Once over land, a hurricane weakens very quickly. Why? Simply, it loses latentheat of condensation. "So", you ask, "what on god's earth is that?" It is heat energy that comes from the sun, and there is a vast amount of thisenergy stored in warm tropical waters where sea surface temperatures are above80 degrees.So, how does this energy get from the ocean into the sky? It happens when waterevaporates from the sea surface. In turn, this heat energy is released into adeveloping tropical storm as condensation occurs.
The resulting temperature gradient between the warm interior of the storm andslightly less warm surrounding environment creates a large pressure gradient.
This is what drives the powerful winds of a hurricane. As long as this systemis undisturbed (and wind shear does not come into play), the hurricane willmaintain its intensity or grow even stronger.
So when a storm like Hermine moves over land, the game is over is far as highwinds go. However, when the winds do slow down, something very importanthappens. The pressure gradient force suddenly overwhelms the outwardcentrifugal force and the storm collapses on itself. The converging air causesmore upward motion, and rainfall rates usually increase.
This is what we are seeing now with Hermine, and the remains of the storm willcontinue to produce very heavy rainfall across the South Central states throughWednesday.
Story by AccuWeather.com Senior Meteorologist John Kocet.
