Sandy strikes Eastern Seaboard
Hurricane Sandy, sometimes called “Superstorm Sandy,” cruised up the East Coast in October, then made a sharp left to cross the New Jersey coast on Oct. 29, causing wind damage, flooding and deaths across the Northeast.
The enormous storm was born in the Western Caribbean on Oct. 22, and at one point was 1,100 miles across, a record for an Atlantic hurricane. Fed by two other weather systems, it created a storm surge and flooding across New York City’s low-lying areas, including parts of Queens and Manhattan. Millions lost electric power, transit systems were shut down, subways and tunnels flooded.
As the region tries to dry itself out and recover, here’s a question: what drives these enormous storms, and how they affect ecosystems and society?.
Hurricanes are born over water, driven by solar energy stored in the ocean. Hurricanes, properly called tropical cyclones, can travel for weeks across the ocean, blasting islands and coastlines with fierce winds, torrential rains and swollen seas.
Hurricanes can also remake land — tearing up barrier islands and dunes while depositing sand on other beaches. But ironically, as soon as a hurricane reaches land, it starts to lose power.
Hurricanes can remake history — the Galveston, Texas, hurricane of 1900 killed 8,000 to 12,000, and practically erased the city, helping convert the inland city of Houston into a petrochemical giant.
These gigantic cyclones can be even more deadly. A 1991 cyclone swept across low-lying Bangladesh, drowning an estimated 139,000 people.
And hurricanes can cost. Early estimates from Florida indicate that Charley and Frances cost $20- $40 billion — more than disastrous Hurricane Andrew in 1991. Sandy’s cost in the New York area is said to top $60 billion.
Packing more wallop than a nuclear bomb?
Once upon a time, hurricanes seemed to come from nowhere. Now, weather satellites track them to their sources. Atlantic hurricanes, for example, originate off the coast of West Africa, where “tropical disturbances” form in low-pressure zones.
A disturbance may intensify into a “tropical depression,” surrounded by a high-pressure zone that helps contain the storm, which is centered on a column of rising air. Winds are moderate: 21 to 35 miles per hour.
Once the winds exceed 35 miles per hour, the system, now called a “tropical storm,” gets an alphabetical name. The storm now has the circular structure of a hurricane, although it may not become one.
Powered by solar heat that was stored in the ocean and then transferred into the warm, moist air, the tropical storm becomes a hurricane once winds exceed 74 miles per hour. (More on hurricane formation.)
You might expect a rotating storm to whirl itself apart, but hurricanes feed on themselves to gain strength. In their energy flow, hurricanes resemble large thunderstorms. But while thunderstorms can start over land or water, hurricanes only start over water. Hurricanes also last much longer, carry far greater energy, and cause much greater destruction. (More on storm comparisons and hurricane formation.)
JUMBO! heat engine
Tropical cyclones are powered by heat engines — “machines” that use heat to do work. In a hurricane, the work comes in the form of driving furious waves and winds. The hurricane sucks in warm, humid air from the lower atmosphere. The air rises and the water vapor condenses, releasing the stupendous amount of heat energy that the moisture absorbed as it evaporated from the ocean. Finally, the storm exhausts this expended air into the upper atmosphere.
Define “stupendous”? The average hurricane releases heat energy equivalent to 200 times the global production of electricity! This released heat drives hurricane winds and powers the upward convection in the storm (convection is the movement of lighter fluids over heavier fluids). The rising air creates a low pressure area near the ocean that draws in more energy-laden air, feeding the continuing storm.
Due to the Coriolis effect, the lower levels of a tropical cyclone start rotating counter-clockwise in the Northern Hemisphere, but clockwise in the Southern.
Eyeball the eyewall
Hurricane winds whirl around the bizarrely calm “eye,” a circular region with little wind, no rain and often a blue sky. The placid eye is surrounded by a circular “eyewall” of furious, thunderstorm-type clouds and the fiercest winds. When Hurricane Camille shredded the U.S. Gulf Coast in 1968, winds in the eyewall reached 200 miles per hour.
Gargantuan winds, combined with extremely low atmospheric pressure near the eye, cause a catastrophic rise in sea level called a storm surge. This destructive mound of water, topped with furious, wind-whipped waves, can hoist the surface 20 feet above average sea level, causing biblical-scale flooding along coastlines. In 1899, the Bathhurst Bay hurricane produced a record record storm surge of 13 meters!
