Around the lower rim of the vortex small tornadoes were constantly forming and breaking away. These looked like tails as they writhed their way around the end of the funnel. It was these that made the hissing noise. I noticed that the direction of rotation of the great whirl was anticlockwise, but the small twisters rotated both ways — some one way and some another.
The opening was entirely hollow except for something I could not exactly make out, but I supposed it was a detached wind cloud.
No meteorologist, not even the great Fujita, could have asked for a better vantage or a better description. The presence of inner vortices that Keller refers to as small tornadoes (which, in fact, they were) wasn’t even guessed at until Fujita first speculated about their existence in 1965, but here is Keller describing them quite lucidly 37 years earlier. While the strong, gassy odor he reports has never been explained, the tubular cloud that snaked up the center of the tornado has since been witnessed.
No other storm exceeds the wind speed of a tornado. The debris field left in the wake of a major one often includes extraordinary testaments to a tornado’s power and bizarre selectivity. Wheat straws are imbedded inches deep in tree trunks. An egg is found perforated with a neat hole that looked as if it had been drilled — inside is a bean. A diesel engine on the Union Pacific Railroad is lifted off its track, spun in midair and put back down again on a parallel set of tracks facing the opposite direction. A mattress with a child sleeping on it is sucked out the window of a Kansas farmhouse and dropped in a field nearby without even waking the child. A lit kerosene lamp is carried hundreds of yards and then set down upright with the flame still burning.
6
Katrina
The Life Story of a Hurricane
“Y’all can’t stop Mother Nature. If she wants
to come ’n get ’cha, she’ll come ’n get ’cha.”
Katrina survivor, New Orleans
I’m a born storm chaser. Growing up as I did in the middle of a summer storm zone at the southern end of the Great Lakes peninsula, I was witness every summer to hot Gulf air masses blowing north, up the Mississippi valley and spawning terrific thunderstorms over my hometown. I prayed for tornadoes. I stood outside in the wind before the rain, searching the blue-black clouds for a funnel, and, as an adult, I drove toward distant towering thunderheads, hoping to catch even the briefest glance of a twister. But my wishes were never fulfilled, at least not in the tornado department.
I’d arrived in Grand Bahama late Sunday afternoon on August 21, 2005. After unpacking, I took a sunset stroll along the beach. The air was hot, even in the early evening, and the ocean was calm enough for me to spot a dark patch in the water. Could it be a coral reef? After breakfast the next morning, I strapped on my mask, crouched in the seawater and pulled on my flippers.
The water was surprisingly warm and not only in the shallows. Just the mild exertion of a facedown breaststroke made me sweat, underwater, a decidedly peculiar sensation. (The purpose of perspiration — cooling the body — is defeated underwater, with sweat only pumping more salt into the ocean.) I had no idea that water this warm was unusual even for Grand Bahama in August and that it would play into meteorological events that were underway long before I landed in Freeport.
Three weeks earlier, around the time I was wondering what I should pack for my trip, the weather in North Africa was conspiring a chain of events that would turn my holiday upside down. And it all had to do with prevailing winds, the northern trade winds in this case, which in turn are entirely a result of the Coriolis effect.
The Coriolis Effect
In the northern hemisphere, anything airborne and moving in a straight path in any direction — east, west, north or south — has a tendency to deflect to the right, while in the southern hemisphere it has the opposite inclination. This tendency, the Coriolis effect, is named after the French engineering professor, Gaspard-Gustave de Coriolis, who discovered it in 1835. It is a result of the Earth’s spin, and the effect is so pervasive that even long-range artillery guns have to be specially calibrated to compensate; otherwise, they would miss their targets.
So why the difference between the northern and southern hemispheres? In both hemispheres, the sun rises in the east and sets in the west, but that’s about it for similarities — because the hemispheres rotate in opposite directions. But how can this be? How can two halves of a single planet spin in opposite directions? It’s one of the profound paradoxes of rotating spheres, but it can be solved with a simple thought experiment.
Imagine you are on a spacecraft approaching Earth from above the southern pole. You would descend on a planet spinning clockwise. But take the same spacecraft and approach the Earth from the North Pole, and you land on a planet spinning counterclockwise. Voila.
High-pressure systems in the northern hemisphere, whose outflow of wind is deflected to the right by the Coriolis effect, rotate clockwise. Northern hemisphere low-pressure systems cause an inflow of wind and so rotate counterclockwise. Just to confuse everything, meteorologists call high-pressure systems (which spin clockwise) anticyclones and low-pressure systems (which spin anticlockwise) cyclones. At least in Australia, their high- and low-pressure systems are less confusing: their anticyclones spin anticlockwise.
The Coriolis effect decreases with latitude. The closer you get to the equator, the less it deviates. Within five degrees above and below the equator, its effect is almost zero. But it definitely has an effect on subtropical weather beyond 15° north of the equator: it delivers hurricanes that hit the Caribbean region and North America.
This is what happens. Air over the equator heats, rises and is pushed north in the upper atmosphere. There it cools and sinks and travels south in the lower atmosphere, forming a kind of tubular vortex — a Hadley cell — that rings the Earth. The southward traveling winds of a Hadley cell are deflected to the right by the Coriolis effect, producing winds blowing from the northeast to the southwest. These are the northeast trade winds.
