Subject: A16) Why do tropical cyclones require 80°F (26.5°C) ocean temperatures to form ?
Contributed by Chris Landsea (NHC)
Tropical cyclones can be thought of as engines that require warm, moist air as fuel (Emanuel 1987). This warm, moist air cools as it rises in convective clouds (thunderstorms) in the rainbands and eyewall of the hurricane The water vapor in the cloud condenses into water droplets releasing the latent heat which originally evaporated the water. This latent heat provides the energy to drive the tropical cyclone circulation, though actually very little of the heat released is utilized by the storm to lower its surface pressure and increase the wind speeds.
In 1948 Erik Palmen observed that tropical cyclones required ocean temperatures of at least 80°F (26.5°C) for their formation and growth. Later work (e.g., Gray 1979) also pointed out the need for this warm water to be present through a relatively deep layer (~150 ft, 50 m) of the ocean. This 80°F value is tied to the instability of the atmosphere in the tropical and subtropical latitudes. Above this temperature deep convection can occur, but below this value the atmosphere is too stable and little to no thunderstorm activity can be found ( Graham and Barnett 1987).
See Question G3 for how this value might change if a significant global warming occurs.
Last updated August 13, 2004
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