Chapter 35: Tropical Cyclones
A tropical cyclone is the most dangerous weather a ship can meet. Recognising the approach and manoeuvring by the semicircle rules is a survival skill.
Formation and structure
Tropical cyclones require sea surface temperatures above about 26.5°C, latent heat, low vertical wind shear and sufficient Coriolis force — hence none form within about 5° of the equator. They progress from tropical disturbance to depression, storm (34 kn) and hurricane/typhoon (64 kn). Structure comprises a calm eye, an eyewall of maximum wind, and spiral rainbands. Warning signs at sea are a long low swell from an unusual direction, a barometer reading 3 mb or more below normal for the area and season, unusual cirrus, and a hard squally wind increase.
- Sea temperature above ~26.5°C and away from the equator
- Long unexplained swell and a falling barometer are the first warnings
- Track typically west then poleward, recurving near 25°–30°
Dangerous and navigable semicircles
In the northern hemisphere the right-hand semicircle relative to the storm's track is dangerous: the storm's forward motion adds to the wind and the wind tends to drive a vessel into the path. The left semicircle is navigable. Both are reversed in the southern hemisphere. Determine which you are in by the wind's behaviour: in the northern hemisphere, a veering wind means the dangerous semicircle, a backing wind the navigable, and a steady wind direction with falling pressure means you are in the path.
Avoidance rules
Northern hemisphere: in the dangerous semicircle, bring the wind on the starboard bow, hold course as far as possible and make all speed — you are turning right as the wind veers. In the navigable semicircle, bring the wind on the starboard quarter and run. In the path, put the wind on the starboard quarter and run into the navigable semicircle. In the southern hemisphere use port bow and port quarter respectively. Plan on the 1-2-3 rule of forecast-track uncertainty and stay at least 34-knot-radius plus margin clear.
Key takeaways
- • Northern hemisphere: right semicircle dangerous, left navigable. Reverse in the south.
- • Veering wind (NH) = dangerous semicircle; steady wind with falling glass = in the path.
- • NH avoidance: wind on the starboard bow in the dangerous semicircle, starboard quarter otherwise.
Knowledge check
Immediate feedback with rationales. Answer each question to see the explanation.
Chapter 35 quiz
0 of 3 answered
1.In the northern hemisphere, a vessel observing a steadily veering wind with falling pressure is in the:
2.A northern-hemisphere vessel in the dangerous semicircle should:
3.Tropical cyclones do not form within about 5° of the equator because: