By – Prof. N. Deva Singh
Hailstorms are a relatively rare climatic phenomenon in Manipur. They typically develop as part of a severe thunderstorm under conditions of atmospheric instability, where strong updrafts and downdrafts develop within convective cells, leading to the formation of hailstones along with heavy rainfall. In milder thunderstorms, only a few such cells may form, depending on localized instability. The devastating hailstorm that struck Manipur on the afternoon of Sunday, 5 May 2024, served as a clear warning of increasing climatic hazards in the region. Earlier, the typical pre-monsoon (March–May) weather instability in the state involved local tropical winds and eddies that produced heavy showers, gusty winds, mild thunder, and lightning from cumulus and cumulonimbus clouds. On a few occasions, strong squalls exceeding 30 km per hour had damaged or uprooted standing crops, trees, and household properties in both hill and valley areas, mostly before the arrival of the southwest monsoon.
The people of Manipur have long lived in a changing natural environment. The state features vast undulating terrain ranging from below 500 m to about 3000 m above sea level, with exposed, deforested hills and sparsely vegetated valley lands, amid the shrinking Loktak Lake ecosystem and dwindling wetlands. The Tropic of Cancer (23.5°N) lies just south of Manipur, meaning the sun will be directly overhead around 21–22 June (summer solstice).
Similar weather patterns occur across the Gangetic plains, Northeast India, and adjoining parts of western Myanmar. In June, solar radiation reaches approximately 650–700 cal/cm²/day in these areas, following the same seasonal pattern dictated by the geometry of Earth’s tilt and its orbit around the Sun. This axial tilt is responsible for the changing seasons, equinoxes, and solstices. Key geographical factors influencing the region’s climate include moisture from the Bay of Bengal to the south, the offshoots of the Himalayas and associated mountain ranges to the north, the strategic position of the southern branch of the circumpolar Westerly Jet Stream near 30°N, and the vast plains to the west. These interact with the southwest monsoon, northeast monsoon, and western disturbances. Pre-monsoon is a period of rising temperatures following winter and the spring equinox (March). In West Bengal, Bihar, Uttar Pradesh, Madhya Pradesh, Odisha, and Bangladesh, this season commonly brings “Nor’westers” (locally known as Kalbaisakhi) — intense thunderstorms. During March–May over the Gangetic plains, intense solar heating creates low pressure over land and high pressure over the Bay of Bengal. Moisture-laden winds from the sea, combined with pre-existing low-pressure systems in the troposphere over northern India, the swift jet stream above 12 km, local topography, and prevailing winds, contribute to the formation of destructive thunderstorms. Western disturbances may not be the primary driver but can enhance local heating and convection during this period. Similar localized thunderstorms with torrential rain and hail, usually occurring in the afternoon before sunset, are also observed in Assam, where they are called Bordoisila. The mechanisms behind Kalbaisakhi and Bordoisila appear comparable.
These same weather instability conditions help explain how the severe thunderstorm developed over Manipur on 5 May 2024, producing large hailstones, high-speed winds, and heavy rainfall. Satellite data from NOAA-related apps showed that from morning until noon, clouds over the northern and eastern regions moved eastward and eventually merged, covering a large area including about two-thirds of Manipur. By that time, surface temperatures had likely risen above 33°C due to intense heating, creating low pressure ahead of the thunderstorm. A similar mechanism of atmospheric instability during the pre-monsoon season led to the hailstorm on 15 March this year. Such devastating climatic events are likely to remain a continuing vulnerability until the onset of the Southwest Monsoon.
Possible Factors for the Unstable Atmosphere
i. Intense solar heating: Extensive areas of low-albedo surfaces, including valley settlements, dry wetlands, exposed construction sites, paddy fields, and heavily degraded forests in the hills, acted as key drivers for generating mesoscale convective currents of warm air. This warm surface air created strong buoyancy, promoting powerful updrafts.
ii. Moisture-laden winds: Cooler, moist air (likely from the west or southwest) added fuel to the developing thunderstorm, in addition to pre-existing clouds. The convergence of two different air masses created a mechanism for strong converging winds. When cooler air overlays warmer air, it generates atmospheric turbulence and buoyancy, leading to the rapid growth of cumulus and cumulonimbus clouds capable of producing severe thunderstorms. Over the vast hill and valley terrain, this resulted in unusually large hailstones falling from storm cells developing roughly between 3 to 6 km altitude. The role of western disturbances (which can persist in India until October) or moisture supply from the Bay of Bengal may have been significant.
Continued anthropogenic activities and global warming, combined with extensive land-use and land-cover changes in Manipur, appear to be increasing the frequency and intensity of such weather-related catastrophic events.
Hailstorm Formation
On that day, hot and dry air over Manipur likely interacted with cooler, moisture-laden air from the west or southwest, triggering a powerful thunderstorm with strong updrafts. This led to the formation of multiple thunderstorm cells, each a few kilometers in diameter. Within these cells, water vapor rapidly condensed into water droplets, then into ice nuclei, and finally into supercooled ice particles at higher altitudes. The release of latent heat (approximately 600 cal/g during condensation from gas to liquid and 80 cal/g during freezing from liquid to solid) added energy to the system, intensifying the storm.
Ice crystals at temperatures below 0°C become positively charged, while those at the base of the cloud tend to be negatively charged. As hailstones grow by colliding with supercooled droplets and fall through downdraft zones, they acquire different sizes. Lightning occurs when positively and negatively charged particles collide, producing a sudden electrical discharge that heats the air explosively, resulting in thunder. Lightning may remain within the cloud or strike the ground, the latter causing casualties and property damage.
The heavy rainfall accompanying the hailstorm indicated high atmospheric moisture content. Winds probably exceeding 50 km per hour suggested powerful updrafts capable of carrying water droplets into the freezing zone. Large hailstones reaching the ground imply that ice particles remained suspended long enough in the upper cold layers or fell from relatively low heights. Localized strong downdrafts and wind shear associated with the storm caused significant damage, including breakage of corrugated iron (CI) sheets on rooftops when struck by large hailstones. (For comparison, hailstones weighing up to 3 kg have been reported in the United States, where thunderstorms are more frequent.)
The entire event was short-lived both temporally and spatially. At the dissipation stage, all convective cells exhibited sinking motion, and precipitation was exhausted through the downdrafts.
Precautions During Pre-Monsoon
Local residents in both hills and valleys should remain alert for thunderstorms, particularly in the afternoon when surface heating is intense. If lightning is observed, it is safer to crouch on dry ground without touching any metal objects (including mobile phones) and to stay away from trees and tall metal structures connected to wires. If a safe RCC (reinforced cement concrete) building is nearby, take shelter there until the storm passes. Drivers should avoid moving vehicles and park in a safe location.
Knowing the fact of global warming, Long-term measures include:
- Avoiding the use of low-albedo materials in construction wherever possible.
- Rejuvenating wetlands and restoring ecosystems.
- Promoting thick vegetation cover and maintaining green paddy fields year-round.
- Preventing further intensification of the urban heat island effect, especially in the valley.
In Manipur,the Southwest Monsoon is expected to arrive in June, marking the beginning of the monsoon cycle. During this season, the region typically experiences heavy rainfall, often accompanied by intense and sometimes harsh climatic events.
