A hurricane's calm eye is warmer, and oddly full of ozone
At the heart of the most violent storms on Earth sits a patch of light winds and sometimes clear sky. The eye of a hurricane holds the storm's lowest pressure, is warmer than its surroundings and carries extra ozone dragged down from the stratosphere. How it forms is still not fully understood.
A typical eye is roughly circular and 30 to 65 kilometres across, ringed by the eyewall, a wall of towering thunderstorms where the fiercest winds blow. Pressure inside can be up to 15 percent lower than outside the storm. In strong cyclones the eye is crisp and symmetric; in weaker ones it may be ragged, filled with cloud, open on one side, or missing entirely. Forecasters read those shapes to judge intensity, and eyewalls occasionally take on polygonal outlines from triangles to hexagons.
Sizes vary enormously. Typhoon Carmen's eye spanned 370 km, while Hurricane Wilma's shrank to just 3.7 km. Tiny pinhole eyes tend to appear in storms that are intensifying fast, and their strength can swing wildly. Eyes under about ten nautical miles often kick off an eyewall replacement cycle: a second ring of storms forms farther out, producing an eye within an eye, then contracts and strangles the inner one. The storm usually weakens during the swap and may strengthen again once the new wall tightens.
The standard explanation starts with a ring of strong thunderstorms around the centre. Rising air piles up aloft, forming a high-pressure zone that sends most of it outward, but some flows back towards the middle. Eventually that weight overcomes the updrafts, and the air sinks, clearing and warming the core. Scientists still cannot say why convection forms a ring rather than a central tower, or why only part of the air escapes. What is certain is that a cyclone needs an eye to reach its highest wind speeds.
Spotting an eye is easy on satellite images when it is clear. When clouds hide it, hurricane hunter aircraft, ships and coastal Doppler radar networks help, as do satellite readings of water vapour and cloud temperature. Because air sinking in the eye is rich in ozone, ozone-sensing instruments can even flag a developing eye before satellite pictures reveal it.
Source: Eye (cyclone)
