Physical Characteristics and Structure of Tropical Cyclones

One weather system, several regional names

A tropical cyclone is an intense, rotating low-pressure weather system that develops over tropical oceans. Hurricanes and typhoons are regional names for the same type of storm: hurricanes occur in areas such as the Caribbean and Gulf of Mexico, while typhoons occur in the western Pacific.

These are large weather systems, often hundreds of kilometres across, rather than a single thunderstorm. Their main characteristics are powerful spiralling winds, heavy rainfall and a central area of very low atmospheric pressure. However, conditions are not equally severe throughout the storm.

Low pressure draws air towards the centre

Atmospheric pressure is the force exerted by the weight of the overlying air per unit area of a surface. A tropical cyclone has lower surface pressure at its centre than in the surrounding atmosphere. This pressure difference drives air towards the centre.

The pressure gradient describes how much pressure changes over a distance. A large pressure difference over a short distance produces a stronger force driving the wind. This helps explain the powerful winds around a cyclone's centre. Low pressure alone does not tell us the wind speed at every point: the eye has the lowest surface pressure, but it does not have the strongest winds.

Earth's rotation makes the winds spiral

Air does not travel straight into the centre. Earth's rotation produces the Coriolis effect, which deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

As a result, surface winds spiral inwards around a tropical cyclone:

• Anticlockwise in the Northern Hemisphere.
• Clockwise in the Southern Hemisphere.

The storm therefore combines inward air movement with rotation around its centre. These directions describe winds within the storm, not the direction in which the whole storm travels.

The eye, eyewall and spiral rainbands

A well-developed tropical cyclone has three recognisable structural features.

The eye is the central area. It has relatively light winds, little or no rainfall and often clearer skies. Air slowly sinks within the eye, warming as it descends and becoming drier, which helps explain the lack of cloud. Although the eye is relatively calm, it lies inside a powerful storm; not every developing tropical cyclone has a clearly defined eye.

The eyewall surrounds the eye. It is a ring of towering storm clouds where air rises strongly. Rising moist air cools and water vapour condenses, forming clouds and heavy rainfall. The eyewall contains the strongest surface winds and torrential rain, so conditions change sharply between the eye and its surrounding wall of cloud.

Beyond the eyewall are spiral rainbands: curved bands of clouds and thunderstorms wrapped around the centre. Air rises within these bands too. Towards the outer edges, winds are generally lighter and cloud and rainfall become more scattered. Air that has risen through the storm spreads outwards at high altitude.

Connecting structure to changing weather

Imagine a well-developed cyclone passing directly over a location. Its outer rainbands arrive first, bringing bursts of rain and wind. Conditions become much more severe as the eyewall arrives. If the eye passes overhead, there may then be a temporary lull, with lighter winds and little rain, before the opposite side of the eyewall brings severe weather again.

This sequence connects the storm's structure to weather experienced at the surface. The key contrast is between the relatively calm, lowest-pressure eye and the surrounding eyewall, where winds and rainfall are most intense.