Tropical Cyclone Formation

What is a tropical cyclone?

A tropical cyclone is a rotating low-pressure weather system that develops over warm tropical or subtropical oceans. It contains organised thunderstorms, heavy rainfall and winds circulating around a central area. These storms transfer heat and energy from the tropics towards cooler regions.

The regional names describe the same type of weather system. At hurricane strength, they are called hurricanes in the Atlantic and eastern North Pacific, typhoons in the western North Pacific, and cyclones in the Indian Ocean and South Pacific. A developing system begins as a tropical depression, becomes a tropical storm as its winds strengthen, and reaches hurricane or typhoon strength when sustained winds reach about 119 km/h.

How global circulation creates source areas

A cyclone's source area is the region where it develops, rather than every place it later crosses. Global atmospheric circulation helps explain why these source areas are concentrated over tropical oceans.

Strong solar heating near the equator warms the ocean and encourages evaporation. Warm, moist air rises, creating a belt of low pressure. This rising air forms part of the Hadley cells. Farther from the equator, air sinks, producing subtropical areas of higher pressure. Near the surface, air flows back towards the equatorial low-pressure belt as the trade winds.

The north-east and south-east trade winds meet at the Intertropical Convergence Zone (ITCZ). Convergence means that air flows together; this forces air upwards and encourages clusters of thunderstorms. These clusters can provide the starting disturbance for a tropical cyclone, but most do not develop into one.

Earth's rotation produces the Coriolis effect, which helps air flowing towards low pressure develop a rotating circulation. It is too weak at the equator, so tropical cyclones normally form at least about 5° north or south. Their winds rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

The conditions and sequence of formation

Several conditions must occur together. Sea-surface temperatures normally need to be around 27°C or higher, with warm water extending to a depth of roughly 50–60 metres. The storm also needs moist air, converging surface winds, sufficient distance from the equator and low wind shear. If winds change greatly with height, they can disrupt the developing storm rather than allowing its thunderclouds to build vertically.

Formation follows a reinforcing sequence:

1. Warm seawater evaporates. This supplies moisture and energy to the air above it.
2. Warm, moist air rises. Surface pressure falls, drawing surrounding air towards the developing low-pressure centre.
3. The rising air cools and water vapour condenses. Towering cumulonimbus clouds form, producing heavy rainfall. Condensation releases heat, helping maintain uplift.
4. More air is drawn in. Continued rising air and the supply of ocean heat and moisture strengthen the circulation. The Coriolis effect helps the inflowing air spiral towards the centre.
5. An organised storm develops. Strong uplift forms an eyewall around the centre, while air spreads outwards high above the storm. In a mature cyclone, some air sinks in the centre, forming the relatively calm eye.

The ocean is therefore the storm's energy supply. Over cooler water, less heat and moisture are available. Over land, the supply from the warm ocean is cut off and friction disrupts the circulation. The cyclone usually weakens, although its remaining rainfall and winds can continue inland.

The structure and weather of a mature cyclone

A tropical cyclone can be hundreds of kilometres across; typical diameters range from about 100 to 1,000 km. It may last around 7–14 days, although individual storms vary.

Conditions change across the storm. The eyewall contains towering thunderclouds, torrential rainfall and the strongest winds. Farther out, bands of cloud and rain spiral around the centre, with generally less intense conditions towards the outer edges.

The eye is relatively calm and often largely cloud-free because air sinks there and becomes drier. Calm conditions do not mean high surface pressure: the centre remains an area of very low pressure. High waves and a storm surge—an abnormal rise in sea level—can accompany the storm.

Where tropical cyclones occur and move

Tropical cyclones normally form over oceans between approximately 5° and 30° latitude in both hemispheres. This combines warm water with a sufficient Coriolis effect.

