Wind Pressure Sys. Structure of a Depression Anticyclone Weather Services TRS




Tropical Revolving Storms


Terms associated with a TRS

The air circulation's of the Northern and Southern hemispheres are in opposite directions, so that disturbances, which form in one hemisphere, cannot cross into the other hemisphere.

The area near the equator is one of convergence as the Northern Hemisphere N.E. Trade wind blows towards it, as does the S.E. Trade of the Southern Hemisphere.

The general name for this area is the INTERTROPICAL CONVERGENCE ZONE (ITCZ), also known as the DOLDRUMS.

When the ITCZ is well to the north or south of the equator the change in direction of the trades after crossing the equator will cause very strong convergence currents and it is possible that a cyclonic disturbance will form in this area. The possibility is increased when the ITCZ is in the vicinity of islands when local surface heating of air of high humidity gives rise to very unstable conditions.

Low pressure areas frequently occur in the ITCZ but cyclonic circulation can only result if the geostropic force is sufficiently large (there is no geostropic force on the equator) and this is unlikely in latitudes less than 5.

The cyclonic disturbance once formed is known as a Tropical Revolving Storm whose diameter varies between 50 to 800 miles, 500 miles being an average.

After formation between 5 to 10 of latitude the storm moves westwards at 10 - 12 knots, until reaching the tropic, where3 it slows down before re-curving eastwards and proceeding at 15 20 knots to the higher latitudes.

TRS do not normally cross-land but when and if they do the supply of warm moist air necessary to their existence being cut off they tend to fill and thus dissipate. However if the TRS do cross over again from land to sea then they again deepen.

Normally these occurrences are few and the land that is crossed is small, but sometimes-powerful TRS can cross wide landmasses as wide as South India.

It is noticed that the T.R.S. is not found in the South Atlantic or the eastern side of the South Pacific. Reference to the charts will show that the ITCZ is at no time south of SOS in these areas.




North Atlantic Ocean Western side


June to November

North Pacific Ocean Eastern side

Hurricane or Cordonazo

June to November

North Pacific Ocean Western side

Typhoon or Raguios

All the year but greatest frequency and intensity June to November.

South Pacific Ocean western side


December to April

South Indian Ocean Eastern side


December to April

South Indian Ocean Western side


December to April

Bay of Bengal and Arabian Sea


June and November but they may occur during the S.W. monsoon season.





The following terms are in common use when reference is made to a T.R.S.:

PATH: The direction in which the storm is moving.

TRACK: The area, which the storm centre has traversed.

STORM FIELD: The horizontal area covered by the cyclone conditions of the storm.

SOURCE REGION: The region where the storm first forms.

VERTEX: The furthest westerly point reached by the storm centre.

EYE OF THE STORM: The storm centre.

BAR OF THE STORM: The advancing edge of the storm field.

ANGLE OF INDRAUGHT: The angle, which the wind makes with the isobars.

VORTEX: The central calm of the storm.

DANGEROUS SEMI-CIRCLE: The half of the storm, which lies to the right of the path in the Northern Hemisphere and to the left of the path in the Southern Hemisphere.

DANGEROUS QUADRANT: The leading portion of the dangerous semicircle where

NAVIGABLE SEMI-CIRCLE: The half of the storm, which lies to the left of the path in the Northern Hemisphere and to the right of the path in the Southern Hemisphere.

TROUGH LINE: A line through the centre of the storm at right angles to the path. The dividing line between falling and rising pressure.

Oceanic depressions usually have one or more fronts extending from their centres, each front representing a belt of bad weather, ac companied by a veer (backing) of wind, which marks the change from the weather characteristic of one air mass to that of the other. During the first two or three days of its existence a depression has a warm and a cold front, the area between the two being known as the warm sector because the air has come from a warmer locality than that which is outside the sector.

This is shown in the following diagram:

Depression Northern Hemisphere:




Warm air is lighter than cold air and rises over the cold air ahead of the warm front as shown in the diagram:



These are intense depressions occurring in tropical latitudes accompanied by high winds and heavy seas. Although the pressure at the centre of a tropical storm is comparable to that of an intense mid- latitude depression, the diameter of a tropical storm is much smaller (some 500 miles compared with 1,500 miles), and therefore the pressure gradients and the wind speeds correspondingly greater.

