How Tropical Cyclones Form and Intensify

Tropical cyclones require a specific combination of ocean and atmospheric conditions to form, first systematized by the meteorologist William Gray in 1968 into a set of genesis parameters still taught today. Sea-surface temperatures must reach at least 26.5°C (about 80°F) to a depth of roughly 50 meters, providing the latent heat that fuels the storm's convection; the mid-troposphere must be sufficiently humid to sustain deep thunderstorm clusters rather than let them dry out and dissipate; and vertical wind shear, the change in wind speed and direction with height, must stay low, since strong shear tilts a developing storm and tears its convection away from its center of circulation. A pre-existing disturbance, most often an African easterly wave drifting off the West African coast into the Atlantic, supplies the initial spin, and the Coriolis effect, the deflection produced by Earth's rotation, must be strong enough to organize that spin into a closed circulation. Because the Coriolis force is essentially zero at the equator, tropical cyclones almost never form within about 5 degrees of latitude of it; Typhoon Vamei, which organized near 1.5°N over the South China Sea in December 2001, is one of the rare documented exceptions and remains a well-studied anomaly in tropical meteorology. When all of these ingredients align, an area of low pressure can organize into a self-sustaining warm-core vortex within a day or two, though the great majority of tropical disturbances observed each year never complete this transition.