THE NATURE OF HURRICANES
Hurricane. A hurricane may be described as a cyclonic storm of great violence in which the winds achieve great speed of motion revolving anti clock wise (in the
northern hemisphere) more or less symmetrically round an area of low atmospheric pressure, while moving slowly over the surface of the sea. The winds of a
hurricane increase in volume the nearer they are to the centre of calm round which they revolve. A hurricane is usually accompanied by high seas, thunder and
lightning and torrential rain.
(ii) Area of Storm. A more or less circular area, covered by the hurricane which at any given moment is subject to storm disturbance.
(iii) Line of Progression. The direction in which a hurricane moves across the earth’s surface represented by an Imaginary line drawn through the centre of the
(iv) Centre of Storm, Eye of Storm, Area of Calm. These various names are given to the central calm area of low atmospheric pressure round which the winds
(v) Right and Left Storm. The line of progression bisects the storm halves, and the terms right and left are applied respectively to portions to the right and left of
this line as an observer would lock along the line facing the direction in which the storm is moving. The point where the centre of the storm strikes is the centre
and the coast line is called right and left of storm as if the observer had his back to the oncoming storm.
(vi) Right Semicircle, Right Half. The part of the storm on the right of the line of progression.
(vii) Left Semicircle, Left Half. The part of the storm on the left of the line of progression.
(viii) Right Front Quadrant, Right Rear Quadrant. The front half and back half, respectively, of the right semicircle of the storm.
(ix) Left Front Quadrant, Left Rear Quadrant. The front half and back half, respectively, of the left semicircle of the storm.
(x) The Trough. During the passage of the centre of the storm the barometer falls to its lowest paint and begins to rise as soon as the centre has passed. If the
centre does not pass over the observer the lowest point reached by the barometer marks the trough. It will occur when the centre of the storm is in a position in
relation to the point of observance, of making a right angle with the line of progression.
(xi) The Track or Path of the Storm. The route the storm has taken, and therefore plotted after the storm has passed.
2. THE GENERAL PATTERN OF HURRICANES
While it is not possible to lay down hard and fast rules as to how a hurricane will behave, they tend to follow a fairly regular pattern and the following paragraphs
will set out the general facts that are known about them.
(i) The speed of hurricanes. It is estimated that the average speed of a hurricane along the line of progression is 12 to 4 knots, but the speed depends on the size,
the more intense the hurricane the slower its advance. It also moves at a greater speed when travelling in a straight line than when it is curving.
(ii) The diameter of hurricanes. There is considerable varia¬tion from 50 to 500 miles have been recorded the average might be taken as about 100 – 120 miles.
(iii) The length of time of a hurricane takes to pass a point of observation depends on the rate of progress and the diameter. There is therefore, wide variation but
the usual time is about nine or ten hours. This is the full time of the storm winds. The duration of the destructive winds will be from two to three hours.
(iv) The average life of a hurricane is about eight days.
(v) The line of Progression in the Northern Hemisphere is straight or parabolic curving North, except for hurricanes occurring in the Western Caribbean the general
line of storm is from east to west curving northwards,
(vi) The area of calm is a period of relative calm, possibly exaggerated by violence, that precedes and follows it. It is marked by the suddenness with which comes
and goes. During this period there is a marked drop in the atmospheric pressure and in the relative humidity, but there is a sharp rise in the temperature. There
conditions last only as long as the period of the calm.
(vii) The diameter of the area of calm varies with the size of the storm, but the average width is estimated to be about 15 – 20 miles.
(viii) The length of time the area of calm takes to pass a point of observation depends on its size and the rate of progress of the storm and also whether the point of
observa¬tion is in the exact centre of the calm. An area of calm moving at 12 miles will take two hours to pass a point of observation on the line of progression.
This time will be progressively shortened until no period of calm is experienced as the point of observation is moved to the right or left of the line of progression.
(ix) The sources of hurricanes which visit the West Indies are extremely varied, and may be almost anywhere between 5 and 23 degrees North over the Atlantic or
the Caribbean. The most common locations are to the east of the Lesser Antilles or in the Western Caribbean.
3. SIGNS OF APPROACH
The main phenomena to look for on the approach of a hurricane are as follows:
(i) Sea Swell. One of the earliest signs of a hurricane is the rising of a swell at sea and this may be noticed when the storm is as much as 400 to 500 miles away.
The period of the swell is long but very definite and causes heavy surf on the coast facing the oncoming storm. The breaking surf has a distinctive booming note
but as the storm approaches the sea becomes more and more boisterous so that the result of the swell is less marked.
(ii) The Tides. As a hurricane approaches a coastline there is a tendency for the t ' ides to be abnormally high and to lack full ebb. The reason has been given that
the storm waves approach tends to reinforce the gravitational tides thus accentuating the high tide and cancelling out the low.
(iii) Barometer Pressure. In the tropics the diurnal rise and fall of the barometer is very marked and very regular. A hurricane approaching will upset this
regularity. It may be preceded by a ridge of high pressure, in which case the barometer will rise higher than the normal, then fall to below normal. As the hurricane
comes nearer, this drop will become increasingly sharp.
(iv) Temperature. As a rise temperature does not give any real indication, but there is a tendency for the day temperature to rise more than normal while the night
temperature is somewhat lower. However, with the dropping of the wind, the feeling of heat may be exaggerated by the oppressive atmos¬phere.
