In the lower few thousand feet, air flows in toward the center of the cyclone, and is whirled upward through ascending columns of air near the center. This inner band is the eyewall, where the storm's worst winds are felt, and where moist air entering at the surface is chimneyed upward releasing heat to drive the storm. Maximum winds run still higher in typhoons. Hurricane winds are produced, as all winds are, by differences in atmospheric pressure, or density. Atmospheric pressure is popularly expressed as the height of a column of mercury that can be supported by the weight of the overlying air at a given time.
Weather maps show atmospheric pressure in millibars, units equal to a thousandth of a bar.
Hurricanes drop the bottom out of those normal categories. It is encountered suddenly. From the heated tower of maximum winds and thunderclouds, one bursts into the eye, where winds diminish to something less than 15 knots. Penetrating the opposite wall, one is abruptly in the worst of winds again. A mature hurricane orchestrates more than a million cubic miles of atmosphere.