Nib asked
How does a hurricane form step by step?
A hurricane is a heat engine fed by warm ocean water: rising vapor condenses and releases heat, pulling in more air, and Earth's spin twirls it into a spiral around a calm eye.
8 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1the engine
Seen from a satellite, a hurricane is a huge spiral of cloud with a hole in the middle. Underneath, it works like an engine: it takes in heat from warm tropical seawater and turns some of that heat into the fast winds. No warm water, no hurricane.
A hurricane is a heat engine, and warm ocean water is its fuel.
2the stages
A storm cluster is just a patch of thunderstorms over warm water. When it starts to spin and winds stay under 39 miles an hour (63 km/h) it is a tropical depression. From 39 to 73 miles an hour it is a tropical storm and gets a name. At 74 miles an hour (119 km/h) or more it is a hurricane. In the western Pacific the same storm is a typhoon, and in the Indian Ocean and South Pacific a cyclone.
Hurricane, typhoon and cyclone are the same storm; it is a hurricane once winds reach 74 mph (119 km/h).
Is a typhoon a different kind of storm from a hurricane?
Show the answer
No. It is the same kind of storm, only named by the ocean basin where it happens.
3warm sea
Hurricanes need ocean water of at least about 26.5 degrees Celsius (80 degrees Fahrenheit) through a layer roughly 50 metres deep. Warm water evaporates quickly, and the vapor carries a huge store of hidden heat, called latent heat, into the air.
Sea water above about 26.5 °C evaporates fast, loading the air with vapor and hidden heat.
4heat released
High up, the vapor cools and condenses into cloud droplets. Condensing releases the heat the water took in when it evaporated. That heat warms the surrounding air, making it lighter, so it rises faster, and rising air leaves lower pressure at the sea surface.
Condensation releases heat, which makes the air rise faster and drops the pressure underneath.
5feeding itself
Air flows from high pressure to low pressure, so wind blows in toward the low from every side along the sea surface. That wind sweeps across warm water and picks up more vapor, which rises and condenses, which drops the pressure further. The loop feeds itself.
Wind rushing in to the low pressure brings fresh vapor, so the storm feeds itself.
Why does air rush toward the middle of the storm near the sea?
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Rising, warmed air leaves low pressure at the surface, and air flows from high to low pressure to fill it.
6the spin
Earth rotates, and that deflects moving air to the right in the northern hemisphere (the Coriolis effect). Air heading toward the low pressure is bent sideways, so instead of falling straight in it spirals counterclockwise. In the southern hemisphere it spirals clockwise. Near the equator the bend is almost zero, so hurricanes almost never form within about 5 degrees of latitude of it.
Earth's spin bends the incoming wind into a spiral, counterclockwise in the north.
a common mix-upHurricanes do not form right on the equator: there Earth's spin gives almost no bend, so no spiral can start.
Why do hurricanes not form right on the equator?
Show the answer
The spin-bend of the wind is almost zero there, so the air cannot start rotating around a centre.
7eye and wall
Where the inflowing air meets, it is forced upward in a ring of tall thunderstorms around the centre. This eyewall holds the strongest winds and heaviest rain. In the very middle, air sinks and warms, clearing the sky: the eye is calm, often 30 to 65 kilometres wide, but the storm is just as dangerous on the other side of the wall.
The eyewall ring holds the strongest winds; the eye in the middle is calm.
8the whole idea
Put it together: sea above about 26.5 degrees Celsius feeds vapor into the air, the vapor condenses and releases heat that makes the air rise, inflow rushes in to replace it, and Earth's spin twirls everything into a spiral with an eyewall and a calm eye. Over land or cold water the fuel is cut off and the storm fades.
Warm sea feeds rising vapor, which releases heat and pulls in air that Earth's spin twirls into a spiral.
A hurricane is just warm sea water turned into wind, and it dies the moment its fuel runs out.