Thunderstorms, Lightning and Hailstorms

At a Glance

• Parent cloud Cumulonimbus
• Stages Cumulus, mature, dissipating
• Lightning temperature About 30,000 degrees Celsius
• Hail Layered ice formed in strong updrafts
• Indian examples Kalbaisakhi, Bordoichila, pre-monsoon storms
• Exam link UPSC Prelims, Geography: Storms, Cyclones, Jet Streams and Air Masses

Where Does It Fit?

This is the first topic of the category on storms, cyclones, jet streams and air masses. The previous category ended with precipitation and the types of rainfall, and this topic describes the violent storms that grow out of convectional rain. The next topic covers tornadoes. Previous topic: Precipitation and Types of Rainfall. Next topic: Tornadoes.

What Is a Thunderstorm and How Does It Form?

In short: a thunderstorm is a short-lived storm from a cumulonimbus cloud, powered by warm, moist, rising air.

A thunderstorm forms when three ingredients come together: moisture in the lower air, an unstable atmosphere in which a rising parcel stays warmer than its surroundings, and a trigger that lifts the air. The trigger may be intense surface heating, a front, a sea breeze or a mountain slope. Warm moist air rises, cools, condenses and releases latent heat, which makes it rise even faster, as in Water Vapour, Evaporation and Condensation. The result is a cumulonimbus cloud that can grow 10 to 15 km tall, as described in Clouds, Fog and Forms of Condensation. Thunderstorms are most frequent in the tropics, where the heating is strong, and over land in the afternoon, and they are the most common form of convectional rainfall, as in Precipitation and Types of Rainfall.

A single-cell storm is brief, but a multicell storm or a supercell, which has a rotating updraft, can last for hours and produce large hail, damaging winds and tornadoes, as in Tornadoes.

How Do Lightning and Thunder Occur?

Inside a cumulonimbus cloud, strong updrafts carry small ice crystals and water droplets, and these collide with heavier graupel, a soft pellet of ice, that is falling. The collisions separate electric charges, so that the upper part of the cloud is mostly positively charged and the lower part negatively charged. The ground below becomes positively charged by induction. When the voltage becomes large enough to break down the insulating air, a huge spark, called lightning, passes between the charged regions.

• Intra-cloud and inter-cloud lightning: the most common kind, between charged regions within or between clouds.
• Cloud-to-ground lightning: the kind that kills people and animals, striking tall objects such as trees, towers and people in open fields.
• Thunder: the lightning channel is heated to around 30,000 degrees Celsius, several times hotter than the surface of the Sun, and the air expands explosively, creating a shock wave that we hear as thunder.

Light travels at about 300,000 km per second and sound at about 340 metres per second, so we see lightning first. Counting the seconds between flash and thunder and dividing by three gives the distance of the strike in kilometres. If the gap is less than 30 seconds, the storm is close enough to be dangerous.

How Does Hail Form and Why Is It Damaging?

A hailstorm is a thunderstorm in which ice pellets, hailstones, reach the ground. Hail forms in the freezing part of a cumulonimbus cloud, where a strong updraft carries supercooled water droplets upward again and again. A frozen nucleus collects a layer of ice each time it goes up, and the stone grows in concentric layers, like an onion, until it is too heavy for the updraft and falls. A larger hailstone needs a stronger updraft, which is why the biggest hail is linked to supercells. Hail is different from sleet, which is frozen rain, as in Precipitation and Types of Rainfall.

In India, hailstorms damage standing crops such as wheat, mustard, fruit orchards and tea, especially in March and April, just before the rabi harvest, and they occur most often in the northwest and in the north-eastern hills. Hail can also dent vehicles and break roofs. Suppression by seeding clouds has been tried in several countries, but its effect is unproven, as in Clouds, Fog and Forms of Condensation, so crop insurance and early warning are the practical defences.

Which Thunderstorms and Cloudbursts Matter in India?

• Kalbaisakhi or nor’westers: violent pre-monsoon thunderstorms over West Bengal, Jharkhand, Odisha and Bangladesh in April and May, called Bordoichila in Assam, as in Planetary, Seasonal and Local Winds. They bring squalls, hail and rain that cools the heat, but also damage crops and houses.
• Pre-monsoon and mango showers: convectional storms in the south and the west that help fruit and coffee.
• Cloudburst: a sudden, very intense rain over a small area; the India Meteorological Department defines it as rainfall of 10 cm or more in an hour over an area of roughly 20 to 30 square kilometres. Orographic lifting over the Himalaya makes cloudbursts common in Uttarakhand, Himachal Pradesh and Jammu and Kashmir, and they trigger flash floods and landslides.

Lightning is a major killer in India. Government data on accidental deaths show lightning as one of the largest single causes of death from natural forces, with well over two thousand deaths a year, and most victims are farmers working in open fields in Bihar, Uttar Pradesh, Madhya Pradesh, Odisha and Jharkhand. The Indian Institute of Tropical Meteorology developed the Damini app, which warns users of lightning within about 20 km, and several states have declared lightning a state-specific disaster. As an analytical point, the toll is far higher than the number of deaths from many headline disasters, but it gets little attention because the deaths are scattered, so low-cost measures such as early warning, lightning arrestors and safe shelters in rural areas give a large payoff.

How Can You Stay Safe in a Thunderstorm?

Move indoors or into a closed vehicle with a metal roof. Avoid open fields, hilltops, isolated trees, water and metal poles. If caught outside, crouch low on the balls of the feet with the feet together and head down, without lying flat. Unplug sensitive equipment, because a surge can travel through wires. Wait for 30 minutes after the last thunder before going out again.

Why Is This Topic Important?

• Concepts Stages and lightning asked
• Disaster management Lightning is a leading killer
• India Kalbaisakhi and cloudbursts

What Came Next?

The next topic describes tornadoes, the most violent rotating storms that grow out of severe thunderstorms. Next topic: Tornadoes.

Key People and Terms

• Cumulonimbus Thundercloud
• Lightning Electrical discharge in a storm
• Thunder Sound of heated, expanding air
• Supercell Long-lived thunderstorm with a rotating updraft
• Cloudburst Very intense rain over a small area
• Kalbaisakhi Nor’wester of eastern India

Exam Corner

Mains Angle

Questions ask about thunderstorms as a hazard. These points add depth.

• Cause Moisture, instability, lift
• Impact Crops, lives, flash floods
• Response Warning and rural safety