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Maintain a Safe Navigational Watch


Weather Routeing

Principle of Weather Routeing


Ship weather routing helps to develop the most efficient track for ocean voyages based on forecasts of:


Sea conditions, and

The ship’s individual characteristics for a particular transit.

Within specified limits of weather and sea conditions, the most efficient is used to mean:

Maximum safety and

Crew comfort,

Minimum fuel consumption,

Minimum time underway, or

Any of one of the above or a mixture of the above factors.

The mariner’s first resources for route planning in relation to weather are the routeing charts and the sailing directions.

These publications give climatic data, such as wave height frequencies and ice limits, for the major ocean seas of the world.

They recommend specific routes based on probabilities, but not on specific conditions.

A ship routing agency, acting in an advisory service, attempts to avoid or reduce the effects of specific adverse weather and sea conditions by issuing:

Initial route recommendations prior to sailing

Recommendations for track changes while underway (diversions), and

Weather advisories to alert the master about approaching unfavorable weather and sea conditions which cannot be effectively avoided by a diversion.

The initial route recommendation is based on a survey of weather and sea forecasts between the point of departure and the destination.

It takes into account the hull type, speed capability, cargo, and loading conditions. The ship’s progress is continually monitored, and, if adverse weather and sea conditions are forecast along the ship’s current track, a recommendation for a diversion or weather advisory is transmitted to the ship.

By this process of initial route selection and continued monitoring of the ship’s progress for possible changes in the forecast weather and sea conditions along a route, it is possible to maximize the ship’s speed and safety.

The greatest potential advantage for this ship weather routing exists when:

(1)The passage is relatively long, about 1,500 miles or more;

(2)The waters are navigationally unrestricted, so that there is a choice of            routes; and

(3)Weather is a factor in determining the route to be followed.

Use of this advisory service in no way relieves the master of responsibility for prudent seamanship and safe navigation.

There is no intent by the routing agency to inhibit the exercise of professional judgment and prerogatives of masters.

Why & How of Weather Routeing

The advent of extended range forecasting and the development of selective climatology, along with powerful computer modeling techniques, have made ship routing systems possible.

The short-range dynamic forecasts of 3 to 5 days are derived from meteorological equations.

These forecasts are computed twice daily from a data base of northern hemisphere surface and upper air observations, and include surface pressure, upper air constant pressure heights, and the spectral wave values.

A significant increase in data input, particularly from satellite information over ocean areas, can extend the time period for which these forecasts are useful.

For extended range forecasting, generally 3 to 14 days, a computer searches a library of historical northern hemisphere surface pressure and 500 millibar analyses for an analogous weather pattern.

This is an attempt at selective climatology by matching the current weather pattern with past weather patterns and providing a logical sequence of events forecast for the 10 to 14 day period following the dynamic forecast.

It is performed for both the Atlantic and Pacific oceans using climatological data for the entire period of data stored in the computer.

For longer ocean transits, monthly values of wind, seas, fog, and ocean currents are used to further extend the time range.

Automation has enabled ship routing agencies to develop realistic minimum time tracks. Computation of minimum time tracks makes use of:

1. A navigation system to compute route distance, time enroute, estimated times of arrival (ETA’s), and to provide 6 hourly DR synoptic positions for the range of the dynamic forecasts for the ship’s current track.

2. A surveillance system to survey wind, seas, fog, and ocean currents obtained from the dynamic and climatological fields.

3. An environmental constraint system imposed as part of the route selection and surveillance process. Constraints are the upper limits of wind and seas desired for the transit. They are determined by the ship’s loading, speed capability, and vulnerability.

The constraint system is an important part of the route selection process and acts as a warning system when the weather and sea forecast along the present track exceeds predetermined limits.

4. Ship speed characteristics used to approximate ship’s speed of advance towards the destination while transiting the forecast sea states.

There are two general types of commercial ship routing services.

The first uses computer simulations and analyses these into direct forecast conditions and routing recommendations.

The second assembles and processes weather and sea condition data and transmits this to ships at sea for on-board processing and generation of route recommendations.

The 1st system allows for greater computer power to be applied to the routing task because powerful computers are available ashore.

The 2nd system allows greater flexibility to the ship’s master in changing parameters, selecting routes, and displaying data.

Ship and cargo as a factor

Ship and cargo characteristics have a significant influence on the application of ship weather routing.

            Ship size,

            Speed capability, and

            Type of cargo is an important considerations in the route selection process prior to sailing and the surveillance procedure while underway.

