Tropical Cyclones – Formation and Conditions

At a Glance

• Sea temperature At least 26.5 degrees Celsius to 50 m depth
• Latitude Roughly 5 to 20 degrees, away from the equator
• Energy source Latent heat of condensation
• Structure Eye, eyewall, spiral rainbands
• Biggest killer Storm surge
• Exam link UPSC Prelims, Geography: Storms, Cyclones, Jet Streams and Air Masses

Where Does It Fit?

This is the third topic of the category on storms, cyclones, jet streams and air masses. The previous topic described tornadoes, and this one explains the large tropical cyclone. The next topic covers the path, intensity scales and breeding grounds. Previous topic: Tornadoes. Next topic: Path, Intensity Scale and Breeding Grounds of Tropical Cyclones.

What Is a Tropical Cyclone?

In short: a tropical cyclone is a giant heat engine over warm seas, whose fuel is the latent heat in sea water vapour.

A tropical cyclone is a warm-core, low pressure storm system, 300 to 1,500 km across, with winds that spiral inward and upward around a centre. In the northern hemisphere the winds blow anticlockwise, and in the southern hemisphere clockwise, because of the Coriolis force, as in Forces Affecting Wind Movement. The same storm is called a hurricane in the North Atlantic and northeastern Pacific, a typhoon in the northwestern Pacific, a tropical cyclone in the Indian Ocean and Australia, and willy-willy in northern Australia. It is thus a regional name and not a different kind of storm. The storm is a major hazard for the coasts of India, Bangladesh, the Philippines, China, Japan, the Caribbean and the southeastern United States.

What Conditions Are Needed for a Tropical Cyclone?

These conditions explain the geography. Cyclones do not form on the equator, in the cold waters of the South Atlantic and the southeastern Pacific where cold currents, as in Effects of Ocean Currents, cool the sea and the ITCZ lies north of the equator, or over land. Most cyclones form in late summer and autumn when the sea is warmest. In the north Indian Ocean there are two seasons, pre-monsoon in April to June and post-monsoon in October to December, because the strong shear of the summer monsoon suppresses cyclones from July to September.

How Does a Tropical Cyclone Develop?

1. Formation and development: warm moist air rises over the disturbance, creating a low pressure; air from the surroundings rushes in, is deflected by the Coriolis force and starts to spiral.
2. Latent heat feedback: as the rising air cools, vapour condenses into cumulonimbus clouds and releases latent heat, which warms the core, lowers the pressure further and draws in more moist air. This positive feedback is the engine, as in Water Vapour, Evaporation and Condensation.
3. Intensification: the pressure falls and the wind increases. The system moves up from a depression to a deep depression, then a cyclonic storm, and so on. Sometimes the storm intensifies by at least 30 knots in 24 hours, which is called rapid intensification.
4. Maturity and decay: the storm reaches its peak while over warm water. On landfall, or over cold water, the supply of moisture is cut off and friction increases, so it weakens in a day or two.

What Is the Structure of a Tropical Cyclone?

• Eye: the eye of the cyclone is a calm, clear and dry central zone, usually 20 to 50 km wide, where air sinks and warms. The pressure is the lowest here, and winds are light. When the eye passes, there is a deceptive lull before the other side of the eyewall arrives.
• Eyewall: a ring of tall cumulonimbus clouds around the eye with the strongest winds and heaviest rain.
• Spiral rainbands: curved bands of thunderstorms that extend outward for hundreds of kilometres.
• Outflow: at the top, air spreads out at high levels, usually in an anticyclonic pattern.

The most destructive effects are the storm surge, the rise of sea water caused by low pressure and onshore winds, which can raise the water several metres above the normal tide and flood low coasts; the winds; and the heavy rain with floods. Their effects are described in Naming, Warning and Effects of Tropical Cyclones.

How Is Climate Change Affecting Cyclones?

A warmer ocean gives more energy and more moisture. Scientific assessments conclude that the total number of tropical cyclones is not clearly rising, but the share of the strongest storms, and the rainfall within them, are increasing. Rapid intensification is occurring more often close to coasts, which leaves less time for warning, and the Arabian Sea, once less active, has produced more intense cyclones in recent years. As an analytical point, India’s cyclone deaths have fallen sharply because of better forecasts and mass evacuation, even as the storms have become stronger, which shows that adaptation matters as much as the hazard.

Why Is This Topic Important?

• Conditions Asked in Prelims and Mains
• Geography Why not near the equator
• Climate Stronger, faster-intensifying storms

What Came Next?

The next topic describes the paths, intensity scales and breeding grounds of tropical cyclones. Next topic: Path, Intensity Scale and Breeding Grounds of Tropical Cyclones.

Key People and Terms

• Tropical cyclone Rotating warm-core storm over tropical seas
• Eye Calm centre
• Eyewall Ring of the strongest winds
• Storm surge Sea level rise driven by the storm
• Rapid intensification Gain of 30 knots in 24 hours
• Wind shear Change of wind with height

Exam Corner

Mains Angle

Questions ask for conditions and structure. These points add depth.

• Energy Latent heat feedback
• Limits Cold currents and equator
• Trend Stronger storms with warming