Ocean routes follow the planet's wind belts. One thing you avoid at any price: the tropical revolving storm, where your hemisphere decides every action.
1 · Wind belts
Trades, doldrums and westerlies
Which way do the trade winds blow in each hemisphere?
NE in the northern hemisphere, SE in the southern: the trades flow from the highs near 30° back towards the ITCZ, the doldrums, turned by Coriolis.
Air rises at the ITCZ, the doldrums: calms and squalls. It sinks near 30° in a belt of highs, the horse latitudes, and flows back as the trades, turned by Coriolis: NE in the northern hemisphere, SE in the southern. Poleward blow the westerlies with their lows, then the polar easterlies.
The ITCZ follows the Sun and is wider in the east: cross it further west, with fuel for the calms. Winter highs reinforce the trades to 25–30 kn; El Niño weakens the Pacific trades and moves the cyclones east. Trade-wind squalls travel with the wind and can double it: reef before they arrive.
2 · Highs and monsoons
Which side of the high to sail
Which side of an ocean high should I sail to avoid the calms?
Keep out of its calm centre: the trades blow on a high's equatorward side, the westerlies on its poleward side. Wind circles it clockwise in the northern hemisphere, anticlockwise in the southern; route by where it lies today.
Wind circles a high clockwise in the northern hemisphere, anticlockwise in the southern. Either way the trades blow on its equatorward side, the westerlies on its poleward side, calms in the centre. Route by where it lies today.
Northern high
Southern high
A monsoon reverses with the season: in the northern Indian Ocean and the South China Sea the NE monsoon (November–March) is fair, the SW monsoon (June–September) blows force 6–7, and between them come calms, squalls and cyclones. The Red Sea is NNW all year, except SSE in its southern half in winter.
3 · Depressions
Let the low pass on the poleward side
Which side should I let a depression pass when sailing east across an ocean?
On your poleward side: lows travel east, and with the centre passing poleward of you the wind stays westerly and fair, veering SW to NW in the northern hemisphere, backing NW to SW in the southern.
Lows travel east, turning anticlockwise in the northern hemisphere and clockwise in the southern. Bound east, let the centre pass on your poleward side: the wind stays westerly and fair.
Northern hemisphere
Southern hemisphere
Centre passes
Northern
Southern
Poleward of you
veers SW → NW
backs NW → SW
Equatorward
backs SE → NE
veers NE → SE
The cold front brings the shift, squalls and a rising glass. Southern Ocean lows come in a train: a brief ridge, then NW wind and a falling glass for the next.
4 · Tropical revolving storms
Where and when, and the first signs
What are the first warning signs of a hurricane or tropical cyclone at sea?
A long swell from the storm's bearing, cirrus fanning from one point, then the barometer: corrected for its daily 3 hPa swing, 3 hPa below normal means suspect a tropical revolving storm, 5 means act.
A tropical revolving storm needs a sea of 26.5 °C and Coriolis, so it forms at least 5° from the equator, in its hemisphere's summer and autumn. It moves west and poleward, then recurves towards the pole and east, faster.
Northern hemisphere
Southern hemisphere
Ocean
Name
Season
North Atlantic
hurricane
June–November
NW Pacific
typhoon
mostly June–November
South Pacific, South Indian
cyclone
November–April
North Indian
cyclone
monsoon changes
South Atlantic
almost none
—
First signs: a long swell from the storm's bearing, cirrus fanning from one point, then the barometer. Tropical pressure swings 3 hPa daily (high 1000 and 2200, low 0400 and 1600); corrected for that, 3 hPa below normal means suspect, 5 means act. In the eye the wind drops, then returns as hard from the opposite side.
5 · Avoiding a storm
Find the centre, then act by hemisphere
Which way should I steer to get clear of a tropical revolving storm?
Keep the wind on the starboard bow, close-hauled, if it veers and you are in the dangerous semicircle; on the starboard quarter, running, if it backs or holds steady with a falling glass. South of the equator it is port, and backing means dangerous.
