When do hurricanes form?
Tropical cyclones are typically a summer phenomenon, since they can't exist without lots of warm, sun-baked sea water. The Atlantic hurricane season officially runs from June 1 through Nov. 30, but the region's cyclones don't always follow the rules, sometimes arriving early and sometimes flouting their curfew. The latest date that a tropical storm was ever recorded in the Atlantic is Dec. 30, which has happened twice: Hurricane Alice in 1954 and Hurricane Zeta in 2005.
 
Since the Pacific Ocean is larger and warmer than the Atlantic, its typhoon season is usually longer and more intense. Some areas are so active that they have no official "seasons," with cyclones sprouting up virtually year-round. Still, most regions see few typhoons in winter, with the vast majority developing between May and December.
 
In both the Atlantic and Pacific basins, cyclone formation shifts into high gear by late summer, hitting its peak around August or September. The Atlantic hurricane season's average peak is Sept. 10, and many of the worst storms in U.S. history have occurred within two weeks of that date.
 
Where do hurricanes go?

Seven ocean basins around the world host tropical cyclones, six of which are in the Pacific or Indian oceans and only two of which directly affect the United States (see map). One, off the coast of western Mexico, presents little risk to U.S. soil aside from rare clashes with Hawaii. The other fills up much of the North Atlantic, and is responsible for almost all hurricanes that hit the United States.
 
All seven basins have at least one big risk factor in common: lots of warm sea water to their northwest or southwest, a problem since tropical storms feed on warm water, travel westward and like to curl away from the equator. But geography alone doesn't make a region susceptible; a wide range of other issues like wind currents, water depth and coastal geology also play a role. The U.S. East and Gulf coasts face a high risk because they're an ideal distance away from the equator, and many of their large port cities are sitting ducks for big storm surges. Throw in the annual blast of easterly waves from Africa, and much of the United States' eastern half finds itself trapped in the middle of a busy hurricane highway for six months every year.
 
Why are hurricanes dangerous?
Hurricane winds can rip off roofs, blow out windows and flatten buildings, and they're notorious for making deadly weapons out of random debris. But the storms also spark an array of other disasters, some of which are even worse than their wind. Coastal storm surges kill more Americans than any other effect from hurricanes, and left an indelible mark on the nation's collective memory by flooding much of New Orleans after Hurricane Katrina. Heavy rain is another killer, especially in hilly parts of Asia and Central America, where rains can spur massive mudslides and sinkholes. And the tail end of a hurricane often generates tornadoes, which feature even stronger winds than the hurricane itself.
 
The deadliest hurricane in U.S. history was an unnamed storm that ambushed Galveston, Texas, on Sept. 8, 1900, killing 8,000 people. But while advances in hurricane forecasting and preparation have reduced U.S. death tolls since then, any given cyclone is now capable of more devastation than in the past. That's mainly due to rising population densities along the world's coasts, especially in developing countries. Yet affluent nations aren't immune from coastal crowding, either: In the United States alone, some 160 million people — more than half the total population — are now crammed into just 673 oceanfront counties, up 46 percent from 1970. Another 7.1 million are expected to join them by 2015, a 5 percent increase in less than five years.
 
To make matters worse, many of those coasts are also much closer to the ocean than they used to be, since dam-building, development and pollution have helped wipe out swaths of coastal marshes that would normally soften a hurricane's blow before it hits land. Without that buffer, storm surges can be more severe, and wind gusts may also be stronger without anything to slow down the storm ahead of landfall.
 
What about global warming?
Not only are some coastal cities upping the ante as they pack in more people, but humans in general may also be stacking the deck in hurricanes' favor. Scientists now widely agree that rising sea-surface temperatures due to manmade climate change will boost the power of hurricanes, but they remain cloudy on details like when or by how much. A study published in Nature Geoscience earlier this year predicts "greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift toward stronger storms," with intensity rising 2 to 11 percent by 2100. Rainfall from hurricanes is also expected to grow by about 20 percent within 70 miles of a storm's center.
 
Global warming may strengthen individual storms, but climate models also suggest it should slash the total number of them. The Nature Geoscience study forecasts a global dip in hurricane frequency of 6 to 34 percent over the next 90 years, and Thomas Knutson, a research scientist at NOAA's Geophysical Fluid Dynamics Laboratory, sums up the issue in a recent report for the GFDL: "Anthropogenic warming over the next century more likely than not will lead to substantial fractional increases in the numbers of very intense hurricanes in some basins, despite a likely decrease (or little change) in the global numbers of tropical storms."
 
hurricane katrinaAs for whether any of this is happening yet, the jury is still out. Atlantic hurricane activity has increased since the 1970s, but there are too many variables to scientifically link any single storm to global warming (although Katrina, pictured, is often seen as a leading candidate). That doesn't mean they aren't related; the situation is just too complex to prove or disprove a connection yet. But as Knutson points out in the GFDL report, it's probably just a matter of time. "It is premature to conclude that human activity — and particularly greenhouse warming — has already had a detectable impact on Atlantic hurricane activity," he writes. "However, human activity may have already caused substantial changes that are either below the 'detection threshhold' or are not properly modeled yet." 
 
What you can do to be safe
Avoiding the lure of seaside property is the easiest way to stay safe from hurricanes, but there are of course millions of people who can't or won't live anywhere else. For them, as well as anyone who picked the wrong beach for a late-summer vacation, there are still plenty of other ways to prepare yourself before landfall:

    * Listen to weather reports if you live near the ocean. Equally important, though, is actually acting on them. Evacuate if you're told to, or if you live in a mobile home, a high-rise building or on a floodplain. If you ride out a storm, stay inside near the center of the house, and don't go near windows or doors.
    * Storm shutters are the best option for protecting windows, but in a pinch, you can also just board them up. Use 5/8-inch marine plywood, cut to fit the window's shape, and make sure it's secure. Don't bother taping up your windows, though — not even duct tape can stop a hurricane.
    * Strong cyclones are known for ripping off roofs, so if you're expecting a big one, you might want to secure the roof to your house's frame using straps or clips. If you have time, it's also wise to make sure nearby trees are trimmed and healthy, since they provide hurricanes with an arsenal of deadly projectiles.
    * Aside from the hurricane itself, you should also prepare for the potential aftermath. Shut off any propane tanks, as well as other utilities if instructed to do so. Turn your refrigerator to its coldest setting and keep its doors closed, because you'll want your food to stay good as long as possible if the power goes out. And ensure a post-hurricane supply of clean water by stockpiling it in jugs, bathtubs, and other large containers before the storm arrives.
    * Don't let the hurricane's eye fool you: Make sure the storm is actually over before you go outside. Even when it has passed, still be careful as you survey the damage. Watch out for downed power lines, dangerous debris and falling trees, and don't wade through floodwaters unless it's absolutely necessary.

上一页  [1] [2]