A watch officer reads the barometer, the swell and the cloud together, then decides whether to hold course, heave to, or route around what is coming. This chapter works through the physics behind each of those calls, then puts numbers to three of the situations most likely to turn up in the exam.
A synoptic chart is a map of pressure at a single instant, drawn as isobars — lines joining places of equal barometric pressure. Air does not sit still under an uneven pressure field; it is pushed from where pressure is higher towards where it is lower, and the closer the isobars are packed together over a given distance, the steeper that push and the harder the wind blows. That is the first reading to take off any chart before looking at anything else: spacing tells you strength.
Left alone, air would flow straight across the isobars, high to low. It never does, because the Earth is turning underneath it. That rotation deflects any moving air to the right of its direction of travel in the Northern Hemisphere, and to the left in the Southern. The deflection grows as the air speeds up, until it balances the pressure push and the air ends up flowing almost along the isobars rather than across them — anticlockwise around a low in the north, clockwise around a high, and the mirror image south of the equator.
That circulation is the whole of Buys Ballot's law, stated as a rule of thumb rather than a diagram: stand on deck facing into the wind, and in the Northern Hemisphere the centre of low pressure lies on your right hand, roughly eight points — 090° — round from the direction you are facing. South of the equator the rule flips: low pressure sits on your left. It costs nothing to check, and it is the fastest way to get a feel for where you sit relative to a system before the chart itself confirms it.
Wind does not blow from high pressure to low pressure in a straight line — it blows almost along the isobars, and that single fact is what makes Buys Ballot's law work.
A front is not really a line on the chart so much as the edge between two air masses that refuse to mix. Warm, moist air is lighter than cold, dense air, so wherever the two meet, one rides up over the other rather than blending — and which one is doing the riding decides what kind of weather follows.
At a warm front, warm air is advancing and climbs gradually up over the retreating wedge of cold air ahead of it. Because the slope is shallow, cloud builds from high, thin cirrus a day or more ahead of the front, thickening down through the layers to low stratus and drizzle as the front itself arrives — a long, slow deterioration a watchkeeper can see coming hours in advance from the cloud sequence alone. Visibility drops steadily, and once the rain starts it tends to set in for hours rather than pass through.
At a cold front the roles reverse: cold air is advancing and undercuts the warm air ahead of it, forcing it up sharply rather than gently. The steep slope builds cumulonimbus rather than layered cloud, and the front passes as a squall — heavy, short-lived rain or hail, a marked veer in the wind, then a rapid clearance to cooler, cleaner air and improved visibility behind it. Where a faster cold front catches up with a warm front ahead of it, the two merge into an occluded front, which behaves like a colder, messier version of whichever type is doing the catching.
A warm front is read in the cloud building up over many hours; a cold front announces itself in minutes, as a squall line advancing on the horizon.
A tropical revolving storm forms over ocean water warm enough, and deep enough, to keep feeding it moisture and latent heat as it grows — broadly why these systems form only in the tropics, in season, and essentially never within a few degrees of the equator, where the Coriolis deflection needed to organise a rotation is too weak to get one started. Once established, the storm organises itself into a calm, often clear eye at the centre, ringed by an eyewall carrying the most violent wind and rain in the whole system, with spiral rain bands winding outward from it and the wind strength falling away, gradually, with distance from the centre.
The warning signs are worth committing to memory, because the chart itself may be hours old by the time you see it: a barometer falling faster than the ordinary daily wobble, a heavy, long-period swell arriving from a direction the local wind cannot explain — because that swell has outrun the storm that raised it — and a true wind that will not settle into a steady direction and strength the way ordinary trade-wind weather does. Any one of these on its own can have an innocent explanation; the three together, and especially a falling glass paired with an unexplained swell, should be treated as a storm until proven otherwise.
The barometer alone is not the warning — it is the falling barometer together with a swell that does not match the local wind that removes the doubt.
Once a tropical revolving storm is suspected near the track, the next job is to find out roughly where, and Buys Ballot's law is the fastest tool available before any plotted fix confirms it. Facing the true wind — not the apparent wind felt on a moving ship, but the true wind, corrected for the ship's own course and speed — the storm's centre lies roughly eight points to the right hand in the Northern Hemisphere, ahead of the point where the storm crosses the ship's meridian; once it has crossed, that bearing opens out further as the storm draws away.
Which semicircle the ship is in decides how urgent the situation is, and it is read from how the wind is changing, not from a single observation. If the true wind is veering — shifting clockwise — while the glass keeps falling, the ship is in the dangerous semicircle, on the side where the storm's own forward speed adds to its rotary wind rather than subtracting from it. If the wind is backing instead, the ship is in the navigable semicircle, where the two partly cancel. A wind holding a steady direction while the barometer falls hard means the ship is close to the storm's own track, near dead ahead of it.
