Tropical revolving storms — track, recurvature & avoiding action

Where they come from, and why never on the equator

A tropical revolving storm is a heat engine running on the latent heat of a warm ocean. It needs four things at once, and the absence of any one of them stops it — which is why, of all the thunderstorm clusters over the tropics, only a handful ever spin up.

Note the second condition. Within about 5° of the equator f = 2Ω sin φ is too small to spin the inflow into a closed circulation — the same Coriolis parameter you met in MT-01, here deciding whether a storm exists at all. Slide the ship's latitude below 5° and watch the check fail.

The track — and the point of recurvature

Born on the equatorward side of the subtropical high, the storm is steered WESTWARD — WNW in the northern hemisphere, WSW in the southern — at a modest 10–12 knots. Somewhere near 20°–25° it reaches the edge of that high, comes under the mid-latitude westerlies, and RECURVES: it slows, turns poleward, and then accelerates away to the east at 15–25 knots. That turning point is the point of recurvature, and it is the single most consequential thing on the chart — because a ship that has placed itself assuming the storm will keep going west finds it coming back over the top.

Northern hemisphereSouthern hemisphere

This storm recurves at 19.2°N and 69.3°E, 132 hours from the start — before that she makes good about 295° at 11 kn; after it, 045° at 18. The forecaster's real uncertainty is WHERE she turns, which is why the danger area grows with time rather than staying a point.

The two semicircles — and why one of them is called dangerous

Split the storm along its own track. On one side the storm's forward motion adds to the circulating wind; on the other it subtracts. In the northern hemisphere the adding side is the RIGHT of the track (the left in the southern), and that side has a second, worse property: the wind there tends to blow a ship towards the storm's path. Stronger wind AND a wind that pushes you into the way — that is the dangerous semicircle. The other half is navigable, which does not mean safe.

The avoiding action

The rules are not arbitrary. In each case you put the wind where holding it there carries you away from the track, and then let the shifting wind steer you: if the wind is veering you are in the dangerous semicircle of a northern storm, if it is backing the navigable one, and if it holds steady while the barometer falls you are in the path.

Dangerous semicircle, northern hemisphere: bring the wind on the STARBOARD BOW, make as much way as the sea allows, and as the wind VEERS keep altering to STARBOARD. You are working out towards the storm's right rear — the only direction that both increases your distance and takes you away from the track.

Finding the centre, and how far off it is

You may never see the centre, but you can always find it. Face the wind: the centre bears about 8 to 12 points (90°–135°) on your right hand in the northern hemisphere, on your left in the southern — 10 points is the working figure, closing towards 90° as you get near. Then the barometer gives the range: compare the corrected reading with the NORMAL for the place and season, not with 1013.

The signs, in the order they arrive: a long low swell from an unusual direction, often 500 nm ahead of the storm; high cirrus fanning out from the same quarter; the barometer falling below its normal diurnal wobble; then wind, and it is too late to be comfortable.