Tropical revolving stormsApr 29, 2016
A look at how the worst storms form and tactics for dealing with them
Powerful storms like hurricanes can add to the strength of their winds via their forward motion, thus one side of such a storm is labeled the dangerous semicircle.
Editor’s Note: The following is an excerpt from sailor and author Tom Cunliffe’s latest book, The Complete Ocean Skipper, published by Adlard Coles, April 2015.
The tropical revolving storm (TRS) is the planet’s most extreme system. Under their various guises of hurricane (North Atlantic), typhoon (China Seas) or cyclone (Indian Ocean), these cyclonic disturbances cause more mayhem every year than most commentators would care to quantify. In 1970, a cyclone rampaged ashore on the Bay of Bengal, killing half a million people and unleashing energy that has been likened to many atomic bombs.
It isn’t only the Third World that suffers from these terrible storms. New Orleans was shattered by a hurricane in 2005, while Hurricane Andrew back in 1992 left upwards of a quarter million homeless in Florida and ran up an unsettled account for US$25 billion. Life aboard a yacht at sea in such conditions is effectively indescribable. Knowledge is power, however. To make certain that it doesn’t happen to you, the nature of the beast must be thoroughly understood. Later in this chapter, we will consider tactics to weigh up if you ever find yourself peering into the wrong end of this grizzly gun barrel. First, we’ll take a look at what makes it tick.
Early life and nature of a TRS
Most TRS activity originates from shallow tropical wave depressions running down the trade winds, or major cloud disturbances associated with the ITCZ. They demand a limitless supply of moist air, for which the critical criterion is a sea temperature of around 80° F (26° C). This is only achieved in tropical seas after they have warmed to late summer and autumn levels. The proposition holds good even in the Indian Ocean and Arabian Sea where, although a TRS can occur at either change of monsoon, they are at their worst in the usual northern hemisphere danger months of July to October.
These storms cannot form any closer than 5 degrees to the equator, because the Coriolis force of the turning Earth that sets all weather systems circulating diminishes as the equator is approached. At anything less than this critical distance there simply isn’t enough “spin” to crank up the action.
As moist air rises over the warm sea, an area of low pressure is formed, which may develop a circulatory motion if it becomes enough of a feature for the Coriolis force to grab hold of it. Once this occurs it is designated a “tropical depression,” defined by winds of force 7 or less, but it now has the potential to wind up into something far worse. The rising air in the center of such a system sheds latent heat as its moisture precipitates out at higher altitudes. The freed heat keeps it rising until it runs out of moisture at anything up to eight miles high. The newly dried air is spun out at the top of the depression as it begins to circulate with real power. The air now travels to the outer limits of the center’s influence, to be sucked down towards sea level again. Close to the water once more, it loads up on moisture, spirals inwards towards the center and repeats its journey at ever-more frenetic velocity.
As this huge heat engine spins ever faster, winds near what is now the eye increase to anything from 35 to 55 knots and, before long, force 12 is reached. This is 60 knots, and a full hurricane, cyclone or typhoon is born.
The storm can be anything from 50 to something approaching 1,000 miles in diameter, although typical sizes range between 150 and 250 miles. Wind speeds may rise into the realms of the unimaginable. Hurricane Gilbert wound up to 139 knots in 1988, Luis managed 120 knots in September 1995 and rampaged through the Caribbean with terrible results for St. Martin and Antigua. Speeds of 180 knots have been mentioned, but a more typical wind speed, for Atlantic hurricanes at any rate, is around a hearty 90 knots.
Life in the storm’s eye, or “vortex,” is remarkable for the fact that an observer may well have clear blue sky overhead while the mighty clouds of the eye “wall” tower up on all sides. Rainfall in the circulatory system will have been phenomenal and seas will have built to enormous proportions. Once in the eye, rain stops while the seas, temporarily bereft of the wind that was giving them some discipline, become chaotic and mountainous. As the eye passes, the wind returns from a different direction, the rain begins once more and the seas slowly sort themselves out, although this is purely relative.
A mature tropical revolving storm with a clearly defined eye.
Mid-life, routes and death of a TRS
A TRS normally grows as it travels. Its pressure may not sound staggeringly low to a sailor from higher latitudes used to winter depressions of 950 millibars. A TRS may only run to 960 mb, although far lower readings, such as the exceptional central pressure of 888 mb achieved by Gilbert, have been recorded. What is extraordinary is the pressure gradient, which can be as much as 10 mb over 50 miles with even greater falls towards the center.
It is impossible to be specific about the movements of a typical TRS, but it can be said, with only rare contradiction, that most Atlantic and Pacific examples begin their lives on the equatorial side of their mid-oceanic subtropical “high,” sometimes a good way east. They then travel west or northwest (southwest in the southern hemisphere), slowly at first, before accelerating to 10 or 12 knots as they approach the 20th parallel. Somewhere around here, many storms “recurve” in a clockwise direction (anticlockwise in the south) towards the pole and move at increasing velocity away from the tropics. This may also carry them further from the continental land mass on the west side of the ocean, but it doesn’t help Bermuda or any Pacific islands in the way. The center of recurvature of a storm is called the “vertex” of its track, which is not to be confused with the “vortex.”
