yam, the god of death. It is associated with violent thunderstorms and lightning that often cause destruction and kill people. The storms are called Kal Baisakhi or nor’westers. The frequency of these storms is usually highest in the North East (30 days to 40 days between March and May).

“Thunderstorms in Kal Baisakh are nothing unusual in our part of the country,” said Debasish Lohar, head of the department of physics at Jadavpur University, Kolkata. “But this year has been particularly confusing. These thunderstorms are refusing to abate, and after every few days we are getting heavy rainfall. The intensity of these storms is also much higher this year.”

Lohar, who specialises in thunderstorms, said eastern India has two giant permanent systems, the Bay of Bengal and the eastern Himalayan range, that help build thunderstorms in this region in April, May and June. “In 2008 also, we had a large number of thunderstorms, but this year the situation is very different,” he explained. “A large number of thunderstorms in our region are due to the northern factors.”

The northern factors are the dust storms and thunderstorms that have battered large parts of North and North West India in the past month.

Thunderstorms and dust storms across North India in the past month have left over 124 people dead and caused massive damage. (Credit: Reuters)

Thunderstorm: Factors and trigger

A thunderstorm, mostly a short-duration phenomenon that seldom lasts over two hours, is always accompanied by thunder and lightning, usually with strong gusts of wind, heavy rain, and sometimes with hail. India recorded a total of 3,620 thunderstorms in 2014 and 5,536 in 2015.

“Genesis of thunderstorm is dependent on four broad factors – intense heating, moisture availability, instability in the atmosphere, and a trigger,” said M Mohapatra, additional director general of meteorology at the Met department’s National Weather Forecasting Centre.

The first condition for a thunderstorm to occur is for the lower level troposphere (the lowest region of the atmosphere) and the surface of the earth to be hot. The heat makes the parcel of air light and lowers the density of the atmosphere. If moisture is available, the air becomes moist. Since hot, moist air is lighter than dry air, it rises.

As the air rises, it transfers heat from the surface of the earth to the upper levels of the atmosphere (the process of convection). The water vapour it contains begins to cool, releases heat, condenses and forms a cloud. The cloud eventually grows upward into areas where the temperature is below freezing and ice particles can be created from freezing liquid drops.

Instability in the atmosphere is the third factor in the building of a thunderstorm and is explained through lapse rate, which means change of temperature with height. The last factor is a trigger.“[The] Trigger helps the air move up rapidly. And this trigger is a weather system; it could be a western disturbance or a cyclonic circulation, or a trough line [an elongated low pressure region],” explained Mohapatra.

Most thunderstorms form in three stages. In the cumulus stage, storm clouds form and are pushed upward by a rising column of air (updraft). In the mature stage, the updraft continues to feed the storm, but precipitation begins to fall out of the storm, creating a downdraft, or a column of air pushing downward. Eventually, a large amount of precipitation is produced and the updraft is overcome by the downdraft, which is when the dissipating stage begins.

“Updraft and downdraft can lead to generation of electricity inside the cloud and sometimes cause cloud to ground lightning, which kills several people every year,” said Devendra Pradhan, deputy director general of meteorology, Regional Meteorological Centre, New Delhi. A downdraft has very strong winds, which leads to destruction during thunderstorms, added Pradhan.

Changing pattern of western disturbances

Experts have attributed the “above normal” thunder activity over North West India this year to the late western disturbances.

As per the Met office, seven western disturbances have already passed over the region since April – including three intense ones in the first half of May. This seems higher than usual as a 2017 study has documented an average of two to three western disturbances a month across North West India in the summer months.

On May 13, the wind gust over Delhi clocked 107 km per hour, the third highest since 2000 (with the highest wind gust recorded in Delhi being 141 km per hour on May 10, 2006), said Mohapatra.

Naresh Krishna Vissa, assistant professor in the department of earth and atmospheric sciences at the National Institute of Technology Rourkela, Odisha, said, “Western disturbances used to be very active in the month of January over North India. But, they seem to be getting pushed to [the] spring season. We don’t have enough past data to reach a conclusion, but this area needs more research as it directly impacts our weather.”

The changing pattern of western disturbances has been documented in some research work. Last year, a study, titled “An investigation of reduced western disturbance activity over North West India in November-December 2015 compared to 2014 – A case study”, showed reduced western disturbance activity over the region.

But, scientific opinion on the changing pattern of western disturbances and their impact on thunderstorms is divided. “There may be some impact of western disturbances on thunderstorms but, to my mind, these disturbances cannot be termed a trigger for thunderstorms,” said ANV Satyanarayana, head and associate professor, Centre for Oceans, Rivers, Atmosphere and Land Sciences at the Indian Institute of Technology Kharagpur. “The trigger has to come from more surface level atmosphere.”

