"eye" is a roughly circular area of comparatively light winds and fair weather
found at the center of a severe tropical cyclone. Although the winds are calm at the axis
of rotation, strong winds may extend well into the eye. There is little or no
precipitation and sometimes blue sky or stars can be seen. The eye is the region of lowest
surface pressure and warmest temperatures aloft - the eye temperature may be 10 C [18 F]
warmer or more at an altitude of 12 km [8 mi] than the surrounding environment, but only
0-2 C [0-3 F] warmer at the surface (Hawkins and Rubsam 1968) in the tropical cyclone.
Eyes range in size from 8 km [5 mi] to over 200 km [120 mi] across, but most are
approximately 30-60 km [20-40 mi] in diameter (Weatherford and Gray 1988). The eye is
surrounded by the eyewall, the roughly circular area of deep convection which is the area
of highest surface winds in the tropical cyclone. The eye is composed of air that is
slowly sinking and the eyewall has a net upward flow as a result of many moderate -
occasionally strong - updrafts and downdrafts. The eye's warm temperatures are due to
compressional warming of of the subsiding air. Most soundings taken within the eye show a
low-level layer which is relatively moist, with an inversion above - suggesting that the
sinking in the eye typically does not reach the ocean surface, but instead only gets to
around 1-3 km of the surface.
The general mechanisms by which the eye and eyewall are formed are not fully understood,
although observations have shed some light on the problem. The calm eye of the tropical
cyclone shares many qualitative characteristics with other vortical systems such as
tornadoes, waterspouts, dust devils and whirlpools. Given that many of these lack a change
of phase of water (i.e. no clouds and diabatic heating involved), it may be that the eye
feature is a fundamental component to all rotating fluids. It has been hypothesized (e.g.
Gray and Shea 1973, Gray 1991) that supergradient wind flow (i.e. swirling winds that are
stronger than what the local pressure gradient can typically support) present near the
radius of maximum winds (RMW) causes air to be centrifuged out of the eye into the
eyewall, thus accounting for the subsidence in the eye. However, Willoughby (1990b, 1991)
found that the swirling winds within several tropical storms and hurricanes were within
1-4% of gradient balance. It may be though that the amount of supergradient flow needed to
cause such centrifuging of air is only on the order of a couple percent and thus difficult
Another feature of tropical cyclones that probably plays a role in forming and maintaining
the eye is the eyewall convection. Convection in tropical cyclones is organized into long,
narrow rainbands which are oriented in the same direction as the horizontal wind. Because
these bands seem to spiral into the center of a tropical cyclone, they are sometimes
called spiral bands. Along these bands, low-level convergence is a maximum, and therefore,
upper-level divergence is most pronounced above. A direct circulation develops in which
warm, moist air converges at the surface, ascends through these bands, diverges aloft, and
descends on both sides of the bands. Subsidence is distributed over a wide area on the
outside of the rainband but is concentrated in the small inside area. As the air subsides,
adiabatic warming takes place, and the air dries. Because subsidence is concentrated on
the inside of the band, the adiabatic warming is stronger inward from the band causing a
sharp contrast in pressure falls across the band since warm air is lighter than cold air.
Because of the pressure falls on the inside, the tangential winds around the tropical
cyclone increase due to increased pressure gradient. Eventually, the band moves toward the
center and encircles it and the eye and eyewall form (Willoughby 1979, 1990a, 1995).
Thus the cloud-free eye may be due to a combination of dynamically forced centrifuging of
mass out of the eye into the eyewall and to a forced descent caused by the moist
convection of the eyewall. This topic is certainly one that can use more research to
ascertain which mechanism is primary.
Some of the most intense tropical cyclones exhibit concentric eyewalls, two or more
eyewall structures centered at the circulation center of the storm (Willoughby et al.
1982, Willoughby 1990a). Just as the inner eyewall forms, convection surrounding the
eyewall can become organized into distinct rings. Eventually, the inner eye begins to feel
the effects of the subsidence resulting from the outer eyewall, and the inner eyewall
weakens, to be replaced by the outer eyewall. The pressure rises due to the destruction of
the inner eyewall are usually more rapid than the pressure falls due to the
intensification of the outer eyewall, and the cyclone itself weakens for a short period of