Thunderstorms and Tornadoes

At a Glance

• Thunderstorm cloud Cumulonimbus
• Tornado Rotating funnel reaching the ground
• Scale Fujita scale for tornadoes
• Tornado Alley Great Plains of the USA
• Indian thunderstorms Kalbaisakhi and mango showers
• Exam link UPSC Prelims, Geography: Climatology

Where Does It Fit?

This topic follows air masses and fronts and covers the small, violent storms they produce. They are brief but cause heavy damage, and they are a frequent topic in disaster management as well as in climatology. Previous topic: Air Masses, Fronts and Extra-Tropical Cyclones. Next topic: Humidity, Saturation, Dew Point, Evaporation and Condensation.

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 westerlies, polar easterlies Seasonal Monsoons, land and sea breezes Local hot winds Loo, sirocco, khamsin, foehn, chinook…">sea breeze or a mountain slope. Warm moist air rises, cools, condenses and releases latent heat, which makes it rise even faster, as in Humidity, Saturation, Dew Point, Evaporation and Condensation. The result is a cumulonimbus cloud that can grow 10 to 15 km tall, as described in Types of Clouds: Cirrus, Cumulus, Stratus, Nimbus and Their Combinations. 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 Types of Precipitation: Convectional, Orographic, Cyclonic, Frontal, Monsoonal.

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.

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 Types of Precipitation: Convectional, Orographic, Cyclonic, Frontal, Monsoonal.

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 Types of Clouds: Cirrus, Cumulus, Stratus, Nimbus and Their Combinations, 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 Seasonal and Local Winds: Land and Sea Breezes, Mountain and Valley 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.

What Is a Tornado?

In short: a tornado is a narrow, violently spinning column of air that links a thundercloud to the ground.

A tornado is a rotating column of air in contact with both the base of a cumulonimbus cloud and the surface. The visible funnel cloud is formed when the low pressure inside the vortex cools the air and condenses vapour; if the funnel does not reach the ground, it is only a funnel cloud. Tornadoes are small compared with cyclones, usually 50 to 200 metres wide and lasting a few minutes, but the strongest are more than a kilometre wide and travel over 100 km. The pressure at the centre is very low, and the pressure difference over a few metres produces extreme winds and the explosive damage of buildings. The parent cloud is a cumulonimbus.

How Does a Tornado Form?

1. Instability: warm, moist air lies near the ground below cold, dry air aloft.
2. Wind shear: the wind changes speed and direction with height, which creates a horizontal rolling tube of air.
3. Tilting: a strong updraft in a thunderstorm tilts the rolling tube upright, producing a rotating updraft called a mesocyclone, typically 2 to 10 km wide. This is the structure of a supercell.
4. Tightening: a downdraft near the mesocyclone and stretching of the vortex make the rotation spin faster, like a skater pulling in the arms, and a funnel descends to the ground.

Not every supercell makes a tornado, and forecasters still cannot say exactly which will. Warnings rely on Doppler radar, which sees the rotation inside the storm, and on spotters. The lead time is only around ten minutes on average.

Where Do Tornadoes Occur?

Tornadoes are found on every continent except Antarctica, but the United States has the most, with more than a thousand a year. The region of the central Great Plains is called Tornado Alley, where warm, moist air from the Gulf of Mexico meets cold dry air from Canada and dry air from the Rocky Mountains, with a strong jet stream overhead, as described in tropopause Minimum speed Roughly 110 km per hour Polar front jet…">Jet Streams: Permanent and Temporary Types and Their Impact on Weather. The season peaks in spring and early summer. Other regions include:

• Bangladesh and eastern India: tornadoes in the pre-monsoon months of March to May, in the same season as Kalbaisakhi storms; the deadliest tornado on record, the Daulatpur-Saturia tornado of 1989 in Bangladesh, killed an estimated 1,300 people.
• Europe, Argentina, South Africa and eastern Australia: smaller numbers in temperate zones.
• Waterspouts: tornadoes over lakes or seas, usually weaker, common near the Florida Keys and the Mediterranean.

In India, tornadoes are rare and smaller than in the United States, but they do occur, in West Bengal, Odisha, Bihar and Uttar Pradesh. In some years they have caused deaths, and the damage is often reported as a storm because there is little tornado monitoring. As an analytical point, the high toll in Bangladesh and eastern India is caused less by strength than by dense population and weak houses.

How Is the Strength of a Tornado Measured?

The original Fujita scale, proposed by Ted Fujita in 1971, rated tornadoes F0 to F5 from the damage. The Enhanced Fujita scale replaced it in the United States in 2007, using damage indicators for a range of structures. Because the rating is based on damage, a strong tornado in an empty field may be rated low. A tornado is different from a tropical cyclone: it is small and short-lived, has no eye and forms over land from a thunderstorm, while a cyclone is hundreds of kilometres wide, lasts days and forms over warm oceans, as in Tropical Cyclones: Formation Conditions, Paths, Naming Conventions, Early Warning Systems.

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 explains humidity, saturation, dew point, evaporation and condensation, which underlie all clouds and storms. Next topic: Humidity, Saturation, Dew Point, Evaporation and Condensation.

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