Winds and Currents of the World

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Ever since man first ventured offshore in craft powered by the wind, he has looked for patterns in the wind's behaviour. Such observations may have led the early fishermen to use an offshore breeze to take their canoes to favourite fishing spots in the morning and the onshore breeze to waft them home later in the day. These patterns are still used in some parts of the world where fishermen continue to use sailing craft as their forefathers have done over countless generations. Discovering a similar regularity for offshore voyaging was more difficult and some remote places in the world might have remained unpopulated until much later if early voyagers had been able to find a favourable wind to return home. The fact that there was a regular pattern to the winds was already recognised in ancient times and seasonal sailing routes were a common feature in the ancient world. The Chinese established such routes in the Far East, the Greeks used them in the Aegean, the Polynesians were helped by them to colonise the far flung islands of the Pacific, the Papuans are still following the traditional Kula route with the help of seasonal winds, while Arab traders used the monsoons of the Indian Ocean to establish a regular link between India and East Africa.

This reliance on favourable winds for most offshore voyages lasted until the last century, when the greatly improved windward going capabilities of sailing ships freed their masters of the shackles imposed by having to follow a route governed by the wind alone. However, in spite of being able to sail closer to the wind, neither did the masters of the old clippers nor do the skippers of modern yachts enjoy battling against head winds, and most would still make a detour to pick up a fair wind. Even in a well designed yacht it is often wiser to cover a longer distance with better winds than to stubbornly try to follow the direct route between two points. This is the reason why it is so important to understand the prevailing wind systems of the world, which dictate most of the cruising routes described in the following pages.

The importance of defining the prevailing winds in certain areas of the world was already recognised by the Portuguese and Spanish navigators of the fifteenth and sixteenth centuries, and their findings were kept a closely guarded secret for a long time. The first transatlantic voyage by Christopher Columbus showed his followers that a detour to the south was the best route to the newly discovered islands of the Caribbean, whereas a northerly sweep was to be preferred for the return voyage to Europe. Similarly in the Pacific, Magellan and other early navigators demonstrated that the voyage from east to west across the South Pacific was a relatively easy matter if one stayed within the southeasterly trades. However, a return voyage against the same trades proved impossible until finally the Spanish navigator Urganeta discovered the westerly winds of higher latitudes, which came to be called the anti-trades or passage winds.

This worldwide wind pattern has thus been known for a long time and countless navigators have made use of this knowledge over the centuries. In his Memoir of the Northern Atlantic Ocean, published in its 13th edition in 1873, Alexander George Findlay succinctly describes this wind pattern in the following words:

'It has been well observed that the wind systems of our globe naturally govern the tracks of ships crossing the oceans, the trade winds carrying them from East to West within the tropics, while anti-trade or passage winds

Continued on page 24

Winds and Currents of the World i Prevailing wind systems January to March

Winds and Currents of the World i Prevailing wind systems January to March

Prevailing Wind Directions Turkey

2. PREVAILING WIND SYSTEMS JANUARY TO MARCH f

Winds and Currents of the World

Prevailing wind systems April to June

Winds and Currents of the World

3. PREVAILING WIND SYSTEMS APRIL TO JUNE

Winds and Currents of the World

Prevailing wind systems July to September

Winds and Currents of the World

Prevailing wind systems July to September

5. PREVAILING WIND SYSTEMS OCTOBER TO DECEMBER

Prevailing wind systems October to December

Prevailing wind systems October to December

will bring them back again eastward beyond the tropics. If it were not for the intervening belt of calms, sailing directions for vessels going into opposite hemispheres would be of the simplest kind; but the well-known Equatorial embarrassments - "the doldrums" - generally make a very different matter of it, and cause many considerations to enter into the problem of shaping a course.'

The three main factors that influence the formation and direction of the wind are atmospheric pressure, air temperature, and the rotation of the earth. The primary cause of wind is a difference of temperature. This in its turn leads to a difference in atmospheric pressure mainly because of the tendency of warm air to rise, which is then replaced by cold air drawn from elsewhere. Air also tends to flow from an area of high pressure to one of low pressure. Permanent areas of high pressure are situated between approximately the latitudes of 200 and 40 both north and south of the equator. On either side of these cells of high pressure there are areas of low pressure. If it were not for the rotation of the earth, the wind direction would be either north or south, from an area of high pressure to one of low pressure, but because the earth is rotating on its axis in an easterly direction, air which is drawn towards a centre of low pressure is deflected to the right in the northern hemisphere and to the left in the southern one. The result of this movement in the northern hemisphere is the anti-clockwise circulation of wind around a low pressure area and the clockwise rotation of wind around a high pressure area. The opposite is the case in the southern hemisphere, where the wind circulates in a clockwise direction around a low pressure area and in an anti-clockwise direction around a high pressure area.

