Warm water fuel: why oceans power storms

A hurricane is a heat engine. Its fuel is the heat stored in warm ocean water, carried into the air as water vapour.

1. Warm seawater evaporates. Each kilogram of water that evaporates takes up energy from the ocean, the latent heat of vaporisation.
2. Moist air near the surface is drawn towards the low pressure at the centre and rises in the eyewall.
3. As it rises it cools, and the vapour condenses into cloud droplets. Condensation gives that latent heat back to the air.
4. The air is warmed by this release, so it is lighter and keeps rising. That helps to keep the pressure low at the surface, which pulls in more moist air. The loop feeds itself.

If you want to see how rising air cools and clouds form, see the Weather Island lab, "Lift air, make a cloud".

What a storm needs. Forecasters list the ingredients for a tropical cyclone to form and grow:

• Sea-surface temperature of at least about 26.5 °C (80 °F), in a layer that goes down to roughly 50 m. A deep warm layer matters, because a storm stirs up the water below it. If cold water lies just under a thin warm skin, the storm churns it up and cuts off its own fuel.
• A moist atmosphere through the middle layers.
• Low vertical wind shear: the wind should not change much with height. Strong shear tilts the storm and mixes dry air into its core.
• Enough distance from the equator for the Coriolis effect to spin it (next lesson).
• A starting disturbance, such as a cluster of thunderstorms.

Cutting off the fuel. A storm weakens when it moves over land, because there is no warm water to evaporate, and friction with the ground also slows the wind. It weakens over colder water, in dry or sheared air, or when it passes over water it has already cooled. Weakening does not mean the danger ends: a decaying storm can still drop a great deal of rain.

Rapid intensification. Sometimes the ingredients line up and a storm strengthens fast. The NHC defines rapid intensification as an increase in maximum sustained winds of at least 30 knots (about 35 mph, 55 km/h) in 24 hours. Rapid intensification is hard to forecast and is one reason forecasters are careful with predictions of how strong a storm will be at landfall.