A ship’s characteristics identify its vulnerability to adverse conditions and its ability to avoid them.

Generally, ships with higher speed capability and less cargo encumbrances will have shorter routes and be better able to maintain near normal speed of advance towards the destination’s than ships with lower speed capability or cargoes.

Some routes are unique because of the type of ship or cargo.

Avoiding one element of weather to reduce pounding or rolling may be of prime importance.

For example, a 20 knot ship with a heavy deck cargo may be severely hampered in its ability to maintain a 20 knot speed of advance towards the destination in any seas exceeding moderate head or beam seas because of the possibility of damage resulting from the deck load’s characteristics.

A similar ship with a stable cargo under the deck is not as vulnerable and may be able to maintain the 20-knot speed of advance towards the destination in conditions, which would drastically slow the deck-loaded vessel.

In towing operations, a tug is more vulnerable to adverse weather and sea conditions, not only in consideration of the tow, but also because of its already limited speed capability. Its slow speed adds to the difficulty of avoiding adverse weather and sea conditions.

Ship performance curves (speed curves) are used to estimate the ship’s speed of advance towards the destination while transiting the forecast sea areas.

The curves indicate the effect of head, beam, and following seas of various significant wave heights on the ship’s speed.

A performance curve prepared for an 18-knot vessel

With the speed curves it is possible to determine just how costly a diversion will be in terms of the required distance and time.

A diversion may not be necessary where the duration of the adverse conditions is limited.

In this case, it may be better to ride out the weather and seas knowing that a diversion, even if able to maintain the normal speed of advance towards the destination, will not overcome the increased distance and time required.

At other times, the diversion track is less costly because it avoids an area of adverse weather and sea conditions, while being able to maintain normal speed of advance towards the destination even though the distance to destination is increased.

Although speed performance curves are an aid to the ship routing agency, the response by mariners to deteriorating weather and sea conditions is not uniform.

Some reduce speed voluntarily or change heading sooner than others when unfavorable conditions are encountered.

Certain waves with characteristics such that the ship’s bow and stern are in successive crests and troughs present special problems for the mariner.

Being nearly equal to the ship’s length, such wavelengths may induce very dangerous stresses.

The degree of hogging and sagging and the associated danger may be more apparent to the mariner than to the ship routing agency.

Therefore, the master may initiate adjustment in course and speed for a more favorable ride when this situation is encountered.

Weather - Wind

Environmental factors of importance to ship weather routing are those elements of the atmosphere and ocean that may produce a change in the status of a ship transit.

In ship routing, consideration is given to wind, seas, fog, ice, and ocean currents. While all of the environmental factors are important for route selection and surveillance, optimum routing is normally considered attained if the effects of wind and seas can be optimized.

The effect of wind speed on ship performance is difficult to determine.

In light winds (less than 20-knots), ships lose speed in headwinds and gain speed slightly in following winds.

For higher wind speeds, ship speed is reduced in both head and following winds.

This is due to the increased wave action, which even in following seas results in increased drag from steering corrections, and indicates the importance of sea conditions in determining ship performance.

In dealing with wind, it is also necessary to know the ship’s sail area.

High winds will have a greater adverse effect on a large, fully loaded container ship or car carrier than a fully loaded tanker of similar length.

This effect is most noticeable when docking, but the effect of beam winds over several days at sea can also be considerable.

Weather - Wave height

Wave height is the major factor affecting ship performance.

Wave action is responsible for ship motions, which reduce propeller thrust, and cause increased drag from steering corrections.

The relationship of ship speed to wave direction and height is similar to that of wind.

increased drag from steering corrections”

This happens because the course steered is not uniform, the auto pilot keeps correcting the course steered too frequently as such, instead of a steady course the ship steers a course that is slightly off on either side of the course.

Vessels speed remaining the same the speed towards the destination suffers.

Head seas reduce ship speed, while following seas increase ship speed slightly to a certain point, beyond which they retard it. In heavy seas, exact performance may be difficult to predict because of the adjustments to course and speed for ship handling and comfort. Although the effect of sea and swell is much greater than wind, it is difficult to separate the two in ship routing

Weather - Fog

Fog, while not directly affecting ship performance, should be avoided as much as possible, in order to maintain normal speed in safe conditions.

Extensive areas of fog during summertime can be avoided by selecting a lower latitude route than one based solely upon wind and seas.