Diagrams 2 to 5 show the way in which winds on the equatorial side of the high pressure belts blow towards the equator from a NE direction in the northern hemisphere and from a SE direction in the southern hemisphere. North and south of those areas of high pressure the winds in both hemispheres are predominantly westerly.

In many areas these systems are distorted by land masses, which are subjected to more pronounced differences of temperature and barometric pressure than the oceans. The wind systems are also affected by the seasons, since the annual movement of the sun causes the areas of high pressure to move towards the poles in the summer. Because of this movement, the wind systems associated with these areas of high pressure, particu larly the trade winds, tend to travel a few degrees south or north with the sun.

Trade winds

These steady winds which blow on either side of the equatorial doldrums were so called because of the assistance they gave to the trade o^ sailing ships. The early Spanish navigators gave'them the more romantic sounding name of alisios. These regular winds are usually NE in the northern hemisphere and SE in the southern hemisphere. They rarely reach gale force and on average blow at force 4 to 5. The weather associated with the trade winds is usually pleasant, with blue skies and fluffy cumulus clouds. The barometric pressure within the trade wind belt is steady, interrupted only by a pressure wave, which causes a slight rise and fall of the barometer every 12 hours. If the diurnal movement of the barometer ceases, or if it is very pronounced, a tropical disturbance can be expected. The entire trade wind belt, including the doldrum zone that lies between the two systems, moves north and south during the year. This movement is influenced by the movement of the sun, although there can be a delay of up to two months between the movement of the sun itself and that of the doldrums. The trade winds are less steady in the vicinity of. the Intertropical Convergence Zone.

Intertropical Convergence Zone

This area of low barometric pressure lying between the trade wind regions of the two hemispheres is known as the Intertropical Convergence Zone (ITCZ), the equatorial trough, or more commonly as the doldrums. The winds in this area are either light or nonexistent and the weather is sultry and hot. The only interruptions are occasional squalls and thunderstorms, when rain can be very heavy. The extent of the doldrums varies greatly from year to year and season to season. Although the doldrums have earned their bad reputation because of the frequent calms that could delay ships for days on end, doldrum weather can sometimes be particularly unpleasant, with violent squalls and raging thunderstorms. Weather in the doldrums tends to be worse when the trade winds blow at their strongest.

Variable winds

A zone of light and variable winds extends on the polar sides of the trade winds, corresponding more or less with the high pressure areas of the two hemispheres, between latitudes 25° and 35° approximately. These zones were given the name of Horse Latitudes, because sailing ships that were becalmed in these areas were sometimes forced to kill the animals on board due to the lack of drinking water.

Westerly winds

The higher latitudes of both hemispheres have a large proportion of westerly winds, which prevail north and south of latitude 35 Westerly winds are stronger and more predominant in the southern ocean, where they often blow with gale force from the same direction for several days. Because of the more extensive land masses in the northern hemisphere, the westerlies of the northern oceans are lighter and less consistent.

Monsoons

Seasonal winds are experienced in several areas of the world, the name monsoon deriving from the Arabic word meaning 'season'. Such winds blow consistently from one direction for one season and after a short interruption blow with equal consistency from the opposite direction. The most important regions affected by such seasonal winds are the Indian Ocean and West Pacific Ocean.

Depressions

A depression is an area of low barometric pressure, which is usually responsible for periods of unsettled weather, although not all depressions are accompanied by strong winds. Depressions occur most frequently in middle and higher latitudes, although the most severe storms encountered at sea are those formed in the low latitudes and of a revolving nature, discussed in the next section.

As stated earlier, winds in the northern hemisphere blow around low pressure areas in an anti-clockwise direction, while in the southern hemisphere the direction is clockwise. Most depressions move in an easterly direction, a few moving in other directions at times. The speed at which they move can vary from very little to 40 knots or more. Usually depressions last about four to five days and their movement gradually slows down as they fill and the pressure rises.

