Tropical Revolving Storm or TRS are intense tropical depressions which develops in tropical latitudes over large sea area. A tropical revolving storm may be defined as a roughly circular atmospheric vortex, originating in the tropics or sub tropics, where in the winds of gale force (34 knots or force 8) blow in spirally inwards (Anti-Clockwise in North Hemisphere and Clockwise in South Hemisphere.
TRS in the Atlantic and pacific are generally more violent than in the Arabian Sea and the Bay of Bengal. With in the circulation of a tropical storm the wind is often very violent and the seas are high and confused, considerable damage may be done even to large and well built ships.
The dangers are prominent when ships are caught in restricted waters without sufficient sea room to manoeuvre. Nevertheless, TRS are a great danger to shipping regardless of where they are encountered and require a special study.
Classification and Nomenclature of TRS
Tropical cyclones are classified into three groups, these groups are based on intensity of the storm and successively its name depends on the region.
tropical storms, and
more intense storms,
For example, if a tropical storm in the North Pacific reaches winds of 64 knots or more it is referred to as a Typhoon. Each area uses a separate system of nomenclature, below are some of the names referred to at various locations as per Beaufort scale.
Formation and Movement
Tropical storms only develop over oceans, and formation is especially frequent near the seasonal location of the ITCZ. In North Hemisphere, TRS forms mostly between 5o – 30o N, in early storm season it is between 5o-15o N, and during late storm season period it is between 10o – 25o N. Only in Atlantic this belt is usually between 25o N and 30o N. In South America most storms develop between 5o – 18o S.
The tracks that a storm follow are not fixed and vary as per areas and sometimes the tracks may be erratic.
In North Hemisphere TRS travels between W and WNW and when near 25o N latitude storms recurve away from the equator and by the time they reach 30o N the storm travels in a NE direction.
In South Hemisphere TRS travels between W and WSW and when between 15o S to 20o S it recurves and follows SE path. When a storm moves inland it weakens and eventually dissipates, but if it does not hits land and continue on ocean passage then it may re-intensify.
Storm in early stage usually move at a speed of 10 Knots increasing slightly with latitude but hardly it will exceed 15 knots before it recurves. A speed of 20-25 knots is usual after recurving, although higher speed have been recorded in the past.
Structure of Tropical Revolving Storm
A developed TRS has 3 Parts as follows
The Vortex or Eye: A circular central area of lowest atmospheric pressure having a diameter usually between 4 to 30 miles.
The Eye Wall: An inner ring or circumference of the vortex or eye having hurricane force winds and width usually between 4 to 30 miles. The pressure gradient in the eye wall is very steep, almost vertical trend. The Winds in the eye wall blow in a perfectly circular path. Winds in eye wall move at a speed of about 130 Knots.
The outer storm Area: The outer area surrounding the eye wall, having radius between 50 to 800 Miles. Winds in this region are strong usually wind force of 6 to 7.
Track: The route over which the storm centre has already passed.
Path: Predicted route which the storm centre is likely to travel.
Trough: The line drawn through the centre of the storm, at right angles to the track. Ahead of the trough, pressure falls whereas behind it, pressure rises.
Vertex or Cord: The westernmost longitude reached by the storm centre when recurving takes place.
Right Hand semicircle (RHSC): It is right half of the storm centre that is, it is the part which lies to the right of the observer who faces along the path of the storm.
Left Hand Semicircle (LHSC): It is Left half of the storm centre that is, it is the part which lies to the left of the observer who faces along the path of the storm.
Dangerous Semicircle: The Right Hand Semi Circle in the NH and Left Hand Semi Circle in the SH is called the dangerous semicircle.
Navigable Semicircle: The Left Hand Semi Circle in the NH and Right Hand Semi Circle in the SH is called the dangerous semicircle.
Dangerous Quadrant: The forward quadrant of the Right hand semicircle in the NH and similarly in the Left hand semicircle is called Dangerous Quadrant.
Ideal Conditions Required for the formation of a TRS
High Relative humidity.
A sufficiently large sea area where in, the sea surface temperature is in excess of 26oC i.e., Tropical Areas.
Low pressure area surrounded by high pressure area.
Convection Current (Day time over large islands). (The Above two points ensure that air rises continuously, so that adiabatic cooling results in condensation that liberates latent heat. This latent heats provides the energy for the TRS).
Fair amount of Coriolis Force i.e., Between Latitude more than 5oN or S (This ensures that when the wind blow, from surrounding areas of HP to LP area inside, they get deflected sufficiently to blow spirally inwards).
Weak prevailing winds, this ensures that if the prevailing winds are strong, the air would not rise vertically. It would be carried off horizontally, thereby not allowing a TRS to form.
So for a TRS to form all above condition should me met. From the past observations it has been observed that TRS does not form in South Atlantic Ocean the reason is simple that the all the above criteria are not complied.
To summarise, the ideal conditions for the formation of a TRS exist during day time over large Tropical islands, in mid-ocean, between latitudes 5o and 20o.
You can Also read the action a vessel can take to avoid Tropical Revolving Storm.