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Old 07-10-2003
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Michael Carr is on a distinguished road
The Science of Hurricanes


The best heavy weather tactic when it comes to dealing with hurricanes is to avoid encountering nature's most powerful storms in the first placeóregardless of your vessel's size.
I rode out a Gulf of Mexico hurricane years ago when I was in the US Coast Guard, and that was quite an experience since we went through the eye and felt both sides of that 80-knot weather system. You might ask why were we out there in the first place? Well a Coast Guard helicopter had lost power during a rescue and crashed, and as members of the Coast Guard Dive Team, we were dispatched to the scene to attempt to salvage of the helo, which had not yet sunk. When we arrived on-scene our hurricane was still hundreds of miles away, and we were very aware of its forecast track toward our position. But we were on a mission and so we dove to rig a strop to the rotor head for lifting and to attach additional flotation bags while we awaited the arrival of a commercial cranebarge. The barge was late arriving on the scene, however, and when it did arrive large swells preceding the hurricane soon capsized the 100-foot barge. We were left to ride out the hurricane aboard a 210-foot, medium-endurance Coast Guard cutter.

After a day of serious rocking and rolling, the hurricane passed and to our wonder the helo was still floating, though upside down and badly damaged. We attempted to salvage it, but soon determined that the fuselage was in such bad shape that recovery was impossible. The chopper soon sank, providing us with yet another good "training exercise" in how not to salvage a downed helicopter!

All of this prompts the question: what are hurricanes and when do they occur? Hurricanes exist because they are expediters of heat from equatorial regions to the Arctic. If heat is not being moved north at a pace sufficient to keep our earth's atmosphere in balance, then hurricanes are likely to form. Under normal atmospheric conditions, ocean currents, as well as low and high-pressure systems, are able to move and mix hot with cold. But from June to November, when the sun is baking the Northern Hemisphere, tropical temperatures often exceed what can be tolerated. And this is when hurricanes are likely to develop and grow.

Most experienced sailors know that it is not so much a hurricane's winds that create problems as it is the large ocean waves and swells that are formed by a hurricane's wind field. These waves propagate outward hundreds and thousands of miles from a hurricane, and they can effect us long before we ever feel a hurricane's wind.

Hurricane Formation    Hurricane formation, development, and intensification depend on a number of intertwined factors. Each of these factors is an indicator of accumulated tropical heatóheat that must be moved northward from tropical regions to polar regions if the earth's atmosphere is to be kept stable.

"Heat must be moved northward from tropical regions to polar regions if the earth's atmosphere is to be kept stable."
Hurricanes need an existing weather disturbance, such as an easterly wave, which contains strong thunderstorm activity, to initiate formation. Hurricanes rely on thunderstorms to vertically transport heat into the upper levels of the atmosphere, and vertical movement of that heat helps build a hurricane's warm core.
  • Hurricanes need warm ocean waters to supply moisture. Warm water (a minimum 79 degrees Fahrenheit or 27 degrees Centigrade), which is mixed down to a depth of 200 feet, allows this source of heat to remain available to developing hurricanes as winds increase and the sea's surface becomes churned. Moisture carries heat and so moist layers in the atmosphere, up to the 500-mb level (10,000-20,000 feet), assist hurricane growth. Dry air does not favor hurricane development.
  • Hurricanes need light winds aloft, which have consistent direction and speed, allowing upward wind flow with little disruption. Light winds aloft enable a core of warm air to remain centered over a developing hurricane. Upper level winds that change direction with height disrupt a hurricane's warm core, inhibiting growth. However, once a hurricane forms it needs a consistent upper level outflow over its core to remove air. Without a method of exhausting air from the top of a hurricane, the surface flow into a hurricane's center is halted and a hurricane smothers.
  • Hurricane formation needs to take place north of approximately 5 degrees north latitude. Formation north of 5 degrees latitude exerts sufficient Coriolis Force so a hurricane rotates counterclockwise. Without Coriolis Force a hurricane is unable to rotate and sustain itself.


