Identifying and Responding to an approaching TRS.

Warning signs of an approaching TRS

Swell

The very violent winds of the eye-wall send swell out in a radial direction. Swell can be experienced as much as a thousand miles away. Swell travels much faster than the speed of travel the storm. Swell, therefore, approaches from the direction of the storm centre. Swell is usually the first indication of an approaching TRS.

Atmospheric pressure

Falls steadily. The approach of a TRS should be suspected if:
a) The ship is in an area where TRS generally occur
b) If the time of the year is within the season when TRS generally occur
c) The aneroid barometric pressure is >3 mb below normal.

The presence of TRS is confirmed if the foregoing conditions are met and the fall of barometric pressure is more than 5 mb below normal.

Weather

a) Cirrus clouds in bands or filaments aligned towards the direction of the storm centre.
b) Unusually clear visibility may occur.
c) Sometimes peculiar dark red/copper colour of sky is seen at sunset before a TRS.
d) Increase of wind force as the pressure falls.
e) Threatening appearance of dense, heavy
clouds on the horizon.
f) Frequent lightning may be seen.
g) Succession of squalls, with or without rain.

Storm warnings

Weather reports based on satellite pictures and observations from other vessels may contain storm warnings which give the position and pressure of the storm centre and also probable direction of movement of the storm.

Action when approach of a TRS is confirmed

1) Obtain the bearing of the storm centre : Face the wind, and according to Buys Ballot’s law, the storm centre will lie 8 to 12 points on your right in the NH, left in the SH. If the pressure has fallen 5 mb below normal, allow 12 points as it means that either the vessel is in the outer fringes of a well developed TRS, or that a new TRS is forming in the vicinity. If the pressure has fallen 20 mb or more below normal, allow 8 points as it means that the vessel is near the eye of a well developed TRS.

Also try: Concentrates, Carriage Hazards of Concentrates and Type of Concentrates.

2) Ascertain in which semi-circle the vessel lies : For a stationary observer, if the wind veers, vessel is in the RHSC and if it backs, LHSC. This holds good for both NH and SH. While determining the semicircle, the following points should be noted:
a) Wind observations, though logged every
hour during bad weather, should be compared with that 2 hours earlier. This is to give time for significant veering or backing and hence weed out errors that may be caused by irregular gusts of wind.

b) Veering or backing, once detected, should be continuous while the observer remains stationary, i.e. a veering wind should continue to veer and a backing wind should continue to back. If the wind veers at first and then backs, or if it backs at first and then veers, the vessel must have passed from one semicircle into another, due to change of path of the storm.

c) During the two-hour interval between observations, while veering or backing of
wind is being decided, the observer must be stationary i.e., vessel should remain hove to. If not, the conclusion arrived at regarding RHSC or LHSC may be wrong and disastrous consequences may result.

3) Avoiding action: Any avoiding action should aim to keep the vessel well out of the storm centre. If a vessel is in port when a storm warning is received, it may be advisable to proceed well out to sea so that the vessel will have plenty of sea room and sufficient depth of water (to prevent the vessel from pounding on the seabed during pitching and heaving). If proceeding out to sea is not possible, it would be advisable for the vessel to anchor outside the port, in whatever shelter she can find, drop both anchors with several shackles of cable out on each. The engines should be ready – bursts of engine movement may be necessary to prevent dragging of anchors.

Out in open sea

The following action is recommended to keep the vessel out of eye/eye-wall:
1) If vessel is in the dangerous quadrant:
Proceed as fast as practicable with the wind 1 to 4 points on the stbd bow (port bow in SH) 1 point for slow vessels (less than 12 knots) and 4 points for fast vessels (more than 12 knots) altering course as the wind veers (backs in SH). This action should be kept up until the pressure rises back to normal i.e., until vessel is outside the outer storm area. If there is insufficient sea room, the vessel should heave to with the wind on the stbd bow (port bow in SH) until the storm passes
over.

2) If vessel is in the path of the storm or in the navigable semi-circle:
Proceed as fast as practicable with the wind about 4 points on the stbd quarter (port quarter in SH), altering course as the wind backs (veers in SH). This action should be kept up until the pressure rises back to normal i.e., until vessel is outside the outer storm area.

Ref: Marine Meteorology by Capt H Subramaniam