The strength of the wind generated by a depres sion is dictated by the closeness of the isobars, which can be seen on a synoptic chart as lines joining areas of equal barometric pressure. The closer the isobars lie together the stronger the wind. The approach of a depression is always indicated by a falling barometer and usually by a change in the aspect of the sky and cloud formation. It may be worthwhile studying this aspect of meteorology, so as to be able to predict the kind of wind and weather to expect both on passage and in port.

Tropical squalls

This is a common phenomenon encountered in the tropics, especially below latitude 20°. These linear disturbances travel from east to west at 20-25 knots and are usually perpendicular to the direction of the prevailing wind. They are accompanied by thundery and squally weather. The first indication of an approaching line squall is a heavy band of cumulo-nimbus to the east. The wind is usually light or calm and the atmosphere oppressive. As the cloud approaches it becomes dark and menacing with occasional thunder and lightning. The bottom of the cloud has the appearance of a straight line but it sometimes changes to an arch as it passes overhead. Suddenly there is a blast of wind from an easterly direction, which on average rises to 25 - 30 knots, although occasionally it can be much stronger. Shortly after the blast of wind, it starts to rain heavily. Such squalls last on average about half an hour, although sometimes they may last longer. The barometer does not indicate their approach, therefore they can only be detected visually, although they also show up on radar. As some of these squalls can be quite vicious, it is prudent in squall prone areas to reduce sail at night, when their approach is more difficult to detect. In the North Atlantic, line squalls occur especially at the beginning and end of the rainy season (May to October) and are particularly violent near the African coast. In the South Pacific squalls can occur at all times, although as a rule they are not as violent as the North Atlantic variety. Line squalls are less of a problem during the NE monsoon of the North Indian Ocean, but can be violent during the opposite SW monsoon. •

Tornadoes

Tornadoes and waterspouts occur in the same areas and during the same season as tropical storms.

World distribution of tropical storms

They usually travel in the same direction as the prevailing wind and their approach can normally be seen, especially as they rarely form at night. The wind generated by a tornado can be extremely violent, but as the actual area covered is very small, the likelihood of being hit by such a whirlwind at sea is quite remote. Waterspouts sometimes occur during afternoon thunderstorms in the vicinity of the coast, the ocean side of Chesapeake Bay being particularly vulnerable during the summer months.

Tsunamis

These are large waves caused by an earthquake which can occur thousands of miles from the place where the destructive effects of the gigantic wave will be felt. Tsunamis occur mostly in the Pacific Ocean, and ports both on the continent and islands have been hit by tsunamis during the twentieth century. There have been six destructive tsunamis in Hawaii in the last 50 years. The most recent major tsunami occurred in 1960 causing great destruction in Hilo where over 60 people lost their lives. Boats are better off at sea in deep water, preferably over 100 fathoms, where the effect of a tsunami will pass almost unnoticed.

Tropical revolving storms

Tropical revolving storms are the most violent storms that can be encountered at sea and it is both prudent and wise to try and avoid the areas and seasons where such storms occur. The extremely strong winds generated by these storms and the huge seas they raise can easily overwhelm a small boat Depending on which part of the world they occur in, these storms are known as hurricanes, cyclones, typhoons, or willy-willies. They blow around an area of low pressure, the rotation being anti-clockwise in the northern hemisphere and clockwise in the southern hemisphere. The wind does not move around the centre in concentric circles but has a spiral movement, being sucked in towards the core of the storm.

Usually these storms occur on the western sides of the oceans, although they are also found in other parts of the world. They usually form between latitudes 7'and 15° on either side of the equator, but there have been many instances when tropical storms formed closer to the equator. The breeding ground of tropical storms is the intertropical

Convergence Zone, where the two opposing trade wind systems converge. Under certain conditions of barometric pressure, temperature, and moisture, the resulting whirlpool of air created at the point of convergence can develop iniib a severe tropical revolving storm. The most dangerous areas affected by such storms are the western North Atlantic from Grenada to Cape Hatleras, the western North Pacific from Guam to Japan, the South Pacific from the Marquesas to the Coral Sea, the north and northwest coasts of Australia, the southwest Indian Ocean, and the Bay of Bengal. In some of these areas tropical storms occur several times a year, while others are only hit about once every ten years.

In addition to their circular motion, tropical revolving storms also have a forward movement. In the northern hemisphere the movement is initially WNW, storms recurving gradually to the N and NE as they reach higher latitudes. In the southern hemisphere the initial movement is WSW, storms recurving to the SE as they approach latitude 20 °S. Sometimes a storm does not recurve but continues in a WNW direction in the northern hemisphere, or a WSW direction in the south-em hemisphere, until it hits the continental land-mass where it gradually breaks up after causing much damage. Occasionally the storm meanders erratically and its direction is often impossible to predict with certainty. The speed at which a storm is moving is normally about 10 knots in the early stages and accelerates after recurving.

