The "eye" is a roughly circular area of comparatively light winds and fair
weather found at the center of a severe hurricane. 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. The eye
temperature may be 18° warmer or more at an altitude of 8 miles than the
surrounding environment, but only 0-3°F warmer at the surface in the hurricane.
Eyes range in size from 8 miles to over 120 miles across, but most are
approximately 20-40 miles in diameter.
The eye is surrounded by
the eye wall, the roughly circular ring of deep convection which is the area of
highest surface winds in the hurricane. The eye is composed of air that is
slowly sinking and the eye wall has a net upward flow as a result of many
updrafts and downdrafts. The eye's warm temperatures are due to compressional
warming 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.
The exact mechanism by
which the eye forms remains somewhat controversial. One idea suggests that the
eye forms as a result of the downward directed pressure gradient associated with
the weakening and radial spreading of the tangential wind field with height.
Another hypothesis suggests that the eye is formed when latent heat release in
the eye wall occurs, forcing subsidence in the storm's center. It is possible
that these hypotheses are not inconsistent with one another. In either case, as
the air subsides, it is compressed and warms relative to air at the same level
outside the eye and thereby becomes locally buoyant. This upward buoyancy
approximately balances the downward directed pressure gradient so that the
actual subsidence is produced by a small residual force.