As a first approximation, a hurricane is simply a very intense depression with the surrounding air spiralling inwards towards the central core, rather like an inverted bath-tub vortex. As it forms over the warm ocean waters in low latitudes, pressure falls near the centre of the disturbance. Then the air, flowing around the low, and approaching nearer and nearer to the centre, increases its speed to conserve angular momentum - in much the same way as a stone whirled on a piece of string will revolve more quickly when the cord is shortened.
A development then occurs that is unique to hurricanes: an "eye" and an "eye-wall" form. The "eye" is a circular calm and cloud-free zone, some 20 to 30 miles in diameter at the centre of the vortex, which can be seen very clearly on satellite pictures. The "eye-wall" is a ring of thick, deep, shower clouds surrounding the eye, and merging together to form what seems almost to be a solid, circular wall.
The eye occurs because, unlike a mid-latitude depression, a hurricane has a zone of sinking, rather than rising, air at the centre of the vortex. The sinking air within the eye is subjected to higher and higher atmospheric pressure on its descent, and so becomes warmer and drier than its surroundings at each level; with no condensation taking place, no cloud exists.
The strongest winds around a hurricane are usually just outside the eye, about 25 miles from the storm centre. By contrast, in the eye itself, the winds are very light, and survivors of severe hurricanes have told of the strange experience of winds of unbelievable strength ceasing suddenly as the eye arrives. They have also told of being able to look upwards at a pure blue sky surrounded by a rolling mass of angry wall-clouds spiralling tens of thousands of feet into the sky. As one observer with a poetic bent described it:
The cyclone's roar grew softer,
As the rain cleared from the sky;
And we felt the eerie quiet
Of the calm within the eye.
But we knew the lull was short-lived,
And that back would come the gales,
To lash from another quarter
With their shrieks and banshee wails.
Ships in such circumstances, in the relative calm within the eye of a hurricane, sometimes find the decks and riggings crowded with exhausted sea-birds seeking refuge. It has also been known for vessels to alleviate the worst effects of such a storm by matching the vessel's speed to that of the hurricane, thus staying for an extended period within the eye.