Although the route may be longer, transit time may be less due to not having to reduce speed in reduced visibility.

In addition, crew fatigue due to increased watch keeping vigilance can be reduced.

Weather - Abnormal Waves

During the northern hemisphere autumn and winter, the waters to the north of the gulf stream in the north Atlantic are at their coldest, while the gulf stream itself remains at a constant relatively warm temperature.

After passage of a strong cold front or behind a developing coastal low pressure system, arctic air is sometimes drawn off the mid-Atlantic coast of the united states and out over the warm waters of the gulf stream by northerly winds. This cold air is warmed as it passes over the Gulf Stream, resulting in rapid and intense deepening of the low-pressure system and higher than normal surface winds.

Higher waves and confused seas result from these winds. When these winds oppose the northeast set of the current, the result is increased wave heights and a shortening of the wave period. If the opposing current is sufficiently strong, the waves will break. These phenomena are collectively called the “north wall effect,” referring to the region of most dramatic temperature change between the cold water to the north and the warm Gulf Stream water to the south.

The most dangerous aspect of this phenomenon is that the strong winds and extremely high, steep waves occur in a limited area and may develop without warning. Thus, a ship that is laboring in near-gale force northerly winds and rough seas, proceeding on a northerly course, can suddenly encounter storm force winds and dangerously high breaking seas.

Numerous ships have foundered off the North American coast in the approximate position of the Gulf Stream’s north wall.

A similar phenomenon occurs in the North Pacific near the Kuroshio Current and off the southeast African coast near the Agulhas current.

Ocean currents

Ocean currents do not present a significant routing problem, but they can be a determining factor in route selection and diversion.

This is especially true when the points of departure and destination are at relatively low latitudes.

The important considerations to be evaluated are the difference in distance between a great-circle route and a route selected for optimum current, with the expected increase in speed of advance towards the destination from the following current, and the decreased probability of a diversion for weather and seas at the lower latitude.

Direction and speed of ocean currents are more predictable than wind and seas, but some variability can be expected.

Major ocean currents can be disrupted for several days by very intense weather systems such as hurricanes and by global phenomena such as the ocean current originating off the West coast of S. America known as EL NINO.


The problem of ice is twofold:

            Floating ice (icebergs) and

            Deck ice.

If possible, areas of icebergs or pack ice should be avoided because of the difficulty of detection and the potential for collision.

Deck ice may be more difficult to contend with from a ship routing point of view because it is caused by freezing weather associated with a large weather system.

While mostly a nuisance factor on large ships, it causes significant problems with the stability of small ships.


Generally, the higher the latitude of a route, even in the summer, the greater is the problems with the weather.

Ships, which are not capable of handling bad weather – like tows, should avoid latitudes in excess of 40°especially during the non-summer months.

Weather Patterns

A ship routing should avoid seas associated with extra tropical low-pressure systems in the mid and higher latitudes and the tropical systems in low latitude.

Seasonal or monsoon weather is also a factor in route selection and diversion in certain areas.

Low-pressure systems though not a severe problem for most ships however, may generate prolonged periods of rough seas, which may hamper normal work aboard ship.

Relatively small ships, tugs with tows, low powered ships, and ships with sensitive cargoes can be significantly affected by weather systems weaker than gale intensity.

Using a routing agency can be beneficial.

Gales and storms in the open sea can generate very rough or high seas, particularly with an adverse ocean current.

This can force a reduction in speed in order to gain a more comfortable and safe ride.

But because of the extensive area covered by these low-pressure systems, once ship’s speed is reduced the ability to get away from these areas is also reduced.

Thus, exposure to potential damage and danger is greatly increased.



Generalized 10% frequency Isolines of gale force winds for October through January.

Weather Patterns

Generalized 10% frequency Isolines of gale force winds for January through April..

Area of initial detection of high percentage of tropical cyclones which later developed to tropical storm or hurricane intensity, NA - 1957-1974.

Area of initial detection of high percentage of tropical cyclones which later developed to tropical storm or hurricane intensity, NA - 1946-1973

A recommendation for a diversion by a routing agency well in advance of the intense weather and associated seas will limit the duration of exposure of the ship.

If effective, ship speed will not be reduced and satisfactory progress will be maintained even though the remaining distance to destination is increased.

Overall transit time is usually shorter than if no track change had been made and the ship had remained in heavy weather.

In some cases diversions are made to avoid adverse weather conditions and shorten the track at the same time.

Significant savings in time and costs can result.

