Tornadoes

At a Glance

• Parent storm Supercell thunderstorm
• Diameter Tens of metres to over a kilometre
• Wind speed Up to about 500 km per hour in the strongest
• Hotspot Tornado Alley in the central United States
• Scale Fujita and Enhanced Fujita (EF0 to EF5)
• Exam link UPSC Prelims, Geography: Storms, Cyclones, Jet Streams and Air Masses

Where Does It Fit?

This is the second topic of the category on storms, cyclones, jet streams and air masses. The previous topic described thunderstorms, from which tornadoes grow, and the next topic turns to the much larger tropical cyclones. Previous topic: Thunderstorms, Lightning and Hailstorms. Next topic: Tropical Cyclones – Formation and Conditions.

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, as in Thunderstorms, Lightning and Hailstorms.

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 Jet Streams. 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 and Conditions.

Why Is This Topic Important?

• Concepts Formation and Tornado Alley
• Comparison Tornado versus cyclone
• India Pre-monsoon tornadoes in the east

What Came Next?

The next topic describes the formation and conditions of tropical cyclones. Next topic: Tropical Cyclones – Formation and Conditions.

Key People and Terms

• Tornado Rotating column of air from cloud to ground
• Supercell Rotating thunderstorm
• Mesocyclone Rotating updraft of a supercell
• Funnel cloud Condensation funnel not reaching ground
• Waterspout Tornado over water
• Fujita scale Damage-based tornado rating

Exam Corner

Mains Angle

Questions compare tornadoes with cyclones. These points add depth.

• Origin Supercell and wind shear
• Scale Small but intense
• Vulnerability Density and housing