Tropical cyclones. low-pressure systems that generally form in the tropics and are classified into three main groups:
1- Tropical depressions
2- Tropical storms and
a third group whose name depends on the region:
Hurricane. Named so in the North Atlantic Ocean, North Pacific Ocean east of the dateline, and the South Pacific Ocean east of 160°E.
Hurricanes take other names in other parts of the world:
Willy-Willies (singular Willy-Willy) in Australia
Baguio in the Philippines.
Chubasco in Mexico
Taino in Haiti
Typhoon in the Northwest Pacific Ocean west of the dateline
Severe tropical cyclone in the Southwest Pacific Ocean west of 160°E or Southeast Indian Ocean east of 90°E
Severe cyclonic storm in the North Indian Ocean
Tropical cyclone in the South-west Indian Ocean
NB. In Europe. Certain severe north-east Atlantic cyclonic depressions are referred to as "Hurricanes," even although they rarely originate in the tropics. These European windstorms can generate hurricane-force wind speeds and may also be given names.
- Tropical storms and depressions
Tropical Depression: an organized tropical disturbance with
winds less than 39 mph
- Tropical Storm - tropical cyclone with winds 39-74 mph
|- Tropical cyclone: with winds over 74 mph (see below)|
|- Major whirlwinds||- Funnel clouds, tornadoes and waterspouts: Helical rotations joined to cumulonimbus clouds, or towering or fast-growing cumulus clouds|
|- Severe Thunderstorms||- Combinations
of the following: winds over 58 mph; large (> 3/4 inch) hail; intense
lightning; intense blinding rain, often producing tornadoes (or tornadic
N.B. Severe thunderstorms are often found within feeder bands (spiral-shaped squall lines) of hurricanes and tropical storms.
monsoon is a seasonal change in the direction of the prevailing
wind. This change produces an alteration of the weather, bringing
heavy rains. (See below)
NB. Can be induced by cyclones in the Indian Ocean
Hurricanes: What, Where and When
Hurricanes, called typhoons or tropical cyclones in certain parts of the world (see above), form over the world's tropical oceans except in the Southern Atlantic ocean and the South-eastern Pacific.
NB. Hurricanes, typhoons or tropical cyclones never form on the equator due to the absence of the Coriolis effect.
Universally a tropical storm has winds of 39 mph to 73 mph. When winds reach at least 74 mph the storm becomes a hurricane or typhoon/tropical cyclone, depending on its location and after forming a hurricane may hit places as far away from the Tropics as New England and Typhoons, from the tropical western Pacific, frequently hit Japan and Korea.
Tropical storms begin forming during and just after a region's summer when the ocean is at its warmest. Most regions can be threatened by tropical cyclones for ± six months of the year, but the largest numbers of storms are usually concentrated in a ± two month-long period. Similar indications in each region, such as warm ocean water, light winds, extended periods of warm temperatures and high humidity, help create tropical cyclones over more than 60 percent of the world's tropical latitudes.
Tropical storms and hurricanes form between ±5 degree north of the equator to ±5 degrees south due to the fact that at these latitudes the Earth's spin has little effect on organizing clusters of thunderstorms into whirling tropical cyclones.
The official hurricane season begins May 15 in the eastern and central Pacific basin and June 1 in the Atlantic basin, which includes the Gulf of Mexico and the Caribbean Sea but there are normally few storms in the early part of the season. September is peak season in the Caribbean, Mexico, U.S. with hurricanes forming and striking land in the Caribbean or northward along Mexico's west coast generally around July.
The biggest threat to the Caribbean and the Bahamas, the U.S. and to eastern Mexico begins in August and lasts through October. In general, places in the southern Caribbean have fewer hurricanes than in the northern Caribbean.
Everywhere in the Atlantic basin is liable to hurricanes, including Bermuda, especially during September when the season reaches its peak. Long-term records show that four times as many storms have formed in the Atlantic Basin in mid-September than during mid-June.
While September is the peak of the hurricane season in the Atlantic Basin, storms don't hit every year. Typically, several years can go by without a hurricane.
(Coriolis effect: Force generated by the Earth's rotation, capable of generating currents and causing moving bodies to rotate clockwise in the Northern Hemisphere and anti-clockwise in the Southern Hemisphere. The "force" is proportional to the speed and latitude (the distance from the equator) of the moving object. It is zero at the equator and maximum at the poles.)
Hurricanes, Cyclones or Typhoons?
In meteorology, a cyclone is an area of low atmospheric pressure around which the winds flow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. However, cyclones are responsible for a wide variety of different meteorological phenomena. Because of this, most weather forecasters avoid using the term cyclone without a qualifying term.
Hurricanes are tropical cyclones with winds that exceed 64 knots (74 mi/hr) and circulate counter-clockwise about their centers in the Northern Hemisphere (clockwise in the Southern Hemisphere)
An intense tropical weather system with a well-defined circulation and maximum sustained winds of 74 miles per hour (±64 knots ) in the Northwest Pacific Ocean, west of the International Date Line.