In the most general terms, a recurving TRS will follow the isobars of the mid-latitude anticyclone, or “high,” but even with modern computer models and all the other wonders of science, TRS tracks remain notoriously unpredictable. Storms have even been known to perform a full loop, which forecasters may at first assume to be the recurving process.
As a TRS travels towards temperate latitudes after recurving, its size increases and ultimately it loses its tropical identity by developing into a frontal depression or becoming absorbed into the circulation of a temperate system that has already formed.
The alternative scenario is that the recurving procedure does not take place and the storm goes barreling ashore with catastrophic results. It is not unusual for this to happen when a North Atlantic hurricane piles into Florida or enters the Gulf of Mexico after marching down the Greater Antilles chain. It then turns right and pulverizes the Deep South of the U.S. Another area where recurvature tends to fail is the Bay of Bengal, leaving the low-lying Ganges delta and the Sunderbunds a helpless victim.
When a tropical storm comes ashore, its days are numbered and it is doomed literally to “run out of steam” from lack of the warm water that fuels its engine. Occasionally, however, they have enough inherent energy to leapfrog the land and reappear back at sea to be rewound. This can occur in Central America, while cyclones have been known to travel clear across India to re-establish themselves in the Arabian Sea.
The primary factor in dealing with the TRS menace is to arrange your life so that the issue of how to survive one never arises. Every passage-planning guide, all the world’s pilot charts and every Internet site worth anything at all will give the times of year when the TRS factor must be considered. Because of the unpredictability of tropical cyclones, you can never be absolutely sure of missing them by working the seasons, but you can easily whittle down the chances of being hit from a measurable possibility to the almost negligible. Nobody is going to put his hand on his heart and say: “Cross the Atlantic from the Canaries to the Caribbean at Christmas and I promise you won’t see a hurricane.” Those of us who have made such crossings in winter have not spent our time nervously scanning the sky and tuning our PCs to the hurricane advisory service. Find out about the local TRS season, and go sailing somewhere else.
Sometimes, it may be necessary to be out on the ocean at times when TRS activity is a measurable possibility. An example would be a decision to sail to Bermuda from the U.S. in late October. You might do this in order to miss the beginning of the New England winter before moving south to the Caribbean a few weeks later when the hurricane season is virtually closed. Hurricanes can form in October and do sometimes recurve towards Bermuda. It happened in 2014, but, on the balance of probabilities, the chance might be felt worth the taking to avoid more certain horrors further north, especially with a clear forecast to start the trip. The bottom line is that unless you plot your whole cruising curriculum with TRS avoidance as the sole item on the agenda you must accept the possibility, however remote, that you may one day be hit.
So what are the signs that a tropical storm is coming your way?
Anatomy of a TRS. Observations are taken along the line of the advancing eye. Note that Ci = cirrus, Cs = cirrostratus, Cu = cumulus and Cb = cumulonimbus.
Courtesy Adlard Coles
Warnings from your own observations
Barometer: Just as it is in more temperate latitudes, the first telltale is usually the dear old barometer. In the tropics, the “glass” generally stands rock steady for days on end within the limits of the phenomenon known as “diurnal variation.” Typically, this involves a rhythmic pulsing of pressure of 1–1.6 mb either side of the mean. The cycle happens twice a day, with “highs” around 1000 and 2200 local time, and lows corresponding at 0400 and 1600. Mean pressure comes an hour or so after noon and midnight.
You soon get used to diurnal variation when sailing the tropics, and if the pressure nudges up a notch outside yesterday’s high, watch out. There might be a TRS 500 to 1,000 miles off. If the glass now starts to drop, note readings regularly. A drop of 3 mb or more means you should take matters seriously. A 5-mb drop means that you could well be within 200 miles of a TRS.
Swell: It will come as no surprise to learn that a TRS shoves a mighty sea in front of it as it moves. A high swell with a rapid period can be a sure-fire sign. It might even be the first indication that all is not well because these seas can extend up to 1,000 miles ahead of the system. They move faster than the storm, so you may become aware of them a day or more before the barometer starts playing tricks.
Don’t make the mistake of assuming that the eye of a TRS is advancing at 90 degrees across the wave pattern. It may be, but, as we shall see, the outer winds do not run directly along the super-tight isobars. In any event, unless you have other information, the storm could well have changed course since it kicked up the waves that are passing you at any particular moment.
Sky: Visibility sometimes becomes crystal clear during the approach of a TRS. Evening and morning skies may be vividly colored as cirrus cloud develops, sometimes in a series of “vees” pointing towards the distant eye. Cirrus is seen 300 to 600 miles from the vortex and, together with the swell, may be the first indication that all is going to be far from well. As the storm approaches, the cloud levels lower, with altostratus and fractocumulus giving way to driving, rain-soaked black scud.
Wind: Because there are no fronts in a TRS, the airstream runs directly around it, more or less without interruption. Winds spiral anticlockwise in the northern hemisphere and clockwise in the southern. In the outer regions, considerable in-draught can be experienced as the air tries to flow across the isobars into the low pressure at the center. At 200 miles from the eye, where the wind will typically be blowing force 6 or 7, this can be up to 45 degrees. As the eye-wall approaches, the centrifugal force of the high-velocity airstream virtually defeats the pressure difference and the winds rattle straight round the circle.
