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.