Any boat lying in the path of a storm, particularly its centre, will be in serious danger. The wind remains constant in direction until the eye has passed, then, after a brief calm, the wind returns from the opposite direction, possibly with greater violence, creating rough and confused seas or putting vessels at anchor on a dangerous lee shore. Every storm has two sides, or semicircles, known as the navigable semicircle and the dangerous semicircle. In the northern hemisphere, the dangerous semicircle is the half of the storm lying'on the right hand side of the track in the direction in which the storm is moving. In the southern hemisphere, the dangerous semicircle is on the left.

The detection and tracking of tropical storms has greatly improved since the advent of weather satellites. Stations WWV in Fort Collins, Colorado and WWVH in Kauai, Hawaii, broadcast hourly reports of tropical storms, their coordinates, speed of movement and wind strength. Tropical storm warnings for the Atlantic Ocean are broadcast at 8 minutes past each hour by WWV on 2.5, 5,10,15, and 20 MHz. Warnings for the Pacific are broadcast by WWVH at 48 minutes past the hour on 2.5, 5,10, and 15 MHz. From the information obtained from these stations it is possible to plot the course of an approaching storm and take the best avoiding action. The path of the storm in relation to the vessel's latest position will show the degree of danger. If one is at sea, the best course of action is as follows:

Northern hemisphere

When facing the wind, the centre of the storm will be between 900 and 1350 on the right of the observer. If the wind veers, i.e. shifts to the right, the boat is in the right hand semicircle which is the dangerous semicircle. A backing wind is associated with the navigable semicircle. If the direction of the wind is constant, its strength increases and the barometer falls, the boat is exactly in the path of the storm. If the direction of the wind is not changing, but its strength decreases while the barometer slowly rises,, the boat is directly behind the centre.

The generally accepted tactic for vessels caught in the path of a tropical storm is to run off on the starboard tack by keeping the wind on the starboard quarter. The same tactic should be applied if the boat is in the navigable semicircle when one should try and follow a course at right angles to the assumed track of the storm. Depending on the boat's behaviour in a quartering sea, one should try and either run under bare poles or storm jib. If the boat is in the dangerous semicircle one should heave to on the starboard tack or, if possible, sail close hauled on the same tack, with the object of moving away from the storm centre.

Southern hemisphere

South of the equator, the centre of a tropical storm is between 90° and 135° on the left of the observer. If the wind is backing, the boat is in the dangerous semicircle; if it veers, the boat is in the navigable semicircle. The vessel is directly in the path of the storm if the wind is constant in direction. An increasing velocity combined with a falling barometer means that the boat is in front of the storm, a decreasing wind speed and a rising barometer means that the observer is behind the centre.

The best tactic if one is directly in front of the storm is to run with the wind on the port quarter.

The same tactic should be applied if the boat is in the navigable semicircle, by trying to run away from the storm at right angles to its assumed track, also with the wind on the port quarter. If the boat is in the dangerous semicircle, one should try and sail close hauled on the port tack so as to proceed away from the storm centre. If this is not possible, the boat should heave to on the port tack.

Although these general rules are applicable in most situations, there can be circumstances when they should not be followed without question. Tropical storm strategy depends on many factors, such as the lack of sea room or the behaviour of a particular boat when hove to in strong winds or running before big quartering seas. Such considerations will dictate a different approach to the problem and there is unfortunately no fast rule that can be applied at all times. There is no doubt that the safest course of action is to avoid altogether the areas where tropical storms are likely to occur. Therefore the most important consideration when drawing up plans for a voyage is to make sure that the boat will not be in an area affected by tropical storms during the dangerous season. If one plans to pass through an area which is never entirely free from tropical storms, one should attempt to sail during the months of lowest frequency. Such a strategy is not too complicated to follow and many boats have spent several years cruising in the tropics without ever being in the wrong place at the wrong time, simply by leaving the hurricane zone during the dangerous season and returning at the end of it. The directions given for various cruising routes mention the hurricane prone months, so that these can be avoided when planning a cruise along those routes.