(v) Cloud Formation. If the hurricane is preceded by a ridge of high pressure the weather will be fine and there will be little or no cloud. The first advance
appearance of the storm may be high flying Cirrus or Mare's Tail clouds. These clouds travel In advance of the storm usually along the line of progression and the
paint on the horizon from which they appear to diverge is paid to indicate the position of the centre of the storm. As the storm approaches a thin haze or veil of
cloud begins to form and spreads from the direction from which the Cirrus clouds have come. The veil called “cirrose veil", thickens rapidly until the Cirrus clouds
are obscured. Dense black clouds masses will then make their appearance.
(vi) Winds. During the early stages, the wind will be high and variable particularly if there is a ridge of high pressure. As the general weather conditions deteriorate
the winds will continue light but become more gusty until the storm is definitely getting close when it will start blowing from a definite quarter. It will then
gradually increase in violence and gustiness, its direction depending on the position of the centre of the storm.
(vii) Other indications. At night there may be a halo round moon and the brighter of the stars and by day a halo is sometimes seen round the sun.
The sunrise and sunset are said to be exceptionally brilliant even by tropical standards and have a marked reddish colouration.
The general atmosphere is oppressive, an illustration of the lull before the storm.
4. PASSAGE OF STORM.
During the approach of the hurricane there is usually little or no rain but as the violent winds of the storm itself arrive there are heavy showers Interspersed with
drizzle and the barometer falls abruptly.
The direction of the wind and its force as has been explained above will vary considerably during the storm and depending on the exact quarter of the storm
passing over. The nearer the centre of the storm the more violent and destructive will, be the wind and the: rain will he torrential during the first half and the seas
mountainous. If the centre passes over the Island there will be a brief lull in which the barometer will reach its lowest paint. After the lull the storm will start up
again with great violence with the winds in the opposite direction. The rain will not be so torrential after the passage of the centre and the winds will gradually die
away and the barometer rise.
5. HURRICANE DAMAGE
The destructive force of the winds of a hurricane are well known but the records show that the greatest loss of life and damage to property occurs in places where
the visits of the are so infrequent as to give rise to the belief that the place is almost immune which breeds an attitude of mind that though disasters happen
elsewhere "It never happens here".
This was amply demonstrated when Hurricane David struck Dominica on 29 August 1979. A generation had almost passed away since the hurricane of 1928 and
except for few eyewitness accounts told now and then, most people of middle age had no recollection of the 1928 hurricane.
It is in areas that suffer from frequent hurricanes, that the most adequate precautions are taken and loss of life reduced to a minimum thereby. Buildings are built to
be storm proof and on sites that are well above the reach of the Storm Waves. Care should be taken to build houses of suitable material and of a design that will
give adequate protection during heavy winds and storms.
The destructive forces of a hurricane are mainly:
(i) The force of the wind
(ii) The storm wave
(iii) The torrential rain
(iv) The lightning
(i) Damage by the wind. Damage by the wind includes the incidental damage that arising from wind action that is damage from flying debris or damage to
building from falling trees that make an opening for the wind to wreck the house. The force of the wind uproots trees and growing crops, breaks branches and
blow over or demolishes flimsy buildings. Roofs that are not built to withstand the storm are torn off especially if they offer large areas to the full force of the
wind and are not anchored. The wind may carry the debris with destructive force for considerable distances and damage other objects that otherwise might have
been unharmed. The chief danger to persons arises from the collapse of buildings, from being blown over by the force of the wind or injured by flying debris.
Shelter should therefore be taken against the wind and by the same taken, from flying debris and the shelter should be as secure as possible in a building likely to
stand. The use of corrugated iron adds greatly to the potentialities of injury and damage.
Once the wind has forced a window or door it exerts a force on the weak part of the structure that is on the underside of the roof and the destruction of the
windows is the first step in the demolition of the house. For this reason all windows and doors should be provided with shutters that can be barred. An eye-
¬witness of the 1931 hurricane in Barbados recommended that if a window was blown in on one side of the house the best thing to do was to open one on the
other side to allow the wind to escape instead of offering resistance.
It must not be forgotten that danger to the person continues after the hurricane has died down, from wounds, however slight, that may become infected and many
persons have been known to die from tetanus which arose out of ill attended wounds received during a hurricane.
(ii) Damage from Storm Waves. It is reported by Tamehill that more than three quarters of the casualties caused in hurricanes have been due to drawing in the
storm waves following inundation of low lying, thickly populated coastal areas. The rising sea sweeps away the buildings, roads and bridges, undermines
founda¬tions of solid buildings that would otherwise have stand and 'throws wreckage of debris with devastating force against all types of structures.
The sea waves meeting the torrential waters from the rain making for the sea will add to the destruction. Drains are likely to be blocked and as the water must find
an outlet the weakest structures will go.
(III) Damage by rain. The damage by rain will be similar to that caused by the sea except that it will not be confined to coastal areas. It is probable that houses
will be undermined and roads and bridges swept away by the rush of water making for lower levels and pouring down from the higher ground. A fall of one inch
of rain over an acre amounts to 113 tons of water and any¬thing from 10 to 15 inches may be expected so that the weight of water per acre may be between
1,130 and 1,695 tons. There will be serious flooding in the low lying ground. The sudden preci¬pitation of water is likely to cause serious landslides due to the
overloading of the earth's surface and the weakening of the substrate. Persons living in low lying valleys near to river courses and low lying beaches should be
prepared to leave at an early stage and seek shelter on high ground. Shelter should therefore not be taken in any low lying area liable to flood.
Damage to water pipes and other underground installations will be extensive and before these can be restored the water is likely to become contaminated and not
safe to drink.
(iv) Damage from Lighting. Lightning may cause fires which the high winds would tend to spread and make control more difficult. However, the general result
of the rain and the flooding should play their part in controlling the spread. Far more serious would be a fire to the petrol storage tanks as the blazing oil will float.