The northeast trade winds dominate a world-girdling belt of ocean and land just north of the Tropic of Cancer, shunting all the weather in their path — highs, lows, tropical depressions — westward. They even blow red dust from the Sahara all the way to Florida. In the first week of August, when I was planning my trip to Grand Bahama, the northeast trade winds nudged an unusually violent storm system out of Chad through Nigeria, then on through southern Mali and Guinea. After drenching Guinea with torrential rain, the system blew out into the Atlantic Ocean just south of Cape Verde on August 8.
If North Africa is the nursery for fledgling hurricanes, then the Cape Verde islands are their finishing school. In the late summer months, these islands oversee the development of dozens of tropical waves (a term for a low-intensity storm) that then often transform into tropical depressions. Under the right conditions, tropical depressions turn into hurricanes. Just south of the Cape Verde islands, the African storm system that had left Guinea was coalescing into a tropical wave — a large area of low pressure studded with several active thunderstorms and sporting a complex, extensive cloud shield. Big enough to get some high-tech attention.
The National Hurricane Center has several satellites keeping an eye on the hurricane marshaling grounds. Aqua, launched in 2002, measures sea surface temperatures with a microwave scanning radiometer, and the TRMM (tropical rainfall measuring mission) satellite, a joint Japanese-American satellite launched in 1997, carries a lightning imaging sensor as well as a precipitation radar that gives scientists a three-dimensional view of rain within a storm. In satellite photographs, tropical disturbances look unorganized and patchy, like a random gathering of clouds. But some are a little swirlier than others, and in them you can vaguely see the outlines of a nascent hurricane, like an embryo in a sonogram. This was the case by August 11. The satellite data clearly showed the tropical wave had signs of convective organization, and two days later, on August 13, it was officially upgraded from
a tropical wave to a tropical depression and given a number: 10. It was beginning to rotate, and wind speeds within the storm were reaching 25 to 38 miles per hour as it began its westerly drift along a strip of tropical Atlantic Ocean that stretches some 2,408 miles from the east African coast to the Caribbean.
Hurricane Alley
During summer, the tropical Atlantic heats up like brine soup and the trade winds push developing hurricanes west. They grow larger and evolve from tropical waves into tropical depressions and then into tropical storms. Hurricane Alley is like the conveyer belt that runs through the oven of an industrial bakery, only the airy loaves that pass through Hurricane Alley’s assembly-line bakery puff up on moist, thermal updrafts from a hot ocean. Water temperatures should be a minimum of 26.5°C down to a depth of at least 150 feet to properly spawn a storm. During a hot summer, Hurricane Alley can build a storm from its fledgling stage off the coast of Africa to a monster hurricane threatening the North American and Caribbean coasts.
Tropical depression 10 was 1,600 miles east of Barbados when it was given a number, but U.S. forecasters at the National Hurricane Center doubted it would get any bigger because mid-level wind shear had developed, likely to cap the ability of the storm to build a central core. Hurricanes are, in essence, warm-core systems that are stacked vertically. Hobbled by wind shear, tropical depression 10 meandered slowly west for a few more days until by August 14, a week before I arrived in Grand Bahama, it had degenerated and the National Hurricane Center had reclassified it as a tropical low pressure. It lost its number. By the 18th, it had dissipated almost completely.
This weak disturbance that was hardly worth tracking — the remnants of tropical depression 10 — would turn into Katrina, and her ability to surprise was about to become evident. Now a disorganized tropical low, it kept drifting westward, wafted by the northeast trades. It entered the ocean north of Puerto Rico a day after I landed in Freeport. There it joined forces with a local tropical low and formed an alarming alliance that caught the attention of the National Hurricane Center. Its QuikSCAT satellite, which measures near-surface wind speeds with a SeaWinds scatterometer (a microwave radar) capable of penetrating cloud cover, had picked up one of the key developmental stages of a hurricane: the lowering of the mid-level circulation to the water’s surface. Wind-whipped ocean spray was feeding warm moisture directly into the hungry storm. On August 23, this new hybrid storm was given a new number, tropical depression 12, and it slid into the Bahamas late the same afternoon.
I had had two lovely sunny days in Grand Bahama before tropical depression 12 struck. On that Tuesday afternoon, the sky clouded over, and in the evening it began to rain. There was some wind but not a lot. During the afternoon, the National Hurricane Center had issued a tropical storm watch. There was a possibility that tropical depression 12 might intensify. The wind was increasing, and the local weather channel was clearly alarmed about the possibility of a tropical storm. Being used to the hysterics of my own local weather channels in Toronto, I was skeptical and figured I’d still be able to go snorkeling the next day.
When steady winds reach 39 miles per hour in a tropical depression, the National Hurricane Center gives it a name and upgrades its status to tropical storm. On Wednesday morning, August 24, tropical depression 12 was given the name Katrina. Well, I thought, at least a tropical storm isn’t a hurricane, and although it was breezy in Freeport, it wasn’t that windy. So I drove to the west end of the island to snorkel at my favorite offshore reef, Paradise Cove. There were few showers on the way, but the air was warm, and, more importantly, when I arrived the waves were not so large that the dive masters had shut down access to the outer reef. I parked, checked in, put on my flippers and mask and swam out with dark clouds looming ahead of me.