NB. The word "Typhoon" has both Western and the Eastern origins. In Greek "Typhoon" is "Tuphon" , meaning whirlwind, and in Arabic "Tufan", meaning Deluge. In Cantonese "Typhoon" is "taaîfung", meaning great wind.
Forecasts for more than 2 or 3 days ahead are not possible and it is not possible to predict where a hurricane is going to hit more than a couple of days before it actually hits.
While it's possible to say that some years are likely to breed more hurricanes than others, hurricane forecasters have no way of saying a week or two ahead of time that a hurricane is likely to form, much likely where it's likely to hit. Forecasters hope to be able, someday, to give the odds of a particular place being threatened a week or two before a storm hits. But for now, such forecasts are only a dream.
Hurricane Force Scale Ranges:
Hurricane scales are calculated using the 'SAFFIR-SIMPSON' hurricane scale ranges*
*The scale was formulated in 1969 by Herbert Saffir, a consulting engineer, and Dr. Bob Simpson, director of the National Hurricane Center. The World Meteorological Organization was preparing a report on structural damage to dwellings due to windstorms, and Dr. Simpson added information about storm surge heights that accompany hurricanes in each category.
From this two general formulas are obtained for calculating wind speeds from the minimum central pressure of a hurricane. However, there is an actual formula using the more accurate cyclostrophic balance, but these general ones can be used more easily because the require knowing only the minimum general pressure:
*AVERAGING THE TWO CAN GIVE A RELATIVELY ACCURATE APPROXIMATION*
|ETCS - Enhanced Tropical Cyclone Scale|
|SCALE #||CENT PRESS (MB)||WIND SPD (mph)||SURGE||DAMAGE||S-S* EQUIV.|
|1||1008 - 1010||5 - 39||<1||None||Under Cat 1|
|2||1004 - 1007||40 - 59||1 - 2||Insignificant||Under Cat 1/td>|
|3||998 - 1003||60 - 74||2 - 3||Minimal||Near Cat 1|
|4||990 - 997||75 - 84||3 - 4||Minor||Middle Cat 1|
|5||980 - 989||85 - 94||4 - 5||Light||High Cat 1|
|6||965 - 979||95 - 109||6 - 8||Moderate||Cat 2|
|7||945 - 964||110 - 129||9 - 12||Extensive||Cat 3|
|8||920 - 944||130 - 154||13 - 18||Extreme||Cat 4|
|9||890 - 919||155 - 184||19 - 25||Devastating||Cat 5|
|10||988 - 889||185 - 219||26 - 35||Catastrophic||High 5|
Generally speaking, high or strong winds are announcers, in tropical/subtropical regions, of the arrival of bad or severe weather such as Hurricanes, Tornados... However, high or strong winds are found in every region of the planet, and as 'Stand alone' phenomena, from the Poles to the deserts, and do not spare the more temperate zones such as Europe.
In December 1999, France and Germany witnessed two severe storms in the space of a week that came in from the Atlantic, hit the French Brittany coast at with winds gusting up to ± 200Kph (107 knots or < Force 12 on the Beaufort scale) crossed into Germany devastating everything in their passage and causing the death of 84 people between Dec 26th and 27th.
The reason? The extremely bad forecasting and announcement of an exceptionally bad storm. Apparently, European agencies were not as used to such 'Severe weather' phenomena as their American counterparts. Weather forecasters, at the time,* had no way of anticipating the storms and they had been handicapped by the loss of one of their traditional sources of information: Fishing boats. The fishing boats**, had been forced to stay at home... due to bad weather!
* Since then the Europeans have launched Geostationary weather satellites to cover such eventualities.
** At the time the fishing boats provided the meteorological services with a ± 2-day weather forecast from their fishing zones but they were incapable of doing any more than informing of what was about to start. They had no way of anticipating or, more importantly, analyzing the causes and no sooner had the 01st storm passed... the 02nd hit leaving the weather forecasters totally unprepared.
Beaufort wind scale for land
Beaufort Wind Scale for the Open Sea
The scale is presently maintained by the British Meteorological Office, who
A version currently used by mariners is shown below. Please refer to the warnings following the table.
Column 1 is the Beaufort Number or Force Number.
Column 2 is the range of wind speeds in knots, measured at a height of 33 feet above sea level.
Column 5 is the probable height of waves in feet in the open sea, remote from land. In enclosed waters, or when near land with an offshore wind, wave heights will be smaller and the waves steeper.
