News

Sep 17, 2005

Hurricanes

The probability of eventually being confronted with a devastating hurricane is a defining feature of most Caribbean islands, including St. Barts, and perks up the community annually during the late summer and fall, the period of the year when these storms are most apt to occur.

The likelihood of a mature storm scoring a direct hit is actually quite small, but the consequences are of such magnitude that every resident has a carefully thought out plan to protect himself and his property from the onslaught.

Hurricanes are born in the tropical latitudes of the North Atlantic Ocean. During their short life they exercise a display of raw power that the best efforts of mankind cannot control, diminish, or influence in any way.

Here's how hurricanes happen:

Visualize the relentless radiance of the sun eternally broiling a small, spinning, and very pretty planet. The clarity of its color is heightened by the absorbing blackness of the surrounding void.

Visualize the planet's surface, some land and much water, and now visualize the whole thing enveloped in a moderately thick coating of restless and transparent air. This air is the raw material from which hurricanes are made. It is the same air you breath every day; perhaps a little warmer; perhaps a little more moist; surely less polluted.

This envelope of air responds to the sun's radiance unevenly, and is incessantly dividing itself into discrete pieces separated by a thin border of difference in temperature, humidity, or both. This is happening all the time, everywhere, and has been going on since the beginning.

Late on a fall afternoon, just off the west coast of Africa, a little north of the Equator, one of these pieces of air, warm and water-saturated, begins to rise. It is warmer than its neighbors: not by much, but by just enough to cause a hesitant, gradually more confident, ascent.

As its upper portion reaches into the cooler upper atmosphere, the burden of its moisture is condensed, first into thick clouds, then into fitful showers, and finally into undulating torrents of rain. Surprisingly, in obedience to one of Nature's many inexplicable rules, this release of moisture generates additional heat, accelerating the process, and pushing the top of this bubble ever upward into still colder reaches causing more clouds, more rain, and that peculiar friction of molecule against molecule that gives birth to bolts of lightning and crashes of thunder.

As the bottom of the bubble rises from the surface, the surrounding air rushes in from every direction to fill the void, becoming a new bottom as the old one dissolves and rises toward the heart of what still remains an ordinary and unexceptional storm.

Think of an inverted mushroom. The cap is the air being pulled in from the surrounding surface of the sea; the stem is the rising column of warm moist air. Where the usually diminutive roots would be, is a tumult of boiling black clouds and roiling sheets of falling water, ragged bolts of lightning and the shattering explosions of thunder.

Now, slowly, unexpectedly, something astonishing begins to happen. This upside down mushroom begins to gently rotate. It begins to revolve around its vertical axis like a carousel: cap, stem, and boiling roots all together, slowly at first, but imperceptibly and gradually quickening. Before long, as it becomes sure of itself, it begins to whirl like an teenage ballerina. This is the moment of epiphany, the moment of definition, the moment when this commonplace, albeit impressive, dance of the elements becomes the birth of a hurricane.

To understand what causes this remarkable event, it is necessary to move back again, to regain our distant perspective, and to re-visualize the earth spinning like a top on its path around the sun.

The earth spins on an axis. No one is quite sure what set it going, but going it is, and there is nothing to indicate that it is going to slow down, or stop, or soon change this aspect of itself in any way. It just keeps spinning, and spinning, and spinning.

Each point on the surface of this spinning sphere makes a full circuit every twenty four hours, but the distance traveled by individual points varies enormously: it depends on how far you are away from the poles. If you were standing next to one of them, a complete rotation would transport you a only few dozen feet. If you were standing on the equator, the same rotation would carry you almost twenty five thousand miles.

There is another way to express this: the farther you are way from the poles, the faster you are going. As you descend from the pole toward the equator, you go faster and faster, just as you do when you walk from the center to the edge of a carousel. Anyone in St. Barts is really zipping along compared with people in Paris, or New York, or Moscow, though you'd never know it to look at them.

It's easy to forget that we're whirling around like this; it's not the sort of thing you notice in the course of daily life, though there is a reminder available every day in every kitchen sink. When you pull the plug, the escaping water forms a spinning vortex as it rushes down the drain. It always turns in the same direction. What makes it spin like this is the rotation of the earth.

One side of the dirty dish water is being transported just a fraction faster than the other side - it's further from the pole - and this tiny inequality of velocity spins the fluid as it descends into the drain.

This phenomena is called the Coriolis Effect, named after Gaspard Coriolis, a French civil engineer who lived until 1843, and was interested in why cannonballs deflected from their geometrically predicted path. Gradually he realized that the motion of the earth effects the motion of everything on it, and his calculations opened the way to accurate intercontinental ballistic missiles, a trip to the moon, and an understanding of what makes an ordinary tropical storm begin to revolve, creating the self-perpetuating beast variously called a cyclone, typhoon, or, locally, a hurricane.

If you think about the direction of the earth's rotation, you'll see why hurricanes and draining dishwater rotate counterclockwise in the northern hemisphere, and in the opposite direction in the southern hemisphere.

Once the rotation of the storm has been established, the whole affair becomes a kind of perpetual motion machine. The centrifugal force of the rotation reduces the pressure at the center even further, and the increased rush of surface air hurrying to fill the deepening void accounts for the phenomenal wind speeds by which storms of this kind are defined.

This same centrifugal force creates and area of relative calm at the core of the storm that is called the eye. Heaven knows how it got the name. It doesn't resemble an eye at all unless viewed from above, and that was not possible until fairly recently.

The eye of a hurricane is an eerie place. The tossing surface of the sea is covered with all kinds of floating junk that powerful winds have pulled into it, and this assortment of flotsam - uprooted trees, chunks of styrofoam, oil drums, and unaccountable debris of every shape and size - serves as a last refuge for thousands of exhausted birds who have suffered the same fate. For them, there is no escape. They are encased by an impenetrable wall of overwhelming wind. They can only hang on, try to gather their scattered wits, and hope the storm will fall apart. Most will not survive.

