Back to Basic Definitions
Page | Back
to Main FAQ Page
Subject: A16) Why do tropical cyclones require
80°F (26.5°C) ocean temperatures to form ?
Contributed by Chris Landsea
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.
References:
- Graham, N. E., and T. P. Barnett, 1987: Sea surface temperature,
surface wind divergence, and convection over tropical oceans.
Science, No.238, pp. 657-659.
- Gray, W.M. 1979 : "Hurricanes: Their formation, structure and likely role in the tropical circulation" Meteorology Over Tropical Oceans. D. B. Shaw (Ed.), Roy. Meteor. Soc., James Glaisher House, Grenville Place, Bracknell, Berkshire, RG12 1BX, pp.155-218
- Palmen, E. H., 1948: On the formation and structure of tropical
cyclones. Geophysica , Univ. of Helsinki, Vol. 3, 1948, pp. 26-38.
Last updated August 13, 2004
Back to Basic Definitions
Page | Back
to Main FAQ Page
|