Chapter 3 · From warm seas to a funnel of wind
Hurricanes and tornadoes
An engine made of sea water
Imagine the ocean in late summer, with the water at 26.5 degrees Celsius or warmer down to a depth of about 50 metres. Water evaporates, and the moist air rises. High up it cools and turns into cloud, and this releases heat that makes the air rise even faster, so the air below rushes in to fill the gap. Because the Earth spins, the air does not go straight in, it curves. This is the Coriolis effect. In the Northern Hemisphere storms spin anticlockwise, in the Southern one clockwise. Very close to the equator the effect is too weak, and storms rarely form. In the picture the warm water glows and the clouds begin to turn.
A calm in the middle
As the storm tightens, a clear and calm hole forms in the centre, the eye, usually 30 to 65 kilometres wide. Around it stands the eyewall, a ring of towering clouds with the strongest wind and rain. A storm with sustained wind of 119 km/h or more is a hurricane. The Saffir-Simpson scale sorts hurricanes into five categories by wind speed: category 1 starts at 119 km/h, category 3 covers 178 to 208 km/h, and category 5 begins at 252 km/h. Worked example: a wind of 200 km/h is category 3. The force of wind grows with the square of its speed, so a category 5 wind presses about 4.5 times harder than a category 1 wind. Enter a speed in the sandbox and read the category.
One storm, three names
The names depend only on where in the world the storm is. Over the North Atlantic and the north-east Pacific it is a hurricane. Over the north-west Pacific, near Japan, the Philippines and China, it is a typhoon. Over the Indian Ocean and the South Pacific it is a tropical cyclone. The physics is the same. The widest storm on record was Typhoon Tip in 1979, about 2,200 km across, and its pressure fell to 870 hectopascals, the lowest ever measured at sea level. When a hurricane reaches land it loses its fuel and fades in a day or two, but the rain and sea surge can still do serious harm.
Inside the supercell
A tornado is a different animal. It starts in a supercell, a thunderstorm whose updraught rotates. Warm moist air near the ground meets cold dry air above it, and changes of wind direction with height tilt a spinning tube of air upright. When a funnel reaches the ground, it is a tornado. The Enhanced Fujita scale, introduced in 2007, rates tornadoes from EF0 (105 to 137 km/h) to EF5 (above 322 km/h) by the damage they cause, because wind speed is hard to measure directly. The United States gets about 1,200 tornadoes a year, most of them in the region known as Tornado Alley, between Texas and Nebraska.
Watching from space
Today a hurricane is tracked from the day it is a cluster of thunderstorms. Weather satellites watch the clouds, special aircraft fly into the storm to measure wind and pressure, and computer models draw the likely path. The cone of uncertainty on a forecast map shows where the centre may go. It does not show how big the storm is, so danger lies outside the cone as well. A hurricane watch means that hurricane conditions are possible within 48 hours, and a warning means they are expected within 36 hours.
Where to shelter
Official advice is the same in most countries. Follow the evacuation orders of your local authorities, because for a hurricane the deadliest danger is often the storm surge, a wall of sea water pushed ashore. For a tornado warning go to the lowest floor of a solid building, into a room without windows, and cover your head. A car or a mobile home is not safe, and nor is a space under a bridge. Storm chasers are professionals with training and special equipment, and a tornado is never something to film from nearby.
In numbers
- ≥ 26.5 °C
- sea temperature needed for a hurricane to grow
- 119 km/h
- wind speed at which a storm is a hurricane
- EF0 – EF5
- tornado damage scale, introduced in 2007