The avoiding action follows directly. In the dangerous semicircle, bring the true wind fine on the starboard bow and drive the ship as hard as she can be made to go — that heading draws her away from the track fastest. In the navigable semicircle there is less urgency: put the wind on the starboard quarter and run, letting the storm's own recurvature open the distance. South of the equator every one of these rules mirrors — backing, not veering, marks the dangerous semicircle, and it lies to the left of the track rather than the right.
Surface currents are driven mainly by the prevailing wind dragging on the water and, just as with the wind itself, the Earth's rotation deflects that drag rather than letting it push the water straight downwind. In deep water, far from any coast, a current theoretically sets about 45° away from the wind driving it, though friction and shallower water usually cut that angle down in practice. The great ocean gyres — the looping current systems in each ocean basin — are simply this wind-driven deflection playing out at ocean scale, carrying warm water poleward on one side of a basin and cold water back towards the equator on the other.
Those currents matter to a watchkeeper for two separate reasons. First, they set and drift the ship, sometimes by a knot or more sustained for a whole passage, which is a real number worth building into a passage plan rather than discovering as an error in the DR. Second, where a cold current runs beneath warm, moist air, it is the direct cause of the thickest sea fog a ship is likely to meet — a link worth holding in mind going into the next section.
Ice is a separate hazard with its own behaviour: icebergs are calved from glaciers, most significantly from Greenland into the North Atlantic shipping lanes, and drift with the current far more than with the wind, since so much of their bulk sits below the surface. Growlers and bergy bits — the low, eroded remnants of larger bergs — are the more dangerous of the two, because they sit low enough to give a poor or intermittent radar return and can be nearly invisible in any kind of sea, which is exactly why a visual lookout is kept as well as the set maintained on the radar.
Fog is simply cloud sitting at sea level, and it forms whenever air is cooled — or has moisture added to it — until it can no longer hold the water vapour it is carrying, and that vapour condenses into droplets. The distinction that matters at sea is where the cooling happens. Advection fog forms when warm, moist air drifts over a colder sea surface and is cooled from below; it is the classic sea fog, it can form regardless of wind strength, and because the whole air mass has been chilled through, it tends to persist until the air mass itself changes — typically a wind shift bringing in different air, rather than the sun burning it off. Radiation fog, by contrast, forms over land on a clear, calm night as the ground radiates its heat away and chills the air sitting on it; it can drift out over an anchorage or a coastal passage on the morning land breeze, but because it was never in equilibrium with the sea beneath it, it usually thins and lifts within an hour or two of the sun getting up.
Weather routeing takes all of the above — pressure systems, fronts, storm risk, currents and the likelihood of fog — and uses it to choose a track, not just a course. The direct, rhumb-line distance is rarely the fastest passage in practice once forecast headwinds, a foul current, or an area of heavy weather are weighed against it; a modest diversion that finds a fair current or ducks a developing low can save more time than it costs in extra distance, and avoids risk a shorter track would have run straight through. The judgement is made and remade as the forecast is updated, not fixed once at the start of the passage.
The shortest course and the fastest, safest passage are rarely the same line on the chart.
Three passages below turn the rules above into numbers: locating a storm and choosing which way to turn, reading what a current is worth over a plotted passage, and separating the wind felt on deck from the true wind the law actually needs.
Your ship is on passage in the North Atlantic hurricane belt in September. At 0600 the true wind is from 320°T and the barometer reads 1008 hPa. By 0900 the wind has drawn round to 350°T and the barometer has fallen to 1002 hPa, with a long, low swell running in from the north-east that the local wind cannot account for. Find the approximate bearing of the storm centre at 0600, decide which semicircle the ship is in, and state the avoiding action.
0600: true wind from 320°T, barometer 1008 hPa 0900: true wind from 350°T, barometer 1002 hPa Long swell from the north-east, out of keeping with the local wind Ship is in the Northern Hemisphere
Find the approximate bearing of the storm centre at 0600, decide which semicircle the ship is in, and state the avoiding action
Confirm this is a developing storm, not routine weather.
The barometer has fallen 6 hPa in three hours — far more than the ordinary diurnal wobble of a hPa or so — and the swell is arriving from a direction the local wind cannot have raised. Both point the same way.
Find the bearing of the centre at 0600 using Buys Ballot's law.
Face into the true wind — the direction it is blowing from, 320°T. In the Northern Hemisphere the storm centre lies about eight points (090°) round to the right hand.
Decide which semicircle the ship is in.
The true wind has veered (shifted clockwise) from 320° to 350° while the glass keeps falling — that combination puts the ship in the dangerous semicircle, where the storm's own forward speed adds to its rotary wind rather than subtracting from it.
Take the avoiding action.
In the dangerous semicircle the rule is to bring the true wind fine on the starboard bow and drive the ship clear as fast as she can be made to go, which draws her away from the track fastest.