By the time I got to the outer reef, I was the only snorkeler and I could hear occasional thunder. The storm was definitely getting closer. Seeing lightning flicker under water, reflected from above, seemed totally worth the risk I was taking. It lent a dramatic, moody ambience to the electric colors of the fish and coral. I finished a circuit of the reef as the wind had begun to pick up. I could hear it whistling in my snorkel. As I came around the southern end of the reef, I noticed a motorboat heading toward me. The driver turned out to be one of the staff from Paradise Cove. “Do you need a ride?” he shouted over the wind. I lifted my mask while treading water and said, “No thanks, I’m fine.” Though actually, it was a bit of a slog getting back to shore. The waves were getting a bit choppy.
On the drive back, I had to negotiate some fairly deep, large puddles — there are no storm sewers on Grand Bahama. Otherwise it didn’t seem very stormy. Late that evening, the local television forecast showed radar images clearly revealing that tropical storm Katrina, which was just south of Grand Bahama, had broken up into three distinct fragments. As a result, the weather advisory was lifted. I went to bed just before midnight on a completely calm evening.
No one was sleeping at the National Hurricane Center though. High above me, careening through the upper thermosphere in the early morning hours of August 24, the NHC’s TRMM satellite was sending some alarming data back to the NHC command center. The radar was detecting the formation of eyewall clouds south of Grand Bahama. This is where the heaviest rainbands (continuous, linked chains of thunderstorms) concentrate during the transformation from tropical storm to hurricane. At around three in the morning, Katrina opened its eye.
The formation of a hurricane’s eye is still not completely understood but the basic mechanics are well-known. A hurricane is a vast heat-sucking machine sustained by strong updrafts. As the rainbands begin to rotate around the hub of the hurricane, a strong ring of vertical convection forms just outside the midpoint. This massive circular updraft of warm air rises to the top of the hurricane, where it creates a layer of high pressure directly above the low-pressure heart of the hurricane. Most of this rising air flows outward over the hurricane in a clockwise spiral. This strengthens the vertical updraft, and consequently the power of the storm, in a mighty feedback loop. But in the small, restricted central core of the high-pressure lens above the center of the hurricane, the air is forced to flow inward. Without anywhere else to go, it begins to descend, creating a rainless, cloudless hole at the hurricane’s center. The eye. Here, barometric pressure is at its lowest.
Generally speaking, the smaller the eye, the more powerful the storm. Hurricane Wilma, which followed Katrina in October 2005, had an eye only 2.3 miles across. It was an intensely destructive category 5 storm. Wilma set another record, one for low pressure, bottoming out at an ominous 26.05 inches. (Household barometers only go down to 27.5.) At the other end of the spectrum, Typhoon Carmen, which hit South Korea in 1960, had an immense 230-mile-wide eye and winds so low it barely qualified as a typhoon. I think you could’ve forgiven people in the path of Carmen for thinking that the storm was finished; it took hours for its eye to pass over them.
Typhoons are just hurricanes that form in the Pacific Ocean west of the international dateline. A hurricane that forms over the Indian Ocean is called a cyclone. Hurricane John, a Methuselah among hurricanes, lasted 31 days in August and September 1994. John crossed the international dateline twice, becoming Typhoon John and then doubling back to become Hurricane John again. If a hurricane skips from the Atlantic basin into the Pacific basin, like Earl did in August 2004, the name has to change. Earl became Frank.
I was woken up around four on Thursday morning by a repetitive banging sound. Someone must have come home late, but what were they doing? I got out of bed and raised the blind to look into the parking lot. Bedlam. Horizontal wind-whipped rain. The banging sound was coming from a metal rain gutter flapping against the ceramic roof tiles as the wind pried it off the building.
The word hurricane is derived from the Caribbean Arawak name for their storm god, Huricán, adopted by Spanish colonialists as huracán. Peak hurricane season for the West Indies, Bahamas and Florida is from m
id-August to late October; during the thousands of years the Arawaks inhabited the Caribbean before the Europeans arrived, they were right in line for most of these storms.
Not all hurricanes are the same. None of them are minor, and some are formidable. As it turns out, I had picked a dilly of a year to visit Grand Bahama. The year 2005 broke all records with an unprecedented 15 hurricanes in one season. Four were category 5s.
Calibrating Catastrophes
The Saffir-Simpson scale measures a hurricane’s strength. Herbert Saffir, an engineer from Coral Gables Florida, and Robert Simpson, then director of the National Hurricane Center, developed their hurricane-intensity measurement system in 1971. Saffir had devised the preliminary calculations earlier when he was commissioned by the United Nations to study the effects of hurricane wind on low-cost housing. Like Fujita before him, Saffir calculated the various thresholds of structural damage according to wind intensity. After some fine-tuning, he brought his 1–5 scale to Simpson, who added in the effects of storm surge and flooding.
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