The wind blows round the centre of a tropical storm in a spiral movement inwards, anti-clockwise in the N hemisphere and clockwise in the S hemisphere, giving rise to the occasional alternative name Revolving Storm.

Within 75 miles of the centre of a tropical storm, the wind is often very violent and the sea high and confused, causing considerable damage even to large and well-found ships. The danger is still greater when ships are caught in restricted waters without adequate room to manoeuvre.

Due to torrential rain and sheets of almost continuous spray visibility near the storm centre (but outside the eye) is almost nil. Within 5 to 10 miles of the centre the wind is light or moderate and variable, the sky is clear or partially so, and there is a heavy, sometimes mountainous, confused swell; this area is known as the eye of the storm.

The localities, seasons, average frequencies and local names of these storms are shown in the notes.

The locating of tropical storms has greatly improved in recent years with the aid of weather satellites.

Once identified by satellite, tropical circulations are carefully tracked and in some areas, e.g. the seas around the West Indies and the Philippines, weather reconnaissance aircraft fly into these circulations to measure characteristics such as wind speed and pressure.

Warnings of the position, intensity and expected movement of each circulation -are then broadcast at regular intervals (see Admiralty List of Radio Signals).

Tropical storms generally originate between the latitudes of 7 and 15, though some form nearer the equator.

Those which affect the W part of the Pacific, South Indian and North Atlantic Oceans are usually first reported in the W parts of these oceans, though there are exceptions, such as in the North Atlantic during August and September when an occasional storm begins near Arquipelago de Cabo Verde.

In the N hemisphere they move off in a direction between 275 and 350, though most often within 30 of due W. When near the latitude of 25 they usually recurve away from the equator and, by the time they have reached a latitude of 30, the track (or path as it is more usually called) is NE. In the S hemisphere they move off in a WSW to SSW direction (usually the former), recurve between latitudes of about 15 to 20, and thereafter follow a SE path. Many storms, however, do not recurve but continue in a WNW (WSW) direction until they reach a large land mass where they fill quickly.

The speed of the storms is usually about 10 knots in their early stages, increasing a little with latitude but seldom achieving 15 knots before recurving.

A speed of 20 to 25 knots is usual after recurving though speeds of over 40 knots have been known. Storms occasionally move erratically, at times making a complete loop, but when this happens their speed is usually less than 10 knots.

Winds of force 7 are likely up to. 200 miles from the centre of the storm and winds of gale force 8 up to 100 miles from the centre, at latitudes of less than 20; but by a latitude of 35 these distances may be doubled though wind force near the centre may be diminished. Hurricane force winds are likely within 75 miles of the storm centre in the tropics and gusts exceeding 175 knots have been reported.

As already stated, warning of the position, intensity and expected movement of a storm is given by radio at frequent intervals.

Sometimes, however, there is insufficient evidence for an accurate warning, or even a general warning to be given and then ships must be guided by their own observations.

The first of the following observations is by far the most reliable indication of the proximity of a storm, within 20 or so of the equator.

It should be borne in mind, however, that very little warning of the approach of an intense storm of small diameter may be expected.

Precursory signs of tropical storms

If a corrected barometer reading is 3 millibars or more below the mean for the time of the year, as shown in the climatic atlas or appropriate volume of the Sailing Directions, suspicion should be aroused and action taken to meet any development. The barometer reading must be corrected not only for height, latitude, temperature and index error (if mercurial) but also for diurnal variation, which is given in climatic atlases or appropriate volume of the Sailing Directions.

If the corrected reading is 5 millibars or more below normal it is time to consider avoiding action for there can be little doubt that a tropical storm is in the vicinity. Because of the importance of pressure readings it is wise to take hourly barometric readings in areas affected by tropical storms.

An appreciable change in the direction or strength of the wind.

A long low swell is sometimes evident, proceeding from the approximate bearing of the centre of the storm. This indication may be apparent before the barometer begins to fall.

Extensive cirrus cloud followed, as the storm approaches, by altostratus and then broken cumulus or scud.

Radar may give warning of a storm within about 100 miles. At times the eye can be clearly seen. It is surrounded by a large area of moderate or heavy rain and outside this area the belts of rain are arranged in bands as shown.