Another feature of the eye is that its floor, the surface of the sea, becomes domed, like a blister, rising into the decreased pressure of the core. This accounts for the flooding tides that inundate the shore when these storms encounter land. During the catastrophic hurricane of 1960, the entire area between Shell Beach and Gustavia Harbor was awash with this mass of elevated water. Presqu'ile became the real thing, cut off from the rest of the island, until the water subsided.

As a self-sustaining equilibrium of whirling wind, rising air, and falling torrents of rain becomes established, the whole thing begins to move. Its path away from the coast of Africa toward the Newer World is renowned, made famous by Christopher Columbus' trio of graceless wooden ships.

This broad and featureless avenue has no sign to indicate its entrance, or barriers to contain and guide its travelers. It is marked only by a warm and friendly wind that rarely fails in either intensity or direction, a wind whose reliability is caused, once again, by the fact of a rotating earth bathed in the radiance of the sun.

Over the center of each great ocean there is a mass of stable high pressure air. It is a result of the relative thermal stability of water, which, once heated, is more reluctant to cool off than is the land. This mass expands in the summer and shrinks in the winter in response to the changing intensity of the sun's rays. At the end of the summer, during the spawning season for hurricanes, this enormous, more permanent bubble is fully inflated. Like its furious little cousin, the hurricane, it too rotates around a central axis, though much more slowly, and, more importantly, in the opposite direction. The North Atlantic High, as it is called, flows outward, and turns, consistent with Monsieur Coriolus' expectations, like a clock. An Atlantic hurricane, with its vacuous core, flows inward, and, responding to the same forces, turns oppositely, or counterclockwise.

Stand back now, and take in the whole picture: Spread across the expanse of the North Atlantic Ocean, a huge mass of fair weather slowly turns, moving to the right between Labrador and Ireland, to the left between Senegal and the islands of the West Indies, rising to the north on the east coast of the USA, and descending to the south past Portugal and the entrance to the Mediterranean Sea.

A hurricane, born in the steaming latitudes adjacent to West Africa, begins to move, and is gently but firmly captured and nudged along westward by the lower edge of this rotating disc of air. As the hurricane makes its way across the Atlantic, it matures, growing to whatever dimensions the conditions of heat and moisture will allow. As it approaches the string of islands that define the Caribbean Sea, it may be tossed off the carousel and left to find its way through the islands, westward to whatever destiny awaits it in the mountain jungles of Central America, or along the American coasts of the Gulf of Mexico. More likely, it will hang on to its free ride, turning northward and striking terror into the hearts of shore dwellers from Florida to the eastern provinces of Canada. They can only guess if it will suddenly reach out for the brass ring and transform their seaside idyll into a nightmare of destruction. If its hold is especially tenacious, it will cross the Atlantic again, this time from west to east, and, gradually weakened by the cold, hurl itself, reduced now to little more than bluster and rain, on the shores of northern Europe.

Sir Francis Beaufort was a nineteenth century English naval officer (an exact contemporary of Monsieur Coriolus, in fact) who devised a scale of wind conditions, still used today, that was expressly meant to be useful to mariners. He divided the range of possibility into twelve parts, gave each a name, and included a few unexpectedly charming descriptive remarks.

A sample from his table: "Force Six - Strong Breeze - wind speed 21 to 35 miles per hour - Large branches in motion; telegraph wires whistle; umbrellas used with difficulty." Occasionally he approaches the poetic: "Force Five - wind speed 19 to 24 miles per hour - Small trees in leaf begin to sway; wavelets form on inland waters."

When he reached the top of the scale, the charm and poetry were usurped by a blunt finality: "Force Twelve - Hurricane - wind speed above 73 miles per hour - Devastation occurs."

Devastation occurs.

Indeed it does. How long did Sir Francis linger over this choice of two words to summarize all that he knew of a wind that could flatten a forest, obliterate the separation between sea and sky, and hurl the remnants of Her Majesty's finest fighting ships onto the High Street of a seaside village ? He was certainly aware of the pivotal event that launched his countrymen on their path to glory:

In 1588, the Spanish Armada sailed northward to humble the English by force of arms. One hundred and thirty one ships put to sea, crowded with twenty thousand disciplined and ferocious halbediers, dragoons, cannoneers, and troops of the line.

South of the Channel entrance, their single formidable formation was assaulted by a revolving storm of devastating force, scattering ships first in one direction, then in another. Dozens were overwhelmed and sank, some were hurled onto rocky shores of France, and others were carried as far as the southern coast of Ireland. When the storm subsided and remnants of the Armada regrouped, they were easy prey for an eager English fleet that had weathered the tempest in the snug shelter of Plymouth Harbor.

Thirty ships returned to Spain, soldiers and crew thanking God for their deliverance, kissing the crowded and dirty soil at the end of each gangplank. Their humiliation ended forever the Spanish domination of the seas, passing the chance to the English, who redirected the course of the exploration and exploitation of the New World, traveled to every corner of the earth, and proceeded to build the greatest Empire the world had ever known.

The Armada was beaten twice: first, by an elderly hurricane, and, finally, by the agile ships and sailors of the Queen Victoria's Royal Fleet. The destiny of our ancestors and ourselves was given direction by a cyclone, born in the tropics, twice traveled across the Atlantic: a whirling heat machine made of moistened air: 78% nitrogen, 21% oxygen, and a 1% mixture of argon, carbon dioxide, helium, krypton, neon, xenon, wood smoke, cow farts, and baby burps, all set into motion by the irrepressible spinning of the earth.

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