Hurricanes pack the power to reduce well-found vessels to mere playtoys in the bathtub of an irate child.
These elements required for hurricane development are interdependent, and an absence or change in one results in a change in other factors. Hurricanes normally form during hurricane season (June 1 to November 30) over warm waters, since this is where excess heat accumulates and is stored. Tropical disturbances emerge every three or four days from the African coast as tropical waves. Obviously, and fortunately, not every tropical wave and disturbance evolves into a hurricane. But when a hurricane does occur it has grown through stages, sometimes slowly, over a week or more, and sometimes very quickly, in just a few days.

Tropical storms and hurricanes are often assisted in forming by cold fronts, which trail into tropical regions. Cold fronts assist in this formation because they are a boundary between warm moist southerly air and cold dry northerly air, and as such are sources of strong convective activity, which encourages mixing of hot and cold.

Each year an average of 10 tropical storms develop in the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico. Of these 10 storms, six will develop into hurricanes, and in an average three-year period, five hurricanes strike the US coastline. Of these five strikes, two are major hurricanes with winds greater than 96 knots.

Here, then, is the sequence of hurricane development;

1. Heat is gathered by tropical disturbances through contact with warm ocean waters.
2. Thunderstorm activity develops and forms a warm core above tropical disturbances.
3. Coriolis Force supports development of counterclockwise circulation. Winds spiral into a disturbance, developing a low-pressure area.
4. Warm ocean waters with a deeper mixed layer continue to add moisture and heat to the air, which rises in updrafts near the disturbance.
5. As moisture condenses, heat is released, adding energy. Thunderstorms organize around a center as clouds grow in height. Light winds at high levels allow a warm core to remain intact and development continues; surface winds soon reach storm and hurricane strengths.


A encounter with a storm like Hurricane Andrew can leave one asking questions reserved for a Zen koan: how strong does the wind have to be blowing for a piece of  plywood to pierce a palm tree?'
Storm Life Cycles    Once a hurricane forms it can last for two weeks or more over the open ocean, generating substantial seas, easily in excess of 50 feet, accompanied by swell trains that extend outward for thousands of miles. In the early stages of development when hurricanes appear as unorganized thunderstorm clusters imbedded in tropical waves, satellite imagery is the best method of detecting development. If weather and ocean conditions are favorable, thunderstorms will strengthen and consolidate to a tropical disturbance and then become a tropical depression (packing winds up to 33 knots).

Tropical depressions take on a familiar spiral appearance due to increasing counterclockwise wind flow around their centers, and if the system continues to strengthen to tropical storm status (with winds from 34 to 63 knots) developing bands of thunderstorms will contribute additional heat and moisture to the storm, which further aids intensification. A storm becomes a hurricane when surface winds reach a minimum sustained speed of 64 knots. At this stage a cloud-free eye appears at a hurricane's center.

A hurricane will continue to grow and sustain itself until one or more of the necessary ingredients is either lost or changes. Wind shear can tear a system apart separating the warm core aloft from the low-level circulation. Movement of hurricanes into regions of drier air can inhibit convection and cause weakening. And movement over cold water or landfall shut down a hurricane's warm energy source, and therefore it's fuel. Landfall also increases friction, reducing intensity, but increasing rainfall.

"Advanced technology in hurricane tracking and forecasting  over the past 30 years has led to a  a one-percent-per-year improvement in accuracy."
Predicting Hurricanes    The meteorologists at the National Hurricane Center (NHC) are making steady improvements in hurricane tracking and forecasting and over the past 30 years there has been a one-percent-per-year improvement in accuracy. An enormous improvement in hurricane forecasting came with the introduction of real-time capabilities that the National Hurricane Center and Mariner Prediction Center have today. These Nowcasts use satellite meteorology, which continuously monitors and tracks Atlantic hurricane location and intensity, as well as tropical weather around the world.

Information from Hurricane Hunter Aircraft and NOAA's computer models are also important for determining hurricane strength, but by far the most valuable information comes from satellite remote sensing technology, specifically NOAA's Polar and Geostationary satellites.

Avoiding Hurricanes    Sailors can reduce the perils associated with encountering a hurricane by using two highly accurate avoidance techniques. The first is called the 34-knot wind radius rule, which says stay outside the radius of 34-knot winds that extend outward from a Hurricane's center. Why avoid 34-knot winds? Because 34 knot winds (Beaufort Force 7) is gale force and within six to 12 hours these winds produce seas with a significant wave height of 12 feet.


