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. (Published 9/15/2004)
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.
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.
But it not only continues. It intensifies.
As more heat is released into the column of air, the column rises faster, pulling in still more air at the bottom of the system -- the sea surface -- with increasing speed. The faster air moves, the more rapidly it evaporates water. This is why a person getting out of a swimming pool into a breeze dries off and cools off much faster than someone getting out into still air.
This fast-moving, inwardly spiraling air picks up only about 10 percent more water vapor than it would normally carry. But it is a crucial 10 percent. It represents the extra energy that is continually being moved from ocean into air, driving the storm.
"You really need this energy source beneath the storm. It seems small in magnitude, but it's vital," said Robert Tuleya, an adjunct professor at the Center for Coastal Physical Oceanography at Old Dominion University in Norfolk. He used to work with the National Oceanic and Atmospheric Administration.
The energy is so vital that when hurricanes go over land, they tend to fall apart, eventually if not immediately. There are numerous reasons for this, among them the increased friction created by land compared with water. A big difference, however, is simply the loss of potential evaporation that comes with landfall. In addition, when the water on land (generally, in the forest canopy or on the ground) does evaporate, it cools the underlying surface much more dramatically than the ocean cools when water is evaporated from it. That cooling, over time, quenches a hurricane's power.
Of course, a lot more than that goes into making a hurricane. Otherwise, there would be more hurricanes.
"To get the heat engine to go, it doesn't happen spontaneously. Hurricanes definitely need to be triggered by some independent disturbance that comes along in the tropics," said Kerry A. Emanuel, a professor of atmospheric science at the Massachusetts Institute of Technology in Cambridge.
Among the other necessary ingredients is the initial low-pressure system that pulls air to a particular spot of tropical ocean. The Coriolis effect, in which Earth's rotation bends a straight-line wind into an arc, helps get the cyclonic motion going. The presence of very cold air 10 miles up creates a temperature gradient that contributes to a nascent storm's intensity.
There also cannot be large differences in wind speed and direction between the lower and upper parts of the atmosphere -- "shear forces" -- or the storm will not organize.
But if all goes "right," the storm can form and for a precarious stretch lasting hours or days is able to concentrate its newfound energy and use it to recruit more.