In the absence of any radio forecasting, this information is critical for locating the storm center as it approaches. At first, therefore, if you face the wind, the eye of the storm will be around 120 degrees over your right shoulder (left in the southern hemisphere). As things hot up, the angle will decrease until the vortex is at hand, at which point its middle should be square across your outstretched arms.
Tactics and storm management
With a TRS moving at 15 knots or so, the idea of the average cruising yacht outrunning one in a straight foot-race is rarely a possibility. It can, however, be entirely realistic to sidestep its worst effects.
First, the storm’s relative position and track should be ascertained as closely as possible by applying the above principles, coupled with regular weather reports when these are available. Now comes the big decision about tactics. The best action will depend upon which ‘quadrant’ of the approaching TRS you seem to be in. For clarity, we will consider a northern hemisphere TRS, but note the opposite wind directions, etc., for the southern hemisphere.
Dangerous semicircle: This is the semicircle on what we might call the storm’s “starboard side” as it advances. Because of the anticlockwise circulation of air and the forward motion of the storm that creates its own apparent wind, the generally easterly wind across the deck of a stationary vessel on this side can be 20 to 30 knots greater than that on the opposite side, where the velocity of the system’s progress is effectively subtractive from the true wind speed. Furthermore, because it is to this “dangerous” side that the storm will recurve if it hasn’t done so already, a boat caught here may be in for a “double whammy.”
Navigable semicircle: Life in this half of the TRS will still be hell, but the chances of a safe exit are better than those dealt out to the poor souls on the side away from the equator. You also have a better probability of being able to give the slip to the worst of the action as it approaches. The danger of a cruelly timed recurvature is almost (though never totally) non-existent, and in order to increase your distance from the track of the approaching center, you can reach bravely with the wind exactly where you want it. You might even be making some sort of progress towards your destination.
Actions to take in the dangerous semicircle: A yacht trying to evade the eye from a position within the dangerous sector will have to attempt to sail close-hauled or pretty near to it, as fast and as far as possible to escape the worst. Anyone who has tried this in 45 knots of apparent wind in open water will understand that even to contemplate it in 60 knots or more is a non-starter for a cruising yacht. Therefore, if it seems that an approaching storm will catch you in the forward quadrant of this semicircle, you need a quick decision. Either you must beat as far beyond the track of the center as you can while you still may, taking your chance on a recurvature hit, or you should put the wind on the starboard quarter and run or broad-reach as fast as you can, trying to reach the slightly less awesome “navigable” semicircle.
A further problem associated with the dangerous semicircle, or at least its forward quadrant, is that a yacht caught in it and not making way will be carried into the track of the eye. So, if you find yourself in this most wretched of circumstances without the option of running out of it, you have little choice but to make what ground you can by sailing or motorsailing with serious dedication on the starboard tack. When either you or the boat can stand it no more, you must heave to on that tack, if you can still carry any rags of canvas, batten down and take a serious look round the accommodation on the assumption that you may well be knocked down or capsized. The deck needs a cynical inspection as well. In one dramatic incident, a pal of mine with a big gaff schooner opted to leave his topmasts up with extreme weather approaching. All his booming-out poles were lashed on deck. When he came out the other side, the deck had been swept clean, but the topmasts were still standing. Makes you think, eh?
That done, you can only settle in to read a good book, ideally strapped into your bunk, then trust in fate, your god or whoever rigged your boat.
Even when the north–south line of the moving storm’s eye has progressed west of your position, your problems are not over in the dangerous semicircle. You are now in its less lethal quadrant, but the wind is still driving you towards the center. Your only comfort is that as long as the brute doesn’t recurve on top of you, the TRS is on its way to bother somebody else.
The nature of the dangerous semicircle is so uncompromising that if it seems likely to become your lot in an approaching storm, you simply must opt for positive action before it is too late, either to avoid it by getting well clear on its windier side and taking your chance on an unlucky recurvature, or by muscling your way across its bows to the navigable semicircle.
Given reasonable warning of the storm’s approach, getting into the navigable semicircle if you aren’t there already may prove a realistic possibility. If you seem poorly placed, remember that, while the TRS is advancing at 10 knots, it could still be 300 to 400 miles away. This means you may have manageable winds for another 15 hours, possibly longer. Most modern cruisers can readily achieve 100 miles in that time; some can do more, and every yard counts. If a 75-knot storm looked like catching you 50 miles from the center plumb in its dangerous semicircle, a determined effort across its approach line would place you at least the same distance on the safer side, experiencing the dismal reality of 60-knot winds rather than the appalling prospect of 90 knots. What is more, since most boats can run in far heavier going than that into which they can beat, you will still be able to sail towards clearer conditions long after you’d have had to give up on the other side.
Tom Cunliffe has sailed his own yachts from the Caribbean to Russia and from Brazil to the Arctic. He’s a regular columnist for U.K. sailing magazines and is the author of Complete Day Skipper and Complete Yachtmaster.