Hurricanes (tropical cyclones)

Giant spinning storms

A hurricane is a huge spinning cloud born over a warm ocean. It can be 500 km wide — as big as all of Uzbekistan! In the middle is the 'eye' — calm and clear — surrounded by 300 km/h winds. In the Northern Hemisphere hurricanes spin anticlockwise, in the Southern, clockwise. In America they're called hurricanes, in Asia typhoons, in India cyclones — all the same thing. On the globe you'll see them over warm seas.

A tropical cyclone forms over ocean warmer than 26.5 °C: evaporation → heat of condensation → rising air → the Coriolis force sets it spinning (none form within 5° of the equator — no Coriolis force there). The eye is 30–60 km across; the eyewall has the strongest wind and rain. Saffir-Simpson scale: category 5 is ≥ 252 km/h. Its energy equals 10,000 atomic bombs a day. Season: Atlantic June–November, western Pacific all year. Deadliest: Bhola (1970, Bangladesh, 300,000+ dead), Katrina (2005, $125 billion), Haiyan (2013, 315 km/h). As oceans warm, hurricanes are getting stronger and slower. Since 1953 they've been given names (alternating female/male). Satellites and 'hurricane hunter' aircraft track them.

Fun facts

• In the centre of a hurricane — the 'eye' — it's calm and sunny.
• One hurricane releases the energy of 10,000 atomic bombs a day.
• Hurricanes never form right on the equator — Earth's spin can't 'twist' them there.