Tropical Storms: Formation, Structure and Climate Change

One weather system, different regional names

A tropical cyclone is a powerful, rotating low-pressure system that develops over a warm ocean. Moist air rises through enormous thunderclouds while surface winds spiral towards the centre. In GCSE geography, ‘tropical storms’ is often used broadly for these systems, including hurricanes, typhoons and cyclones.

The regional names describe the same type of weather system. Hurricanes occur in the Atlantic and eastern North Pacific, typhoons in the western North Pacific, and cyclones in regions such as the Indian Ocean and South Pacific. Technically, a developing system becomes a tropical storm when sustained winds reach about 63 km/h; hurricane or typhoon strength begins at about 119 km/h.

Where tropical storms develop

Tropical storms usually form over oceans between approximately 5° and 30° north and south of the equator. Their distribution is uneven: they need both warm water and suitable atmospheric conditions, so they do not develop everywhere within these latitude bands.

Important regions include:

• The North Atlantic, Caribbean Sea and Gulf of Mexico, where hurricanes can affect Caribbean islands and the USA.
• The eastern North Pacific, where hurricanes can affect Mexico.
• The western North Pacific, including the South China Sea, where typhoons can affect the Philippines and other parts of East and South-East Asia.
• The Indian Ocean, including the Bay of Bengal, where cyclones can affect India, Bangladesh, Madagascar and nearby islands.
• The waters around northern Australia and the South Pacific, where cyclones can affect Australia and Pacific islands.

Tropical cyclones are very rare in the South Atlantic. They do not normally form directly on the equator because the Coriolis effect is too weak there to organise their rotation.

Storms are seasonal because oceans need time to warm. Most Northern Hemisphere storms occur between June and November, with a September peak, although western North Pacific storms can occur outside this period. The Southern Hemisphere season usually runs from November to April.

How atmospheric circulation creates favourable conditions

Intense solar heating warms tropical oceans. Near the equator, warm, moist air rises in the ascending part of the Hadley circulation, producing a belt of low pressure. The north-east and south-east trade winds converge in the Intertropical Convergence Zone (ITCZ), encouraging further uplift and thunderstorms.

This provides a favourable setting for tropical storm development, but an ordinary tropical thunderstorm does not automatically become a cyclone. A cluster of thunderstorms must become organised over sufficiently warm water, far enough from the equator for rotation to develop, and with little change in wind speed or direction with height.

Atmospheric circulation also helps steer storms. The trade winds generally carry developing tropical cyclones westwards. Some later curve towards the poles and enter the mid-latitude westerlies, which can turn their tracks eastwards. Tropical cyclones also transfer heat and energy away from the tropics towards cooler regions.

From warm ocean to a developing storm

Several conditions must occur together. Sea-surface temperatures are normally around 27°C or higher, supplying warm, moist air. Surface winds must converge, encouraging uplift. Low vertical wind shear allows the thunderclouds to grow upwards without being disrupted. Sufficient distance from the equator allows the Coriolis effect to support rotation.

The formation sequence is a reinforcing cycle:

1. Warm seawater evaporates. This supplies water vapour and transfers energy from the ocean into the atmosphere.
2. Warm, moist air rises. A surface low-pressure centre develops, drawing in more air from the surrounding area.
3. The rising air cools and water vapour condenses. Towering cumulonimbus clouds form, producing heavy rain. Condensation releases latent heat, warming the surrounding air and encouraging further uplift.
4. More warm, moist air is drawn in. Continued heat release strengthens the circulation, and winds accelerate towards the low-pressure centre.
5. The system becomes organised and rotates. The Coriolis effect helps the inward-flowing winds spiral anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. A developing depression can strengthen into a tropical storm and then a hurricane, typhoon or intense cyclone.

The ocean is therefore both a moisture supply and an energy supply. When a storm moves over cooler water or onto land, that supply is reduced and it weakens. Over land, friction also disrupts and slows the surface airflow.

Inside a mature tropical storm

A mature tropical cyclone can be several hundred kilometres across, but conditions vary sharply between its centre and outer edges.

The eye is the relatively calm centre. Air sinks here, becoming drier, so the eye is often cloud-free with little rain and light winds. Despite these calmer conditions, pressure is lowest near the centre.

Around it is the eyewall, a ring of towering cumulonimbus clouds where air rises vigorously. This contains the strongest winds and most intense rainfall. Further out, curved bands of cloud and thunderstorms spiral around the centre. Conditions are generally less intense towards the storm's outer edges. At high altitude, air spreads outwards from the storm.

The distinction between the eye and eyewall explains why a location can experience violent winds, a temporary calm as the eye passes, and then violent weather again. Strong winds also generate large waves and can drive a storm surge towards the coast.

How climate change might alter tropical storms

Climate change can affect distribution, frequency and intensity in different ways. These should not be treated as interchangeable.

Distribution — where storms occur: warmer oceans may allow favourable conditions to extend further from the equator or persist for longer during the year. However, warm water alone is insufficient: wind shear, moisture and circulation also matter. In the north-west Pacific, observations show a poleward shift in the latitude where storms reach their maximum intensity; this does not establish an identical shift in every ocean basin.

Frequency — how many storms occur: a longer period of warm water could provide more opportunities for development, but it does not guarantee more storms. Most climate-model studies project that the total global number of tropical cyclones will decrease or change little. Regional changes may differ, and natural variation makes long-term trends difficult to identify.

Intensity — how powerful storms become: warmer oceans can supply more energy, while a warmer atmosphere can contain more moisture. Average storm intensity and rainfall rates are projected to increase, and a larger proportion of storms is expected to reach the most intense categories.

The important conclusion is therefore not simply ‘more warming means more storms’. Future tropical cyclones may be stronger and wetter even if their total global number does not increase.

Distribution and circulation

• Usually develop over warm oceans between 5° and 30° N/S, not directly on the equator.
• Hurricanes: Atlantic and eastern North Pacific. Typhoons: western North Pacific. Cyclones: Indian Ocean and South Pacific.
• Hadley-cell uplift and converging trade winds provide warm, moist rising air and low pressure.
• Trade winds generally steer storms westwards; some later curve polewards and eastwards.

Formation and weakening

Conditions: sea temperatures around 27°C or above, moist air, converging surface winds, low vertical wind shear and sufficient Coriolis effect.

Sequence: evaporation → moist air rises → cooling and condensation → latent heat released → stronger uplift and inward airflow → organised rotating storm.