In very intense low-pressure systems, with high winds and long duration over a long fetch, seas will be generated and propagated as swell over considerable distances. Even on a diversion, it is difficult to effectively avoid all unfavorable conditions.

The areas, seasons, and the probability of development of tropical cyclones are fairly well defined in climatological publications.

In long range planning, considerable benefit can be gained by limiting the exposure to the potential hazards of tropical systems.

However, when transiting the tropical cyclone generating area, a ship under routing may provide the first report of environmental conditions indicating that a new disturbance is developing.

Thus a reverse case of information flows.

Of course, avoiding an existing tropical cyclone takes precedence over avoiding a general area of potential development.

It has proven equally beneficial to employ similar considerations for routing in the monsoon areas of the Indian Ocean and the South China Sea.

This is accomplished by providing routes and diversions that generally avoid the areas of high frequency of gale force winds and associated heavy seas, as much as feasible.

Ships can then remain in satisfactory conditions with limited increases in route distance.

Seasonal Isolated Weather

In addition to the synoptic weather considerations in ship weather routing, there are special isolated problems that can be avoided by following recommendations and advisories of ship routing agencies.

These problems generally cover smaller areas and are seasonal in nature, but are still important to ship routing.

In the north Atlantic, because of heavy shipping traffic, frequent poor visibility in rain or fog, and restricted navigation, particularly east of Dover strait, some mariners prefer to transit to or from the North Sea via Pentland firth, passing north of the British isles rather than via the English channel.

Weather routed ships generally avoid the area of dense fog with low visibility in the vicinity of the grand banks off Newfoundland and the area east of Japan north of 35°N.

Icebergs are a definite hazard in the North Atlantic from late February through June, and occasionally later.

The hazard of floating ice is frequently combined with restricted visibility in fog.

International ice patrol reports and warnings are incorporated into the planning of routes to safely avoid dangerous iceberg areas.

It is usually necessary to hold south of at least 45°n until well southeast of Newfoundland.

In winter, other seasonal wind systems include Pampero, mistral, and bora, which are best avoided.

In the summer, the semi-permanent high-pressure systems over the world’s oceans produce strong equator ward flow along the west coasts of continents.

This feature is most pronounced off the coast of California and Portugal in the northern hemisphere and along Chile, western Australia, and southwest Africa in the southern hemisphere. Very rough seas are generated and are considered a definite factor in route selection or diversion when transiting these areas.

Routeing Advice

An initial route recommendation is issued to a ship normally 48 to 72 hours prior to sailing, and the process of surveillance begins.

Surveillance is a continuous process, maintained until the ship arrives at its destination.

Initial route recommendations are a composite representation of experience, climatology, weather and sea state forecasts, operational concerns, and the ship’s seagoing characteristics.

A planning route provides a best estimate of a realistic route for a specific transit period.

Such routes are provided when estimated dates of departure are given to the routing agency well in advance of departure, usually a week to several months.

Long- range planning routes are based more on seasonal and climatological expectations than the current weather situation.

While planning routes is an attempt to make extended range (more than a week) or long range (more than a month) forecasts, these recommendations are likely to be revised near the time of departure to reflect the current weather pattern.

An initial route recommendation is more closely related to the current weather patterns by using the latest dynamic forecasts than are the planning route recommendations.

These, too, are subject to revision prior to sailing, if weather and sea conditions warrant.


A Departure advice is a recommendation for delay in departure, or early departure if possible, and is intended to avoid or significantly reduce the adverse weather and seas forecast on the first portion of the route, if sailing on the original departure date.

The initial route is not revised, only the timing of the ship’s transit through an area with currently unfavorable weather conditions.

Adjusting the departure time is an effective method of avoiding a potentially hazardous situation where there is no optimum route for sailing at the originally scheduled time.


A diversion is an underway adjustment in track and is intended to avoid or limit the effect of adverse weather conditions forecast to be encountered along the ship’s current track.

Ship’s speed generally is reduced by the encounter with the heavy weather.

In most cases the distance to destination is increased in attempting to avoid the adverse weather, but this is partially overcome by being able to maintain near normal speed of advance towards the destination.

Diversions are also recommended where satisfactory weather and sea conditions are forecast on a shorter track.

Speed of advance towards the destination

Adjustment of speed of advance towards the destination is a recommendation for slowing or increasing the ship’s speed as much as practicable, in an attempt to avoid an adverse weather situation by adjusting the timing of the encounter.

