This is a good illustration of the dynamices of the picture below. The air movment is in blue. The 'hot' air is driven into the cooler water resulting in an intense low pressure as the water vapor rises into the ciruclation of the storm. The sole purpose of these storms are to deliver hot air to the cooling oceans. The issue with Global Warming is that this exchange of hot and cool is chronic to the oceans and the ocean temperatures are rising. The picture of Katrina below is clearly illustrated. The other storms I have witnessed were not that HUGE. There was space between the surface of the land and the bottom of the storm and not greater than Cat 2 by the time I saw the trailing edge. Katrina was huge and 145 mph at landfall.