A storm surge in New Orleans after Hurricane Katrina in 2005 spelled disaster — the city is well below sea level, and the catastrophic flooding killed about 1,000 people.
Although storm surges are the most dangerous element of these storms, water causes another problem: All that condensing moisture eventually falls as torrential rain. Although hurricane winds slow as they move inland and become deprived of energy, rains can still be drenching. The record rainfall inundated Reunion Island in the Indian Ocean in 1966. In 12 hours, a ‘cane dumped 45 inches of water!
Hurricanes come, and hurricanes go, but the overall trend is periodic lulls, followed by a series of gangbuster years, like the present.
Even though hurricanes don’t seem to be getting more intense, damage is increasing, mainly because of development and torrid population growth in prime hurricane country: which in the United States includes the Carolinas, Florida and the Gulf Coast.
The threat extends further up the Atlantic Coast. A huge storm around New York City could funnel a 30-foot storm surge toward the city, flooding auto and subway tunnels and causing fearsome destruction.
Cloudy picture is clearing
Hurricanes having such destructive habits, it’s no surprise that emergency managers want better predictions. Since evacuations are slow, costly and disruptive, it’s critical to know where the storm will be, and how strong it will be, a couple of days in the future.
Predictions must start from data on present conditions, and scientists have some hair-raising technology for gathering data. They fly above storms and parachute miniature weather stations into the maelstrom to get precise data. They peer down from satellites to get the big picture. Then they crank up computers that would humiliate that box on your desk.
Satellite data is always valuable over the oceans, where hurricanes form and travel, but where observers are uncommon. Satellite data can take many forms, from simple, visual-light images of the clouds to data from individual wavelengths of light.
Temperatures at altitude, as seen in different wavelengths
Data on water vapor are particularly useful, says Christopher Velden, a hurricane researcher at the Space Science and Engineering Center at the University of Wisconsin-Madison. “Only in the water vapor channel can you get information in clear, cloud-free areas.”
Instruments on the Geostationary Operational Environmental Satellites measure heat radiation released by water vapor in the upper atmosphere. By tracking movement, researchers calculate wind speed and direction. Thus the satellite can see invisible winds in the hurricane’s environment that are crucial to steering the storm.
A newer source of data, the NOAA-15 and NOAA-16 satellites, gives temperature readings from oxygen molecules in the air. Again, the advantage is more, better data to feed computer models of hurricane movement. “We send it to forecasters, so they visually have information on current behavior,” says Velden. “It’s very important to know how it is doing now, so that can be fed into prediction models. The better they are initialized, hopefully the better the prediction will be.”
Where will you be?
Over the past decade or so, improved software, computers, satellites and aircraft have meant a steady improvement in track forecasts, but the improvement is incremental, not revolutionary. Today’s three-day forecast is as accurate as a two-day forecast during the late 1980s.
Average errors in the hurricane tracks are now:
24-hours: 80 miles
48-hours: 110 miles
2 hours: 230 miles
Instead of offering one probable track, modern hurricane predictions are shaped like a funnel. A line down the center indicates the most likely track, but the cone shows the probability of a strike based on similar hurricanes in the past. Anybody within that cone, stresses Timothy Olander, a hurricane researcher at the Space Science and Engineering Center at the University of Wisconsin-Madison, could come face-to-face with nature’s worst storm.
The improved accuracy comes from better computer models and better knowledge of atmospheric conditions in and near the storm. Once the data are gathered, “We know pretty much what moves a hurricane,” said Edward Zipser, a meteorologist now at the University of Utah, and who talked to us in 1998. “If you get accurate data to start, the models are far more capable of forecasting the flow patterns tomorrow or the next day.”
As Zipser indicated, predicting a hurricane’s path is relatively straightforward, although it does involve massaging a gob of data. Generally speaking, tropical cyclones move in response to “steering winds” in their environment. The direction of these winds depends on the high and low pressure zones near the storm. Latitude also plays a role. Between about 10 and 30 degrees north latitude, steady trade winds blow toward the west, pushing Atlantic hurricanes from their origin near Africa toward the Caribbean and United States. North of 30 degrees, prevailing westerlies push the storms back toward the east.
