Anatomy of a hurricane: eye, eyewall and spiral bands

A tropical cyclone is a huge, rotating system of thunderstorms that forms over warm tropical oceans. Seen from space it looks like a pinwheel. Cut it in half from top to bottom and a clear structure appears.

• The eye: a roughly circular area at the centre. Air there sinks, so the sky is often clear or partly cloudy and the wind is light. The eye is typically about 30 to 65 km across, though small ones can be under 10 km and big ones over 200 km.
• The eyewall: a ring of tall thunderstorms around the eye. This is where the strongest winds and the heaviest rain are found.
• Spiral rain bands: curved lines of showers and thunderstorms that wind in towards the centre. Squalls and even tornadoes can sit in them.
• The outflow: at the top, about 12 to 15 km up, the air that rose in the eyewall spreads out and flows away from the storm.

How the air moves. Near the sea surface, air spirals inwards towards the low pressure at the centre. At the eyewall it is forced upwards in a powerful ring of rising air. High up it flows outwards. A smaller amount of air sinks in the middle, which is what clears the eye. Pressure at the centre is lower than the surroundings by a few tens of hectopascals in a modest hurricane and by about a hundred or more in the strongest ones. The lowest sea-level pressure ever measured in a tropical cyclone is 870 hPa, in Typhoon Tip in 1979.

The wind does not grow steadily towards the middle. Moving in from outside, wind speed rises to a peak at the radius of maximum wind, which sits in the eyewall and is typically tens of kilometres from the centre (often 15 to 50 km). Inside that radius it drops quickly to calm in the eye.

One storm, three names. Scientists use the neutral term tropical cyclone. What it is called depends on where it forms:

• Hurricane: North Atlantic, and the central and eastern North Pacific.
• Typhoon: the northwest Pacific.
• Cyclone (often "tropical cyclone" or "severe tropical cyclone"): the Indian Ocean and the South Pacific.

These are the same kind of storm. Only the name changes with the ocean.

Rating the wind. In the Atlantic and eastern Pacific, the US National Hurricane Center rates hurricanes on the Saffir-Simpson Hurricane Wind Scale from Category 1 to 5. It uses the one-minute sustained wind at 10 m above the surface. A storm becomes a Category 1 hurricane at 74 mph (about 119 km/h). Below that, a storm with sustained winds of 39 to 73 mph is a tropical storm and is given a name. The next lessons look at what the scale does and does not tell you.