Cyclones & Anticyclones – NDA Geography Notes

Among all weather phenomena, cyclones are the most dramatic and the most destructive. A tropical cyclone can release more energy in a single day than all the nuclear weapons ever built. A temperate cyclone can bring weeks of unsettled weather to an entire continent.

NDA tests this chapter heavily, from cyclone rotation direction and regional names to the structure of the eye, the conditions for formation, and comparisons between tropical and temperate cyclones. The Coriolis Effect plays a central role throughout.

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What is a Cyclone?

A cyclone is a large system of winds rotating around a centre of low pressure. Air flows inward toward the low pressure centre and rises.

Direction of rotation: [NDA 2009-I | NDA 2014-II]

In the Northern Hemisphere, cyclones rotate anticlockwise (counterclockwise): the Coriolis Effect deflects air to the right as it flows toward the low centre, creating an anticlockwise spin.

In the Southern Hemisphere, cyclones rotate clockwise: the Coriolis Effect deflects air to the left.

Air movement: Air flows inward and upward. Convergence at the surface forces air upward, creating clouds and precipitation. This is why cyclones bring cloudy and rainy weather.

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What is an Anticyclone?

An anticyclone is a large system of winds rotating around a centre of high pressure. Air sinks and flows outward from the high pressure centre.

Direction of rotation: Exactly the opposite of cyclones in each hemisphere.

In the Northern Hemisphere, anticyclones rotate clockwise. In the Southern Hemisphere, anticyclones rotate anticlockwise.

Weather associated with anticyclones: Calm, clear, and stable, because sinking air suppresses cloud formation and precipitation. In summer, anticyclones bring heat waves. In winter, they bring clear, cold, frosty conditions.

Feature	Cyclone	Anticyclone
Pressure	Low pressure centre	High pressure centre
Air movement	Inward and upward	Outward and downward
N. Hemisphere rotation	Anticlockwise	Clockwise
S. Hemisphere rotation	Clockwise	Anticlockwise
Weather	Cloudy, rainy, stormy	Clear, calm, stable

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Regional Names for Tropical Cyclones

The same weather phenomenon is called by different names in different parts of the world. This is one of the most heavily and most repeatedly tested facts in this entire chapter.

Name	Region	PYQs
Cyclone	Indian Ocean and Bay of Bengal	—
Hurricane	Atlantic Ocean and Caribbean	—
Typhoon	Western Pacific Ocean (China Sea, Philippines, Japan)	[NDA 2009-II
Willy-Willy	Australia	[NDA 2021-II]
Tornado	USA inland (completely different: much smaller)	—

Hurricanes, cyclones, and typhoons are all the same phenomenon: just in different ocean basins. They share the same formation mechanism and structure. A tornado is entirely different, much smaller and not ocean-related.

★ IMPORTANT NDA has tested the regional name for tropical cyclones in at least three separate papers (2009-II, 2017-I, 2021-II). The pattern: Typhoon = Western Pacific. Willy-Willy = Australia. These two are the most commonly confused options. [NDA 2009-II | NDA 2017-I | NDA 2021-II]

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Conditions for Tropical Cyclone Formation

Not every tropical ocean area produces cyclones. Specific conditions must exist. [NDA 2010-I]

1. Warm ocean water: sea surface temperature must be at least 26–27°C to a depth of about 60 m. This provides the heat and moisture that powers the cyclone. [NDA 2010-I]

2. Coriolis Effect: must be strong enough to initiate rotation. Cyclones do NOT form within about 5° of the equator where the Coriolis Effect is too weak. [NDA 2010-I]

3. Low vertical wind shear: winds at different altitudes must blow in the same direction and at similar speeds. High wind shear tears the cyclone apart.

4. Pre-existing atmospheric disturbance: a cluster of thunderstorms or an atmospheric wave gives the cyclone a starting point.

5. High humidity: through a deep layer of the atmosphere, providing sufficient moisture for sustained convection.

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Structure of a Tropical Cyclone

The Eye: at the centre of a mature tropical cyclone is the eye: a region of calm, clear skies and relatively low winds. Typically 20–50 km in diameter. The eye is calm because air is gently sinking there: in contrast to the violent uplift around it. [NDA 2010-I]

The pressure is lowest at the centre (the eye).