Satellite imagery can help mariners to stay out of the 34-knot wind radius and the 12-foot seas found there. Increased winds and seas limit the choices available to a crew.
So why are 12-foot seas important? This is the maximum height that most small craft can tolerate for extended periods and limits course and speed options available to a boat's crew. How to do you determine the radius of 12-foot seas?  First read official NHC Hurricane Advisories, which are produced every six hours. These Advisories provide radius of winds, in nautical miles and by quadrant, surrounding a hurricane. Radiuses are given for 64-knot (hurricane), 50-knot (storm), and 34-knot (gale) winds. Plot these radiuses and note each Advisory's valid time. Update your plot at least every six hours and take immediate evasive action if your present position shows you entering the 34-knot wind radius. If you have access to satellite imagery then the outer edge of solid hurricane cloud cover is also the edge of 34-knot wind radius.

The second rule I call the 1-2-3 rule, which states that for each 24-hour hurricane forecast position there is a 100-mile track error left and right of the forecast position's latitude/longitude. What does this mean? It means that at the 24-hour-forecast position a hurricane could be at that given position or 100 miles to the left or right of that position. At the 48-hour position a hurricane could be 200 miles to the left or right of the forecast 48-hr lat/long, and at 72 hours a hurricane could be 300 miles left or right of its forecast position.

This technique is proven and conforms to well-analyzed track errors calculated from previous hurricane seasons. Always plot the 24, 48, and 72-hour positions and construct an "Area to Avoid" around those positions using 100, 200, and 300-mile track error. And remember to also construct the radius of 34-knot winds because you want to avoid the radius of these winds, not the hurricane center!

For additional information on how to use both rules visit the National Hurricane Center's website, accessed via the Marine Prediction Center(www.mpc.ncep.noaa.gov) and download their manual titled "Mariners Guide to Hurricane Avoidance in the North Atlantic Basin." Here's a direct link: http://www.nhc.noaa.gov/marinersguide.pdf

Remember the time to take hurricane avoidance measures is early on in their development, when you are able. Don't wait until you are limited by wind and waves. With today's advanced communication and timely hurricane reports there are abundant ways to be well informed.

Hurricane Building Blocks

Hurricanes are low-pressure systems that have no cold or warm fronts, and they develop over tropical waters. Hurricanes originate from tropical waves, which are breeding grounds for tropical disturbances. Disturbances, given the right growing conditions, can blossom into tropical storms, and if there is sufficient energy, they eventually form hurricanes.

Tracking tropical waves and disturbances is important since they are the precursors to hurricanes. Hurricanes do not just appear, they take days and often weeks to form, and it is during this gestation period that we should be alert and ready with a pre-determined hurricane avoidance strategy.

Tropical or Easterly Waves    The seeds from which hurricanes grow are areas of low pressure that form in the Atlantic's easterly trade winds. Significant tropical waves contain thunderstorms, which can be seen on satellite imagery. Each year approximately 60 waves move from east to west across the tropical Atlantic. A majority of these waves bring no significant hurricane development, though they do bring squalls and gusty rain showers. Tropical disturbances are tropical waves that have maintained their identity for at least 24 hours and show organized convective activity, i.e. thunderstorms and squalls. These disturbances are generally 100 to 300 miles in diameter and are easily detected on infrared (IR) satellite images as their thunderstorm cloud tops grow colder with time.

Tropical Depressions    Disturbances grow into depressions that have maximum sustained surface winds, using a one-minute average, of 33 knots or less. Depressions also show enclosed isobars and consistent, counter-clockwise flow of surface winds.

Tropical Storms    The next rung on the ladder up from depressions is tropical storms. These systems show maximum sustained surface wind speeds (again, using again a one-minute average) from 34 knots to 63 knots.

Hurricanes     With minimum sustained surface wind speeds (measured again by way of a one-minute average) greater than 64 knots, hurricanes often have much stronger winds, with strengths of over 200 mph (172 knots) having been recorded.



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