By KEAY DAVIDSON
San Francisco Chronicle
September 22, 2005
That's how Hurricane Rita has grown with frightening speed from a puny low-pressure cell and swelled into a massive, potentially city-busting Category 5 monster as it bears down on the Gulf Coast.
The difference between the formation of ordinary clouds and the generation of hurricanes is partly a matter of degree: Both owe their existence to rising bubbles of warm, moist air. For hurricanes, though, an additional factor is the formation of a huge, spinning low-air-pressure cell that continually refuels itself by sucking in more and more warm, moist air.
Being of lower density than cool air, warm air is buoyant and rises like a hot-air balloon. If it's a humid day, the rising warm air hoists large amounts of water vapor into the heavens. As a moist, warm air parcel ascends, the moisture condenses and cools into large water droplets and clouds - e.g. the fluffy, sheep-like little cumulus clouds that meander innocently across the sky.
But on exceptionally warm, humid days, that process goes into overdrive, sometimes with scary results. On such days, the intense heat and humidity continually pump parcels of warm, moist air skyward.
As the air parcels rise, they continually release latent heat that propels the rising warm air even higher - as if the warm air were pulling itself up by its own bootstraps, as the saying goes.
Exceptionally warm, moist air can rise so high that it forms extremely tall thunderstorms or hurricanes. The taller they are, the more violent they tend to be.
This atmospheric scenario plays out most dramatically in tropical waters. Especially at this time of year, tropical waters are a vast reservoir of heat and moisture waiting to be transformed into hurricane-like energies. As warm air continually rises, it drains enough air from the surface to form large low-pressure cells into which outside warm air and moisture spiral, reinforcing the process.
Temperature is the key: "Ocean water must remain above 82 degrees Fahrenheit" for a hurricane to thrive, according to a NASA Web site.
Because ocean water is warmest from mid- to late year, "the peak of the season is from mid-August to late October. However, deadly hurricanes can occur anytime in the hurricane season," says a Web site run by the U.S. National Oceanic and Atmospheric Administration
Thanks to greatly improved computer models, forecasters' ability to anticipate the direction of hurricanes is better than ever. But they still struggle to forecast how intense a hurricane will be.
Still, they're counting their blessings because intensity forecasts, while imperfect, are better than they used to be. "Five years ago, accurate intensity predictions weren't even possible," weather expert Chris Davis of the National Center for Atmospheric Research said in a press statement Wednesday. Some important factors behind hurricane formation include:
- The formation of a low-pressure cell over warm water. As air is sucked into the low-pressure cell, it begins spinning from the large-scale rotation of the Earth (a phenomenon known as the Coriolis effect). That's why hurricanes resemble spinning wheels on the Fourth of July.
- The absence of an El Niqo in the Pacific. In the 1990s, researchers discovered that El Niqos, which cause Pacific waters to become unusually warm, are associated with high-altitude winds that blow east toward Africa. Those winds tend to disrupt rainstorms forming off the African coast, causing them to diminish.
If there's no El Niqo, the high-altitude winds aren't always strong enough to disrupt the rainstorms. In that case, some of the rainstorms expand into hurricanes - and a few veer toward the Americas.
- Distance from land. As a hurricane passes over land - which tends to be cooler and less humid than the adjacent ocean - the storm typically begins to die out.
On a futuristic note, some experts in the 1960s and '70s tried to control hurricane severity by seeding the storms. The "seeds" were fine particles and chemicals akin to those used for triggering artificial rainfall via traditional cloud seeding, a technique championed from the 1940s for less ambitious forms of "weather modification."
The hurricane-modifying project, dubbed Stormfury, triggered much excitement at the time. Stormfury ended in the early 1980s and is nowadays generally regarded as a scientific failure.
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