The Eye Wall: surrounding the eye is the eye wall: a ring of the most intense winds, heaviest rainfall, and most violent weather in the entire cyclone.

The eye of a tropical cyclone is WARMER than the surrounding regions, not cooler. Sinking, compressing air in the eye warms it. NDA 2010-I tested “temperature in the eye is lower than surrounding regions” as a FALSE statement. [NDA 2010-I]

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Movement and Energy of Tropical Cyclones

Movement: tropical cyclones typically move westward initially (driven by trade winds), then curve poleward, then eastward as they enter the westerlies.

Energy source: the energy source of a tropical cyclone is the latent heat of condensation: released when water vapour condenses inside the storm. This is why warm ocean water is essential.

Tropical cyclones weaken after landfall, because they are cut off from their energy source (warm ocean water).

Destruction: cyclones cause damage through: Storm surge (the most lethal: a wall of seawater pushed inland, causing most deaths) | Strong winds (over 300 km/h in intense cyclones) | Heavy rainfall (flooding far inland) | Spawned tornadoes.

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Temperate Cyclones

A temperate cyclone is a low pressure system that forms in the mid-latitudes (30°–60°) along the polar front: the boundary between cold polar air and warm tropical air. Unlike tropical cyclones, they do not need warm ocean water. They are powered by the temperature contrast between two air masses.

Also called wave cyclones or mid-latitude depressions.

Formation: a wave develops in the polar front. Cold air forms a cold front advancing on one side; warm air forms a warm front advancing on the other. Between them is a wedge of warm air, the warm sector. As the cold front (faster) catches the warm front, the warm air is lifted off the surface, creating an occluded front. After occlusion, the cyclone weakens as its energy (the temperature contrast) is consumed.

Characteristics:

• Forms along the polar front (~60° latitude)
• Moves from west to east: steered by the jet stream
• Much larger than tropical cyclones (2,000–3,000 km)
• Slower-moving; brings prolonged unsettled weather
• Has warm and cold fronts; no eye
• Winds rotate anticlockwise in the Northern Hemisphere (same as tropical cyclones: both are low-pressure systems)

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Tropical vs Temperate Cyclones

Feature	Tropical Cyclone	Temperate Cyclone
Location	Tropics (5°–20°)	Mid-latitudes (30°–60°)
Energy source	Latent heat from warm ocean	Temperature contrast between air masses
Size	Smaller (500–1,000 km)	Larger (2,000–3,000 km)
Wind speed	Higher (can exceed 300 km/h)	Lower (50–150 km/h)
Eye	Has a clear, calm eye	No distinct eye
Fronts	No fronts	Has cold and warm fronts
Movement	Westward then poleward	West to east
Season	Summer/autumn	Year-round; more frequent in winter
Symmetry	Roughly symmetrical	Asymmetrical

[NDA 2010-I]

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Tornadoes

A tornado is a violently rotating column of air extending from a cumulonimbus cloud to the ground. It is the most intense wind system on Earth. Wind speeds can exceed 500 km/h.

Key characteristics: Very small (typically 100–500 m wide) | Short-lived (minutes to an hour) | Funnel-shaped cloud | Extremely destructive along a narrow path.

Most common in: The central United States: Tornado Alley: where warm, moist air from the Gulf of Mexico meets cold, dry air from Canada.

Tornadoes are completely different from tropical cyclones: much smaller, much more intense per unit area, and not ocean-related.

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Thunderstorms

A thunderstorm is a storm with thunder, lightning, heavy rain, and strong winds, produced by a cumulonimbus cloud.

The most favourable conditions for a thunderstorm: [NDA 2020-I]

All four of the following are favourable: Very cold air at high altitudes (creates strong instability) | Very warm air at the surface (energy for convection) | Hot and humid conditions near the surface (high moisture for latent heat) | Strong upper-level winds (organise and sustain the storm).

Answer: All four conditions are correct. [NDA 2020-I]

Lightning is a massive electrical discharge. Thunder is the sound caused by rapid heating and expansion of air along the lightning channel. We see lightning before hearing thunder because light travels far faster than sound.

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