The Storm Engine

How cyclones are measured

After each season, forecast agencies reconcile every observation into a single best track: the storm's position and intensity every few hours from genesis to decay. The tracks on this map are that record.

• Saffir-Simpson: a five-step scale set by peak wind. Category 1 begins at 64 knots; Category 5 at 137 knots and above.
• Central pressure: the lower the pressure at the core, the more intense the storm. Often a better size and strength signal than a single wind reading.
• ACE: accumulated cyclone energy adds up a storm's intensity over its whole life, so a long, strong storm scores far above a brief one.

One peril, three names

Hurricane, typhoon and cyclone are the same phenomenon under different basin names. Every warm ocean spins storms along its own repeatable corridors.

• Hurricane: the North Atlantic and eastern Pacific, the corridor that runs into the US Gulf and seaboard.
• Typhoon: the western Pacific, the most active basin on Earth, striking the Philippines, Japan and China.
• Cyclone: the Indian Ocean and South Pacific. The shallow Bay of Bengal funnel makes the north Indian basin the deadliest for storm surge.

What makes them form, and what drives loss

Tropical cyclones are heat engines. They need a warm ocean, roughly 26 degrees or more through a deep layer, plus moisture and low wind shear, which is why they cluster in late summer and autumn.

Peak intensity out at sea makes headlines, but the loss record keys on landfall intensity and, above all, on what sits behind the coast. The costliest coastline on Earth is the US Gulf and Atlantic; the deadliest is the Bay of Bengal. Same peril, opposite exposure.