Reportedly related to the western disturbances are jet streams, or river-like currents of air that circulate in the upper levels of the troposphere (30,000 feet). A jet stream exists because of the temperature difference between the Poles and the tropical regions. But, because the Arctic is warming at double the rate of the rest of the world, this temperature difference is reducing, thereby affecting the jet streams.

According to Jadavpur University’s Lohar, “Changes in western disturbances and jet streams seem to be impacting our weather and we need scientific examination of the same.”

Experts say the intensity of the storms this year is much higher. (Credit: AFP)

CAPE factor

Satyanarayana and his team are working on a hypothesis of the strong variability in a conducive environment for the generation of thunderstorms. “We are broadly looking at two factors, CAPE [Convective Available Potential Energy, or simply put, the amount of energy available to speed up a parcel of air vertically] and wind shear [change in wind speed and/or direction with height],” he said. “More heat means higher CAPE, which causes higher updrafts linked to thunderstorms. Less horizontal wind shear means vertical lifting would be higher, which supports thunderstorms. Thus, a higher CAPE and less horizontal wind shear is conducive to thunderstorms,” he explained.

Satyanarayana added that from a reading of available data, it has been observed since the 1970s that the conducive atmosphere for thunderstorms over Gangetic West Bengal is increasing and so is CAPE. “But only a higher CAPE does not ensure thunderstorms,” he cautioned. “There is another important factor called CIN [convective inhibition, which prevents an air parcel from rising from the surface]. Only when CAPE is high and CIN is less we get thunderstorms. So, all these factors need to be studied closely.”

Rising heat

According to Sandeep Sahany, assistant professor at the Centre for Atmospheric Sciences, Indian Institute of Technology Delhi, rising heat is another contributing factor. “The likelihood of increased buoyancy-related instability in the atmosphere, combined with greater moisture in the air due to increased temperature, and conducive vertical wind shear could play a very important role. This, in turn, would help deep cloud formation linked to thunderstorms,” he said. He agreed that factors such as CAPE need to be studied more thoroughly to understand the linkages between changes in CAPE and the occurrence of severe thunderstorms.

Satyanarayana said the urban heat island effect would have an impact on thunderstorms, as surface temperatures have changed over Indian cities because of land-use and land-cover changes. “This impacts the lower atmosphere and can influence the trigger mechanism of thunderstorms,” he said.

Research and data collection

In 2005, the Department of Science and Technology had launched the Severe Thunderstorm Observation and Regional Modelling or STORM project “to improve understanding and prediction of severe thunderstorms”. The project was later known as SAARC STORM, after it was extended to all the countries in the South Asian regional bloc, and complimented the World Meteorological Organisation’s Severe Weather Forecast Demonstration Project. “Research progressed under SAARC STORM project is helpful to better forecast thunderstorms over different parts of [the] Indian subcontinent. There is a significant improvement in forecasting thunderstorms over India in the past decade,” said Bhishma Tyagi, assistant professor in the department of earth and atmospheric sciences, National Institute of Technology Rourkela. “However, we still do not have adequate observational data to feed into the models that forecast these severe weather events. And, no forecast model can be made perfect without observational datasets,” he added.

What is needed, according to Tyagi, is regular observations of atmospheric variables and parameters – for example, water vapour, temperature, wind speed and direction, pressure. A study of the thermodynamic structure (related to heat and temperature) of the atmosphere over a particular place and an analysis of associated indices – CAPE, lifted index (a measure of tropospheric instability), humidity index, Severe Weather Threat Index (which incorporates instability, wind, shear and wind speeds) – would also give a better understanding of the occurrence of thunderstorms and help weather agencies forecast weather conditions expected in the short term (which is known as nowcasting), he added.

Tyagi said the collection of such data would require more equipment, and hence more financial support for thunderstorm research. “This lack of high spatial and temporal resolution observations is mainly responsible for so called vague forecasts of thunderstorms,” he added. In the wake of the death and damage caused by the thunderstorms this past month, the Met department and other weather agencies have been criticised for issuing vague forecasts.

However, the Regional Meteorological Centre’s Pradhan said that while research was imperative, even with available knowledge, it was possible to issue specific alerts and save lives. “For minimising damages, three things are needed,” he said. First, the Met department’s forecast has to be specific and conveyed in a people-friendly language. Second, state governments, disaster management groups, community groups and the Met department have to work together for larger and quicker dissemination of information. And finally, people need to be prepared in how to respond to severe weather events.

Nidhi Jamwal is a journalist in Mumbai.