Nature. Tropical storms are violent storms with very strong winds and heavy seas.

Origin. They occur mostly in the late summer and early autumn (December to April in the southern hemisphere). They originate in the tropics between latitude 5° and latitude 10° north or south and develop only over the oceans.

Named. Tropical storms are known by various names, ie Hurricanes in the North Atlantic, Typhoons in the Eastern Indian Ocean and West Pacific and Cyclones in the Western Indian Ocean. They do not occur in the South Atlantic.

Grading. They are graded internationally according to the following:

1. A Tropical Depression – when the winds up to 34 knots are experienced.
2. A Tropical Storm – when winds between 35 and 64 knots are experienced.
3. A Tropical Cyclone/Hurricane – when winds of 65 knots and above are experienced.

Composition. Tropical Cyclones are relatively small, intense, low pressure areas with a more or less circular shape. They comprise the circulation of a single air mass (tropical marine) without any fronts. They are rarely more than 300 miles in diameter and the most intense part is much smaller. The pressure gradient within the cyclone is very steep and the fall from the outer edge to the centre varies between 20 and 70 millibars. The wind reaches its maximum intensity near the centre. Because of the intensity of the wind, it is increasingly deflected until it travels in a circular path, never reaches the very centre. The centre itself is approximately 10 miles in diameter and is relatively calm. On opposite sides of the centre the winds blow in opposite directions.

Danger to shipping. The danger to shipping is the abrupt change in the direction of the wind and the sudden increase in intensity when the centre passes over a vessel. Winds of between 100 and 200 knots around the centre are not uncommon.

Eye of the storm. Although there is little wind in the centre, the sea is very rough and the waves very confused. The cloud cover increases from high cirrostratus at the edge of the storm, through altostratus and nimbostratus to cumulonimbus around the centre. Above The centre itself, the sky is usually clear.

Power of storm. To give some idea of the power and violence of a cyclone/hurricane, it has been calculated that the daily release of energy within the cyclone/hurricane is equal to approximately one hundred thousand bombs of megaton strength.

Warning signs. Before the space age, the Americans used to fly a specially equipped aeroplane into the cyclone to record the parameters of the storm. As you can imagine this was likely to be a very bumpy ride with a fair amount of risk to the aircraft and its aircrew. These days special weather satellites are launched and positioned to monitor the weather in the lower atmosphere. By means of these satellites tropical storms can be detected early and appropriate warnings issued to mariners and coastal areas lying in the path of the storm. Apart from these warnings, how else can the mariner be made aware of their presence? The following paragraphs contain the indicators of an approaching tropical storm:

1. A fall in atmospheric pressure is the most reliable indicator within 20° of the equator. In hurricane/cyclone areas the barometer should be read hourly. There are usually three phases in the fall of the barometer:
   • A slow fall (diurnal range still visible0 between 500 and 120 nautical miles from the storm centre.
   • A distinct fall (no diurnal variation) between 120 and 60 miles from the centre.
   • A rapid fall between 60 and 10 miles from the centre.
   

   Example of a barograph trace for a hurricane.

   

   Example of a barograph trace for a normal depression.

2. An appreciable change in the direction and/or strength of the wind.
3. A long, low swell is sometimes evident, proceeding from the approximate bearing of the centre of the storm.
4. Extensive cirrus cloud followed as the storm approaches, by altostratus and then broken cumulus and then scud.
5. Radar may give warning of the storm within about 100 miles.

Track of the storm (southern hemisphere). In the southern hemisphere initial movement is between west-south- west and south-south-west (usually the former). Between 15° and 20° south, the storms re-curve and follow a south-easterly path. Many storms do not re-curve but continue in a south-westerly direction. After re-curvature or when a storm moves inland, it weakens and dissipates.

Finding the path of a storm. To find the best course of action, the following knowledge is necessary:

1. The bearing of the centre of the storm.
2. The path of the storm.

The bearing of the centre of the storm can be found by Buys Ballot’s law which states; “When facing the wind in the southern hemisphere, the centre of the low pressure will lie 90° to 120° on your left hand”. The path can then be found by taking the above bearing at two to three hour intervals, allowing for the movement of the ship in the interval. An indication of the bearing of the centre of the storm can also be determined by noting the direction of the sea swell.

Rules for avoiding tropical storms (southern hemisphere). The following rules for avoiding tropical storms should be followed:

1. Navigation within the dangerous quadrant. A power driven vessel should proceed with all available speed with the wind 10° to 45° on the port bow. The vessel should haul round to port as the wind backs. Keeping the wind on the port bow, the vessel will follow the curved track in the above sketch.
2. Navigation within the navigable quadrant. A power driven vessel should proceed with all available speed with the wind well on the port quarter. Vessels should alter course to starboard as the wind veers. If there is sufficient sea room to alter course and it is not practical to seek shelter, ie a leeward size of some land, a vessel should lie heaved to with the wind on the port bow.
3. When taking either of the above actions, the course of the ship should be similar to that on the diagram.