Tropical storms are most frequent during the late summer or early autumn in both hemispheres. The safe season in the northern hemisphere is from mid-November to mid-June, whereas the safe season for the southern hemisphere lasts from about May until mid-November. The only tropical area entirely free of hurricanes is the South Atlantic. In the Western North Pacific no month is considered to be entirely safe, although typhoons are extremely rare in winter.- In the Coral Sea, extraseasonal cyclones are not uncommon and have been recorded as late as June and even July. In the Arabian Sea cyclones do not occur in summer, but at the change of the monsoon, either in May-June or in October-November. Diagram 6

Run with wind on starboard quarter

Heave to on starboard tack

Run with wind on starboard quarter

Sail close hauled on, starboard tack at right anglesto assumed track or heave to on starboard tack

Run at right angles to assumed track

Heave to on starboard tack

7. TROPICAL STORM TACTICS NOR THERN HEMISPHERE

Run with wind on port quarter

Sail close hauled on port tack to'assumed track or heave to on port tack

Run with wind on port quarter

A

:

Heave to on port tack

Run at right angles to assumed track

8. TROPICAL STORM TACTICS SOUTHERN HEMISPHERE

shows the world distribution of tropical storms and the months when they are most likely to occur. It is not uncommon for tropical storms to

Prevailing winds

Prevailing weather conditions will be described in greater detail when dealing with regional routes, but it may be useful to summarise here the wind patterns of the principal six areas of the world as they appear on diagrams 2 to 5.

North Atlantic: The NE trade winds blow roughJ ly between latitudes 2 °N and 20 aN to 25 °N in winter, between 10°N and 30°N in summer. In the northern part of the ocean, the winds are predominantly W becoming SW near the North American coast. Between the trade wind and westerly wind belts there is an area of variable winds.

South Atlantic: The SE trade winds cover a wide belt roughly from the equator to 30 °S during the southern summer. They move north during winter (July) when they are found between 3 °N to 5 °N and 25 °S. There are virtually no doldrums south of the equator. Constant westerly winds are to be found in higher latitudes, but they tend to become NW and even N on the South American side of the ocean, especially during summer.

North Pacific: During the summer months the NE trade winds blow between latitudes 12°N and 30 TSf, but move down to an area comprised between latitudes 4 °N or 5 and 25 °N in winter. Between latitudes 35 °N and 55 °N the winds are W or NW. The doldrums are less well defined.

South Pacific: The SE trades are less constant and reliable than in other oceans. At the height of winter (June to August) they blow in a belt stretching approximately from 5aN to 25 "5. During the southern summer the trade winds are even less constant and blow south of the equator as far as latitude 30°S. Westerly winds blow consistently south of 30 °S in winter and 40 °S in summer.

develop outside the official seasons and the early part of the safe season should be treated with caution. World tropical storm seasons follows:

North Indian: The winds are dominated by the two monsoons, NE in winter (November to March) and SW in summer (May to September). The NE monsoon becomes well established in January and it is most consistent until early March. The winds are much stronger during the SW monsoon, at the height of which in July and August they often blow at over 30 knots.

South Indian: The SE trade winds extend from the equator to latitude 25°S in winter (July). During the southern summer (January), the SE trade winds can be found between about 10 °S and 30 eS, the NE monsoon also makes itself felt south of the equator possibly as far as 10°S, but is deflected by the rotation of the earth and becomes the NW monsoon. To the south of the SE trade wind belt there is a zone of variable winds. The higher latitudes are known for their strong westerly winds.

Currents of the world

Currents occur at all depths of the oceans, but the only ones of real interest to the small boat voyager are the surface currents. Because the main catase of surface currents is the direction of the wind, there is a close relationship between their direction and that of the prevailing wind. Constant winds, such as the trade winds, create some of the most constant currents, although these do not always follow exactly the direction of the wind that has generated them. As in the case of the winds, the rotation of the earth has an effect on currents too and therefore in the northern hemisphere currents tend to flow to the right of the direction of the wind, in the southern hemisphere to the left. This is the reason why in the northern hemisphere the currents flow in a clockwise direction, while in the southern hemisphere currents generally tend to follow an anti-clockwise direction. Currents will be described in more detail in relevant routes.

tjfrj

West Indies NE Pacific NW Pacific Bay of Bengal Arabian Sea S Indian S Pacific

Season

June to November May to November All year

May to December April to December November to May November to April f/l\'AI'.-i ttftjHi'iH u

September y

July-September /

July-October

Octobcr-Novcmber

April-May October-November

Decern ber-Ma rch

January-March

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