AnswerStorm centre bears about 050°T at 0600; the veering wind and falling glass place the ship in the dangerous semicircle; alter to about 315°T, wind fine on the starboard bow, and drive at maximum safe speed clear of the track.
The trap: plotting the storm's bearing once at 0600 and holding that course — the centre is moving too, so both the bearing and the veer/back check must be repeated with every new wind and pressure observation.
A 320 nm passage is planned in two legs to make use of the pilot chart's current data. For the first 180 nm the ship can hold 15.0 kn through the water with a 3.0 kn fair current under her; for the remaining 140 nm she can only make 14.0 kn through the water and must cross a 2.0 kn foul current. Find the passage time by this routeing, and compare it with simply steaming the same total distance at a steady 14.0 kn without reference to the current at all.
Leg 1: 180 nm, 15.0 kn through the water, 3.0 kn fair current Leg 2: 140 nm, 14.0 kn through the water, 2.0 kn foul current Total distance: 320 nm Comparison: same 320 nm run at a flat 14.0 kn, current ignored
Find the passage time by this routeing, and compare it with simply steaming the same total distance at a steady 14.0 kn without reference to the current at all
Work up the speed made good on each leg.
Where the current runs directly along the course there is no vector triangle to solve — a fair current simply adds to the speed through the water, and a foul current subtracts from it.
Convert each leg to time and add them.
Compare against the naive plan.
The same 320 nm steamed at a flat 14.0 kn without reading the current at all.
Find what the routeing is worth.
The saving looks modest against a single passage, but it costs nothing beyond reading the pilot chart properly, and it compounds over every ocean passage a ship makes.
AnswerRouted passage time ≈ 21 h 40 min, about 1 h 11 min faster than ignoring the current — for no extra distance run.
The trap: treating current as something that only matters close to the coast — a modest current sustained for many hours across an ocean passage moves the ship as many miles as a squall moves her in minutes, and it is that sustained effect a routeing plan has to capture.
Steering 000°T at 10.0 kn, the bridge wind instrument reads the apparent wind as from 060°T at 25.0 kn (the ship's own heading has already been added into that bearing by the instrument; the speed shown is still the apparent, on-deck speed). Find the true wind's speed and direction, and say by how much the storm bearing in Example 1 would have been wrong had the officer of the watch applied Buys Ballot's law to this apparent reading instead of the true wind.
Ship's course 000°T, speed 10.0 kn Apparent wind: from 060°T, 25.0 kn (true bearing, apparent speed) Northern Hemisphere
Find the true wind's speed and direction, and say by how much the storm bearing in Example 1 would have been wrong had the officer of the watch applied Buys Ballot's law to this apparent reading instead of the true wind
Set up the two vectors that combine to give the apparent wind.
| Vector | Direction (towards) | Speed | East | North |
|---|---|---|---|---|
| Ship's motion | 000°T | 10.0 | 0.0 | 10.0 |
| Apparent wind | 240°T | 25.0 | −21.7 | −12.5 |
| Sum (true wind) | — | — | −21.7 | −2.5 |
The wind felt aboard a moving ship is the true wind together with the headwind of the ship's own passage; to recover the true wind, the ship's velocity is added back onto the apparent wind's vector.
Convert the resultant back to a speed and a true bearing.
The resultant vector's length is the true wind speed; its direction, converted from a 'towards' vector to the usual 'from' bearing, is the direction the true wind is blowing from.
Compare it with the bearing Example 1 used.
That example took its storm bearing from a true wind of 320°T. Here the true wind (083°T) sits about 23° round from the apparent 060°T the instrument displayed before the ship's own motion was removed from it.
AnswerTrue wind ≈ 21.8 kn from 083°T; reading Buys Ballot off the uncorrected apparent wind (060°T) instead would have misplaced the storm's bearing by about 23°.
The trap: reading Buys Ballot straight off the wind vane or anemometer without first removing the ship's own course and speed — on a ship making any way at all, the apparent wind can sit tens of degrees from the true wind, which is the only wind the law is valid for.
Buys Ballot's lawFace the wind: low is on your right in the north, on your left in the southIsobar spacingClose together = strong wind; wide apart = light windVeer / backVeer = wind shifts clockwise; back = anticlockwise, in both hemispheresWarm frontSlow approach, thickening cloud, steady rain, drizzle, poor visibilityCold frontFast approach, squally showers, marked wind veer, sharp clearanceTRS centre bearingFacing the true wind, centre bears ≈8 points (090°) to the right, north of the equatorSemicircles (N hemisphere)Veering wind + falling glass = dangerous (right of track); backing = navigable (left)Avoiding action (N, dangerous)Bring wind fine on the starboard bow, drive clear at maximum safe speedOcean currentsWind-driven, deflected ≈45° from the driving wind in deep waterFogAdvection: warm air over cold sea, persists. Radiation: land-formed, usually lifts by mid-morning