TROPICAL REVOLVING STORMS
A tropical revolving storm aka TRS, is a small area of very low pressure, around which winds of gale force (34 knots or force 8) or more blow spirally inwards, anticlockwise in the Northern Hemisphere (NH) and clockwise in the Southern Hemisphere (SH). In the case of violent tropical revolving storms, wind speeds up to 130 knots have been experienced wit occasional gusts up to 150 knots.
Origin, movement and life span of TRS
TRS originate in latitudes between 5° & 20° and travel between W and WNW in the NH and between W and WSW in the SH, at a speed of about 12 knots. Somewhere along their track, they curve away from the equator curve to N and then recurve to NE in the NH; curve to S and then recurve to SE in the SH. The recurving is such that the storm travels around the oceanic high (situated at about 30°N and 30°S in the middle of large oceans). After recurving, the speed of travel increases to about 15 to 20 knots.
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Sometimes, a TRS does not curve or recurve at all, but continues on its original path, crosses the coast and dissipates quickly thereafter due to friction and lack of moisture. It is important to note that all TRS do not follow such definite paths and speeds. In their initial stages, occasionally, storms have remained practically stationary or made small loops for as long as four days.
The life span of a TRS in anything from one to nineteen days, the average being about six days.
Structure
A well-developed TRS has three distinct parts:
1) The eye or vortex: A calm central area of lowest pressure, having a diameter between 4 miles and 30 miles, the average being about 10 miles.
2) The eye-wall: An inner ring of hurricane force winds having a width usually between 4 miles and 30 miles. The winds in the eye-wall blow in a perfectly circular path with a speed as high as 130 knots with occasional gusts up to 150 knots. The pressure gradient in the eye-wall is very steep and, therefore, the barograph would register a near vertical trend, downward before the eye and upward behind it.
3) The Outer storm area: The area surrounding the eye-wall, having a diameter between 50 miles and 800 miles, the average being about 500 miles. Winds in this region are strong (about force 6 or 7) and the pressure gradient is much less than in the eye-wall.
Ref: Marine Meteorology by Capt. H Subramaniam, Cult of Sea & Marine gyaan website