Important regions include:

• The North Atlantic: the Caribbean Sea and Gulf of Mexico, with hurricanes affecting the Caribbean and USA.
• The eastern North Pacific: hurricanes that can affect Mexico's Pacific coast.
• The western North Pacific: typhoons that can affect the Philippines, China and other parts of East and Southeast Asia.
• The Indian Ocean: cyclones affecting India and Bangladesh, including around the Bay of Bengal, as well as Madagascar, Mauritius and parts of East Africa.
• The waters around northern Australia and the South Pacific islands.

Tropical cyclones are very rare in the South Atlantic. They cannot form over the cold seas around the UK, although the remnants of former tropical cyclones can reach higher latitudes.

Prevailing winds steer their movement. Tropical easterly winds commonly carry newly formed storms westwards and slightly towards the poles. Some then enter the mid-latitude westerlies and curve towards the east. Formation areas and later storm tracks are therefore not identical.

Frequency and seasonal patterns

On average, more than 80 tropical storms develop worldwide each year; not all reach hurricane or typhoon strength. The Pacific has the greatest number, followed by the Indian Ocean and then the Atlantic. The Atlantic accounts for less than 15% of global tropical cyclone activity.

Storms are most common when the oceans are warmest, usually in late summer and autumn. In the Northern Hemisphere, most occur from June to November, with a peak in September. The western North Pacific can also experience storms outside this period. In the Southern Hemisphere, the usual season is November to April.

How patterns change over time

Storm activity varies between seasons, years and decades. Natural climate variations are one reason. For example, El Niño can increase wind shear over the Atlantic, disrupting developing storms and reducing activity there.

Long-term change must be considered separately for frequency, intensity and distribution. The strongest North Atlantic tropical cyclones have increased in frequency and intensity since the 1970s, but this regional pattern does not demonstrate an increase in total storm numbers worldwide. Evidence for long-term global changes in overall activity is less certain.

Warmer seas can supply more energy to storms that develop, while a warmer atmosphere can hold more moisture. Future projections therefore suggest higher maximum wind speeds and heavier rainfall, even though the total global number of tropical cyclones is likely to stay similar or decrease.

Warming could also alter the areas with sufficiently warm water, potentially extending suitable conditions farther from the equator. However, formation still depends on circulation, wind shear and other conditions. Future geographical changes are expected to vary by region, and precise regional patterns remain uncertain.

Formation: recall the chain

Warm ocean → evaporation → moist air rises → surface low pressure → condensation releases heat → further uplift and inflow → stronger spiralling winds.

Conditions: sea surface around 27°C or above; warm water roughly 50–60 m deep; moist air; converging winds; low wind shear; sufficient Coriolis effect.

Global circulation: Hadley-cell uplift and converging trade winds create the tropical low-pressure belt and thunderstorm clusters. Coriolis helps organise rotation away from the equator.

Structure and characteristics

• Eye: sinking air; relatively calm and often cloud-free; very low central surface pressure.
• Eyewall: strongest winds and heaviest rainfall.
• Outer rainbands: spiralling cloud and rainfall.
• Hurricane/typhoon strength: sustained winds of about 119 km/h or more.
• Typical diameter: 100–1,000 km; duration: around 7–14 days.
• Cooler water and land weaken storms by reducing their ocean energy supply; land also increases friction.

Distribution and frequency

• Usually form over warm oceans at 5°–30° north and south; rarely at the equator.
• Hurricanes: Atlantic and eastern North Pacific.
• Typhoons: western North Pacific.
• Cyclones: Indian Ocean and South Pacific.
• Very rare in the South Atlantic; cannot form around the UK.
• Over 80 tropical storms annually worldwide; not all reach hurricane strength.
• Most occur in the Pacific, then Indian Ocean, then Atlantic.
• Northern season: June–November, peaking in September. Southern season: November–April.
• Tracks often begin westwards; some curve eastwards in the westerlies.

Change over time

• El Niño can increase Atlantic wind shear and reduce activity.
• Strongest North Atlantic storms: increased frequency and intensity since the 1970s.
• Future global projections: stronger winds and heavier rainfall, but total storm numbers likely unchanged or lower.
• Suitable source areas may shift with warming; precise regional changes remain uncertain.