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 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.
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.
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."|
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.
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 BlocksHurricanes 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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