Column 6 is the probable maximum wave height in feet in the open sea (rules for column 5 apply here as well).
|0||<1||Calm||Sea like a mirror.||-||-|
|1||1 - 3||Light air||Ripples with the appearance of scales are formed but without foam crests.||¼||-|
|2||4 - 6||Light Breeze||Small wavelets, still short but more pronounced. Crests have a glassy appearance and do not break.||½||1|
|3||7 - 10||Gentle Breeze||Large wavelets. Crests begin to break. Foam of glassy appearance. Perhaps scattered white horses.||2||3|
|4||11 - 16||Moderate Breeze||Small waves, becoming longer; fairly frequent horses.||3½||5|
|5||17 - 21||Fresh Breeze||Moderate waves, taking a more pronounced long form; many white horses are formed. (Chance of some spray.)||6||8½|
|6||22 - 27||Strong Breeze||Moderate waves, taking a more pronounced long form; many white horses are formed. (Chance of some spray.)||9½||13|
|7||28 - 33||Near Gale||Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind.||13½||19|
|8||34 - 40||Gale||Moderately high waves of greater length; edges of crests begin to break into spindrift. The foam is blown in well-marked streaks along the direction of the wind.||18||25|
|9||41 - 47||Strong Gale||High waves. Dense streaks of foam along the direction of the wind. Crests of waves begin to topple, tumble, and roll over. Spray may affect visibility.||23||32|
|10||48 - 55||Storm||Very high waves with long overhanging crests. The resulting foam in great patches is blown in dense white streaks along the direction of the wind. On the whole, the surface of the sea takes a white appearance. The tumbling of the sea becomes heavy and shock-like. Visibility affected.||29||41|
|11||56 - 63||Violent||
Exceptionally high waves. (Small and medium-sized ships might be for a time lost to view behind the waves.) The sea is completely covered with long white patches of foam lying along the direction of the wind. Everywhere the edges of the wave crests are blown into froth. Visibility affected.
The air is filled with foam and spray. Sea completely white with driving spray; visibility very seriously affected.
NB.Estimating wind force by the sea criteria is difficult at night.
An increase of wind does not immediately produce an increase of sea.
Fetch, depth, swell, heavy rain and tide effects should be considered when estimating the wind force from the appearance of the sea.
WARNING: For a given wind force, sea conditions can be more dangerous near land than in the open sea. In many tidal waters, wave heights are liable to increase considerably in a matter of minutes.
1 knot = 1 nautical mile per hour = 6076 feet per hour = 1.15078 mph
Beaufort Wind Scale on Land
In June 1939, the International Meteorological Committee adopted a correlation with wind speeds measured at a height of 6 meters. Great Britain and the United States, however, had already standardized on measuring the wind speed at 11 meters above ground level, which is shown in column 2 below, in miles per hour.
|American description||British description||Land criteria|
|0||<1||Light||Calm||Smoke rises vertically.|
|1||1 - 3||Light||Light air||Direction shown by smoke but not by wind vanes.|
|2||4 - 7||Light||Light||Wind felt on face; leaves rustle; ordinary vane moved by wind.|
|3||8 - 12||Gentle||Gentle Breeze||Leaves and small twigs in constant motion; wind extends light flag.|
|4||13 - 18||Moderate||Moderate Breeze||Raises dust and loose paper; small branches are moved.|
|5||19 - 24||Fresh||Fresh Breeze||Small trees in leaf begin to sway.|
|6||25 - 31||Strong||Strong Breeze||Large branches in motion; umbrellas used with difficulty.|
|7||32 - 38||Strong||Near gale||Whole trees in motion; inconvenience felt when walking against the wind.|
|8||39 - 46||Gale||Gale||Breaks twigs off trees; generally impedes progress|
|9||47 - 54||Gale||Strong Gale||Slight structural damage; chimney-pots and slates removed.|
|10||55 - 63||Whole Gale||Storm||Trees uprooted; considerable structural damage.|
|11||64 - 72||Whole Gale||Violent Storm||Widespread damage; very rarely experienced.|
|12||73 - 82||Hurricane||Countryside is devastated.|
|13||83 - 92|
|14||93 - 103|
|15||104 - 114|
|16||115 - 125|
|17||126 - 136|
Weather Map Wind Symbols:
Particular to the Indian sub-continent, Monsoons stretch from the Northern tip of Australia
- Dry Monsoon
The dry, or winter, monsoon of Asia is largely the result of an area of high pressure that develops over S Siberia. From this area dry winds blow outward, crossing India from northeast to south-west and SE Asia from north to south the dry, or winter, monsoon of Asia is largely the result of an area of high pressure that develops over S Siberia. From this area dry winds blow outward, crossing India from northeast to south-west and SE Asia from north to south.
- Wet Monsoon
The wet, or summer, monsoon is caused by low pressure that develops over S Asia as the landmass warms. Moisture-laden air over the oceans is drawn toward this center of low pressure. The air cools as it ascends the slopes of mountain barriers; it can no longer retain moisture, resulting in heavy rainfall.
NB. The Indian Ocean version of the hurricane, called a "cyclone", can also occur and move ashore in association with the onset of the monsoon. These cyclones have at times killed thousands of people who live in the low-lying areas along the eastern coast of India and Bangladesh.
© Nicholas Richards 2004, revised 2015