This is also an effective means of maintaining maximum ship operating efficiency, while not diverting from the present ship’s track.

By adjusting the speed of advance towards the destination, a major weather system can sometimes be avoided with no increase in distance.

The development of fast ships gives the ship routing agency the potential to “make the ship’s weather” by adjusting the ship’s speed and track for encounter with favorable weather conditions.

Evading Bad weather

Evasion is a recommendation to the master to take independent action to avoid, as much as possible, a potentially dangerous weather system.

The ship routing meteorologist may recommend a general direction for safe evasion but does not specify an exact track.

The recommendation for evasion is an indication that the weather and sea conditions have deteriorated to a point where ship handling and safety are the primary considerations and progress toward destination has been temporarily suspended, or is at least of secondary consideration.

Weather Notices

Weather advisory is a transmission sent to the ship advising the master of expected adverse conditions, their duration, and geographic extent.

It is initiated by the ship routing agency as a service and an aid to the ship.

The best example of a situation for which a forecast is helpful is when the ship is currently in good weather but adverse weather is expected within 24 hours for which a diversion has not been recommended, or a diversion where adverse weather conditions are still expected.

This type of advisory may include a synoptic weather discussion, and a wind, seas, or fog forecast.

The ability of the routing agency to achieve optimum conditions for the ship is aided by the master adjusting course and speed where necessary for an efficient and safe ride.

At times, the local sea conditions may dictate that the master takes independent action.

Routeing for Southern hemisphere

Available data on which to base analyses and forecasts is generally very limited in the southern hemisphere. Weather and other environmental information obtained from satellites offers the possibility of improvement in southern hemisphere forecast products.

Passages south of the Cape of Good Hope and cape horn should be timed to avoid heavy weather as much as possible, since intense and frequent low pressure systems are common in these areas.

In particular, near the southeast coasts of Africa and South America, intense low-pressure systems form in the lee of relatively high terrain near the coasts of both continents.

Winter transits south of cape horn are difficult, since the time required for transit is longer than the typical interval between storms.

Remaining equator ward of about 35°s as much as practicable will limit exposure to adverse conditions.

If the frequency of lows passing these areas is once every three or four days, the probability of encountering heavy weather is high.

Tropical cyclones in the southern hemisphere present a significant problem because of the sparse surface and upper air observations from which forecasts can be made.

Satellites provide the most reliable means by which to obtain accurate positions of tropical systems, and also give the first indication of tropical cyclone formation.

In the southern hemisphere, ship weather routing services is available, but are limited in application because of sparse data reports, from which reliable short and extended range forecasts can be produced.

Strong climatological consideration is usually given to any proposed southern hemisphere transit.


A vital part of a ship routing service is communication between the ship and the routing agency. Reports from the ship show the progress and ability to proceed in existing conditions.

Weather reports from the ship enrich the basic data on which analyses are based and forecasts derived, assisting both the reporting ship and others in the vicinity.

A vital part of a ship routing service is communication between the ship and the routing agency. Reports from the ship show the progress and ability to proceed in existing conditions.

Weather reports from the ship enrich the basic data on which analyses are based and forecasts derived, assisting both the reporting ship and others in the vicinity.


The benefits of ship weather routing services are primarily in cost reduction and safety.

The savings in operating costs are derived from reductions in transit time, heavy weather encounters, fuel consumption, cargo and hull damage, and more efficient scheduling of dockside activities.

The savings are further increased by fewer emergency repairs, more efficient use of personnel, improved topside working conditions, lower insurance rates as preferred risks under weather routing, and ultimately, extended ship operating life.

An effective routing service maximizes safety by greatly reducing the probability of severe or catastrophic damage to the ship, and injury of crewmembers. Avoiding heavy weather also enhances the efficiency and health of the crew.

This is especially important on modern, automated ships with reduced crews.


The success of ship weather routing is dependent upon the validity of the forecasts and the routing agency’s ability to make appropriate route recommendations and diversions.

Modern ship designs, exotic cargoes, and sophisticated transport methods require individual attention to each ship’s areas of vulnerability.

Any improvement in the description of sea conditions by ocean wave models will improve the output from ship routing and sea keeping systems.

Advanced planning of a proposed transit, combined with the study of expected weather conditions, both before and during the voyage, as is done by ship routing agencies, and careful on board attention to sea keeping (with instrumentation if available) provide the greatest opportunity to achieve the goal of optimum environmental conditions for ocean transit.