How wild will you be?
Even if you can predict where a hurricane will go, how strong will it be when it gets there? Forecasting intensity is more problematic than path prediction, says Velden. The changes in intensity may result from changing ocean conditions, Velden explains. If the storm goes over a deep pool of warm water, it gets a surge of power.
Atmospheric conditions such as wind shear (a change in wind speed and/or direction with altitude) also matter. “The atmospheric environment must be just right,” says Velden. “A lack of vertical wind shear between the surface and upper-level winds allows it to convect without tilting over, and feed over the energy source.” In those conditions, the hurricane’s heat engine can be self-sustaining. If wind shear is too great, however, the storm can be blown apart.
At any rate, a run-of-the-mill hurricane can become a rip-roaring monster with little warning. This year, Velden says, “Charley jumped two categories in 24 hours,” an increase in maximum winds of 40 to 60 miles per hour. “It was a contracting eye, much like a twirling skater who brings in the arms; it gets smaller, but the spin gets faster.”
Damage goes up exponentially as wind speeds increase, making a category 4 storm many times as destructive as a category 2 storm. Building damage, for example, increases from “moderate damage to houses” in category 2 to “extreme structural damage.”
Will you rain cats and dogs?
Another problem is predicting rain intensity. “What’s the difference between a storm that drops five inches — and one that drops 20?” Zipser asked. “It’s not related to the rates of wind.” In fact, he notes, tropical storm Charlie — which was too sluggish to qualify as a hurricane — dropped 15 to 20 inches of rain on Del Rio, Texas in 1997.
Better precip predictions call for more understanding of the physics of convective systems in thunderstorms and hurricanes, Zipser noted. Precisely modeling the condensation of water particles into bazillions of rain droplets is not easy, Zipser said. “We have more to learn in terms of fundamental understanding of what controls the intensity.”
Which data mata?
If you like data, you have to love meteorology. Satellites download deluges of data, but is more data better? Usually, but quantity can raise problems, says Tim Olander of the University of Wisconsin-Madison. With each technological advance in meteorology, new sources of data have threatened to inundate forecast models. The problem arose with the first weather balloons, the first weather satellites, and looms again with the growing number of instruments perched on modern satellites.
The GOES-8 satellite, for example, reads visible light and 18 other wavelengths. But a new series of weather satellites, Olander says, will send “hyperspectral” data on thousands of wavelengths. And that will raise thousands of questions. Before any wavelength is fed into a computer model, should it be averaged, deleted, combined or subtracted, or used as is?
Those questions will be tackled with an ancient tactic, he says. “We’ll do what we’ve already done. We’ll use trial and error to find out what works best. Using all that data is extremely difficult, it’s a very important problem to solve.”
Most improvements will be incremental rather than revolutionary, however. No way does Olander expect to ever predict that a particular tropical depression near West Africa will some day hit Miami Beach rather than Palm beach. Weather is way too chaotic and complicated for that.
Yet steady improvements in accuracy have lead the National Hurricane Center to begin issuing five-day forecasts, and those, notes Velden, accurately predicted landfall in Florida for both Charley and Frances. “This is remarkable since just two years ago, the NHC did not even predict out five days due to the uncertainty! Can we make a precise forecast out four or five days? No. But the ability to put out a credible alert to folks and industry that much in advance is a testament to recent gains in research and operational hurricane prediction.”
- David J. Tenenbaum
- Historic storm tracks ↩
- Oodles of hurricane images from space ↩
- Online meteorology guide ↩
- Hurricanes in motion ↩
- National Hurricane Center ↩
- Miami Museum of Science hurricane page ↩
- Tropical twisters for kids ↩
- An overview of Hurricane Charley ↩
- Coming to terms with a hurricane ↩
- How are hurricanes formed? ↩
- Tracking hurricanes ↩
- Lots and lots of info about hurricanes. Enjoy! ↩
- Here’s the answer to that age-old question does the damage caused by a hurricane increase exponentially with wind speed ↩
- How much energy does a hurricane release? ↩
Tags: atmosphere, Christopher Velden, Edward Zipser, hurricane, meteorology weather, predict prediction, Timothy Olander, tropical cyclone, tropics, University of Wisconsin Madison UW-Madison, weather forecast prediction