By the time the exact position of the storm is given by radar, the ship is likely to be already experiencing high seas and strong to gale force winds. It should be in time, however, to enable the ship to avoid the eye and its vicinity where the worst conditions exist.

To decide the best course of action if a storm is suspected in the vicinity, the following knowledge is necessary.

The bearing of the centre of the storm

The path of the storm

If an observer faces the wind, the centre of the storm will be from 100 to 125 on his right hand side in the N hemisphere when the storm is about 200 miles away, i.e. when the barometer has fallen about 5 millibars and the wind has increased to about force 6.

As a rule, the nearer he is to the centre the more nearly does the angle approach 90.

The path of the storm may be approximately determined by taking two such bearings separated by an interval of 2 to 3 hours, allowance being made for the movement of the ship during the interval.

It can generally be assumed that the storm is not traveling towards the equator and, if in a lower latitude than 20 its path is most unlikely to have an E component.

On the rare occasions when the storm is following an unusual path it is likely to be moving slowly.

Diagrams below show typical paths of tropical storms and illustrates the terms dangerous and navigable semicircle. The former lies on the side of the path towards the usual direction of recurvature, i.e. the right hand semicircle in the N and the left hand semicircle in the S hemisphere.


The advance quadrant of the dangerous semicircle(shown in red) is known as the dangerous quadrant as this quadrant lies ahead of the centre. The navigable semi- circle is that which lies on the other side of the path. A ship situated within this semicircle will tend to be blown away from the storm centre and re-curvature of the storm will increase her distance from the centre.

Avoiding tropical storms.

In whatever situation a ship may find herself the matter of vital importance is to avoid passing within 50 miles or so of the centre of the storm. It is preferable but not always possible to keep outside a distance of 200 miles.

If a ship has at least 20 knots, at her disposal and shapes a course that will take her most rapidly away from the storm before the wind has increased above the point at which her movement becomes restricted, it is seldom that she will come to any harm. Sometimes a tropical storm moves so slowly that a vessel, if ahead of it, can easily outpace it or, if astern of it, can overtake it.

If a storm is suspected in the vicinity, the vessel, whilst observing her barometer, should continue on her course until the barometer has fallen 5 millibars (corrected for diurnal variation) below normal, or the wind has increased to force 6 when the barometer has fallen at least 3 millibars.

Then she should act as recommended in the paragraphs below, until the barometer has risen above the limit just given and the wind has decreased below force 6.

Should it be certain, however, that the vessel is behind the storm, or in the navigable semicircle, it will evidently be sufficient to alter course away from the centre.

In the N hemisphere (ship initially moving Slowly).

(a) If the wind is veering the ship must be in the dangerous semicircle. The ship should proceed with all available speed with the wind 10 to 45, depending on speed, on the starboard bow. As the wind veers the ship should turn to starboard, thereby tracing a course relative to the storm as shown in the above diagram.

(b) If the wind remains steady in direction, or if it backs, so that the ship seems to be nearly in the path or in the navigable semicircle respectively, the ship should bring the wind well on the starboard quarter and proceed with all available speed. As the wind backs the ship should turn to port as shown.

In the S hemisphere (ship initially moving slowly).

(a) If the wind is backing the ship must be in the dangerous semicircle. The ship should proceed with all available speed with the wind 10 to 45 depending on speed, on the port bow. As the wind backs the ship should turn to port thereby tracing a course relative to the storm as shown.

(b) If the wind remains steady in direction, or if it veers, so that the ship seems to be nearly in the path or in the navigable semicircle respectively, the ship should bring the wind well on the port quarter and proceed with all available speed. As the wind veers the ship should turn to starboard as shown.

If there is insufficient sea room to run, when in the navigable semicircle, and it is not practicable to seek shelter, the ship should heave to with the wind on her starboard bow in the N and on her port bow in the S hemisphere.

If in harbour when a tropical storm approaches, it is preferable to put to sea if this can be done in time to avoid the worst of the storm. Riding out a tropical storm, the centre of which passes within 50 miles or so, in a harbour or anchorage, even if some shelter is offered, is an unpleasant and hazardous experience, especially if there are other ships in company. Even if berthed alongside, or if special moorings are used, a ship cannot feel entirely secure.