Cyclones these are regions of low atmospheric pressure and are basically of two types. The first is a depression characteristic of temperate latitudes, the second being a much more violent phenomenon, though generally covering a smaller area and typical of the tropics – usually called a Tropical Cyclone.
The two types are similar in that their winds in the Northern Hemisphere circulate in an anti-clockwise direction around its center or eye and, in the Southern Hemisphere, in a clockwise direction.
Over N.W. Australia, in late summer, the tropical cyclones experienced are called Willy-Willies (January-April). They then re-curve to the south-east, cross the coast and bring heavy rain moving overland to the Great Australian Bight, e.g. Darwin, Port Hedland. I
n the China Seas the tropical cyclone is referred to as a Typhoon, generally experienced between May to December but with maximum frequency between July and October. The Philippine Islands are directly affected, as is southern China.
Like all tropical cyclones, the typhoons bring winds of tremendous strength and torrential rain and possibilities of tidal waves affecting ports such as Shanghai and Manila.
In the Arabian Sea, the cyclone of this area is called a Monsoon, derived from the Arabic word Mawsim meaning season, applied by Arabs to the seasonable winds of the Arabian Sea which blow for about six months from the south west and six months from the north east.
The term is now generally applied to the type of wind system in which there is a complete or almost complete reversal of prevailing direction from season to season. It is especially prominent within the tropics on the eastern side of the great landmasses, although it does occur outside the tropics.
For eastern Asia it reaches as far north as about 60 N. South East Asia is pre-eminently a monsoon region. Monsoon gales in the Arabian Sea occur generally from June to August, although tropical storms in the same area may last from May to November.
Extremely heavy rains are experienced and also strong winds and heavy seas. Ports affected include Bombay and Bedi on the west coast of India.
The tropical cyclones or revolving storms of the West Indies and Gulf of Mexico are known as Hurricanes. They usually originate east of the islands, occasionally as far east as the Cape Verde Islands and take a westwards course, sometimes causing extensive destruction on one island after another before generally recurving to the north-east.
Besides the West Indies, the Gulf coasts of the United States and the eastern side of Central America, as far as the Mosquito Coast of Nicaragua, are affected by Hurricanes.
Costa Rica, Panama and the northern coast of South America, however, lie outside the hurricane region. The Hurricane season is between June and November, with a maximum frequency August to October.
The name hurricane is also given to the tropical cyclone experienced off the coast of Queensland, Australia. As an example of the strength of winds, they can have a mean velocity of over 75 miles per hour or equivalent to a wind force 12 on the Beaufort Scale*. Havana (Cuba) and Miami (Florida) are among ports affected.
The Beaufort Scale is the series of numbers devised by Admiral Beaufort at the beginning of the nineteenth century to differentiate approximately between various wind strengths. For example ‘0’ on the Beaufort Scale represents calm, the air being practically motionless, whilst the upper end of the scale, 12, represents a hurricane, the surface wind speed being greater than 75 miles per hour. Between these two extremes, the numbers indicate the intervening wind speeds. The various wind velocities to which numbers on the Beaufort Scale are equivalent have been internationally agreed.
Often, within the terms of a time charter, the speed of a vessel will be related to a certain number on the Beaufort scale – usually 3 or 4 – and the object of this is that the vessel should perform at the prescribed speed with prevailing winds 3 or 4. Should she not perform at this speed, then the charterer may well make a speed claim against the owners for non-performance.
Another scale used for reporting conditions at sea and sometimes used in vessel performance descriptions (consequently, on occasions, in subsequent disputes) is the Douglas Sea State Scale. As the name implies it deals with the state of the sea in terms of the height of the waves. It is a fact that the surface of the sea can often be sufficiently rough to affect the speed of the vessel without the wind in the immediate vicinity being particularly high, due to stormy weather several miles away. Yet another climatic condition, especially affecting shipping, occurs in the Southern Hemisphere between latitudes 40 and 60 S where there are very few land masses and, consequently, the winds in this region are very strong and are often referred to as the Roaring Forties.