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Correction to This Article
A Sept. 13 article on the physics of hurricane formation incorrectly said that the transformation of water from liquid to gas releases heat. That "phase change" consumes heat. The reason it consumes heat, and the implications that has for hurricanes, was correctly described in the article.

Recipe for a Hurricane Relies on Happenstance

The Storm's Powerful Force Stems From Elements of Weather That Are Always Present but Only Sometimes Aligned

By David Brown
Washington Post Staff Writer
Monday, September 13, 2004; Page A09

Hurricane Ivan, which is set to strike western Cuba today, may have a name, path and personal narrative never to be repeated. But it shares a long list of features with every tropical cyclone, named or forgotten, that has formed over warm oceans since time immemorial.

Hurricanes are assembled from the usual building blocks of weather -- air, water, heat, wind, differences in pressure and temperature, and the contours of Earth. In the case of these destructive storms, however, the pieces come together in a way that, while not exactly rare, is always somewhat unlikely.

In a satellite image taken at 1:15 p.m. Friday, Hurricane Ivan churns toward Jamaica. (National Oceanic And Atmospheric Administration Via AP)

_____Hurricane Explainer_____
Why a Hurricane: Detailed information on what constitutes a hurricane.

The result is a phenomenon that is self-feeding and self-reinforcing. A hurricane's size and power allow it to grow even larger and more powerful. For a while at least, hurricanes defy the universe's natural tendency toward disorder -- its pieces becoming more ordered and less random, its energy concentrated rather than dissipated.

Exactly how that happens -- and in particular what the initial conditions of hurricane formation are -- is still the subject of much speculation and research. But the general mechanisms by which hurricanes become engines that turn heat into wind are known.

The "fuel" that drives and sustains a hurricane is evaporated water. Unlike conventional fuels, however, water does not undergo combustion to release heat. What it undergoes is a phase change -- the transformation from liquid to gaseous form. That physical change releases heat as predictably as the rapid sundering of chemical bonds that reduces a piece of burning wood to a pile of ash.

The source of that heat resides in the strong natural tendency water molecules have to stick to one another. This cohesiveness is why water can mound up slightly in a spoon without spilling over. When a molecule of water leaves the liquid and becomes airborne -- when it evaporates -- this physical attraction must be broken.

The energy needed to break those molecule-to-molecule bonds is called the "heat of vaporization." It is no trivial amount.

It takes 100 calories to heat a gram of water from the freezing point to the boiling point. At the boiling point, it takes an additional 540 calories to disrupt the attractive forces and set the water molecules free as vapor.

This process extracts the needed energy from the environment around the evaporating liquid. That is why sweat cools the body. Every drop of sweat that evaporates takes away a large amount of heat from the skin under it.

The opposite occurs when water molecules in the air start to stick together and condense back into water. They release energy equal to what they absorbed to become vapor. It is called the "heat of condensation," and it appears as detectable heat, just as the evaporating water produced measurable cooling.

In hurricanes, air moving over the ocean picks up water through evaporation. Warm water appears to be a necessary condition; hurricanes almost never form when the sea surface is cooler than 80 degrees.

Early in a hurricane's genesis, warm air that has been drawn into a region of low pressure -- and that has just picked up a lot of water vapor -- begins to rise. As it rises, it expands. As it expands, it cools. As it cools, it is less able to hold moisture. The water vapor begins to condense. This releases the heat of condensation, which restores some of the lost heat to the column of air, causing it to rise some more.

Eventually, nine or 10 miles above the ocean surface, the rising air becomes cold and largely depleted of moisture, and begins to sink back to Earth.

The rising air, however, leaves a relative vacuum -- an area of low pressure -- at the ocean surface. Air is pulled into that space, picking up water vapor and heat from the sea surface when it arrives. It then rises, cools, releases water as condensation, and the heat of condensation that always accompanies it, and the process continues.

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