Buys Ballot's law: face the wind and the centre is about 10 points (112°) to your right in the northern hemisphere, to your left in the southern.
Northern hemisphere
Southern hemisphere
In the dangerous semicircle the storm's speed adds to the wind, and the wind carries you into its path: right of the track in the northern hemisphere, left in the southern. The other half is navigable. The wind shift tells you which half you are in; so does the centre's bearing: moving clockwise, you are right of the track.
Northern, dangerouswind on starboard bow
Northern, navigablewind on starboard quarter
Southern, dangerouswind on port bow
Southern, navigablewind on port quarter
Wind
Northern hemisphere
Southern hemisphere
Veers
dangerous: close-hauled, starboard tack
navigable: run, port quarter
Backs
navigable: run, starboard quarter
dangerous: close-hauled, port tack
Steady, glass falling
on the track: run, starboard quarter
on the track: run, port quarter
Remember
North: wind to starboard. South: wind to port. Bow in the dangerous half, quarter in the navigable.
The 1-2-3 rule adds 100, 200 and 300 M to the 34-knot radius at 24, 48 and 72 hours: forecast error grows. The gale arrives at (distance − radius) ÷ speed. Move early; a stalled storm can go anywhere.
6 · Currents, swell and fog
What the wind does to moving water
Why should I not cross the Gulf Stream with a northerly wind?
Wind against current raises short, steep, breaking seas, and the Gulf Stream sets NE at 3 kn: cross it, or ride the Agulhas, only with the wind going its way.
Current
Sets
Note
Gulf Stream
NE, 3 kn
sharp cold north wall
Agulhas
SW, 2–4 kn
SW gale: go inshore of 200 m
Kuroshio
NE, 2–4 kn
Japan's Gulf Stream
Humboldt
N, cold
cool, cloudy, dry
North Equatorial
W, 0.5 kn
fair westbound
Wind against current raises short, steep, breaking seas: cross the Gulf Stream or ride the Agulhas only with the wind going its way. Moist air chilled over a cold current gives advection fog that wind does not clear. Swell outruns a distant storm at 1.5 × its period, in knots. Steer across a waterspout's track.
7 · Pilot charts and forecasts
Climate for the plan, forecasts for the day
Can I trust a GRIB forecast when a tropical storm is near?
No: trust the advisory, not the GRIB. A GRIB shows the mean wind: gusts run 40 % higher, squalls are missing and storms blunted. Where models disagree, plan for the worse.
A pilot chart shows one month's averages: wind roses, gale and calm percentages, ice limits, past storm tracks. It is history: an empty route means rare, not impossible.
Reading a pilot chart
Arrows fly with the wind, to the circle
Arrow length: how often from there
Feathers: the mean force
Figure in the circle: % of calms
Gales 15 % × 8 days = 1.2 days of gale
Source
Limits
NAVTEX
300–400 M from the station
SafetyNET, HF weatherfax
ocean-wide; a fax chart is hours old
GRIB
mean wind: gusts 40 % more, no squalls, storms blunted
Router
advises; the skipper decides
Rule
Models disagree: plan for the worse. Tropical storm: trust the advisory, not the GRIB.
Practice questions
Questions from this topic, as the app asks them. Open one to see the answer.
Leaving Gran Canaria, late November
Azores High 1028 hPa near 34°N 30°W
Air flows south towards the equator
Your wind at 20°N: NE 5
You are three days out of Las Palmas on the ARC route, in the trade-wind belt. Why does the wind blow from the NE and not straight from the N?
The Earth's rotation turns it to the right
The Sahara's heat pulls it in from the east
The Azores High spins it anticlockwise
The Canary Current drags it to the west
Show the answer
A. The Earth's rotation turns it to the right
Coriolis turns moving air to the right in the north: the flow back to the equator becomes the NE trade.
All 100 questions of this topic are in the app: practice by topic with the reason after every answer, your mistakes brought back, and a timed mock exam. Free, no sign-up, in the browser or in Telegram.
