Offshore there is no harbour to run for: read the chart yourself, weigh each forecast, decide early.
How can I tell from the wind which way the low is?
By Buys Ballot's law: stand with your back to the wind, and the low is on your left in the northern hemisphere, on your right in the southern.
Northern hemisphere: the wind turns clockwise round a high, slightly outwards, anticlockwise round a low, slightly inwards; in the south both reverse. Buys Ballot: back to the wind, the low is on your left in the north, your right in the south.
A high's centre is nearly calm: route round its fair-wind side. A blocking high sits for days and steers the lows round it. A col, the saddle between two highs and two lows, gives light, shifting airs, fog or thunder. In summer the Azores high and Iberia's heat low drive a northerly down Portugal.
Will the wind veer or back as a depression passes me?
Face the way the low is travelling: in the northern hemisphere the wind veers on the right of its track and backs on the left. On the right its motion also adds to its wind.
The jet stream steers the lows: under a strong, straight jet they race through in series; a cut-off low wanders unpredictably. Face the way a low travels (northern hemisphere): the wind veers on the right of its track, backs on the left.
On the right its motion adds to its wind. Under the centre the wind dies at the lowest pressure, then returns hard from the opposite side.
| Low | Expect |
|---|---|
| Explosive | 24 hPa in 24 h at 60°N; a comma cloud |
| Ex-tropical | Re-deepens over cooler water |
| Polar low · medicane | Small, fierce, poorly forecast |
Where does a secondary low form, and what does it do to the wind?
A secondary low forms at the triple point, where the fronts meet, or as a wave on a trailing cold front: the wind backs again, rain returns and the glass falls. Its strongest wind lies away from the parent.
Triangles mark a cold front, semicircles a warm one; both on one side an occlusion, on opposite sides a stationary front. An occlusion brings one rain band and one veer, no warm sector.
A secondary low forms at the triple point, where the fronts meet, or as a wave on a trailing cold front: the wind backs again, rain returns, the glass falls. Its strongest wind lies away from the parent.
After the cold front's veer the old sea crosses the new; time the front by closing speed.
How do I work out the wind speed from isobar spacing on a chart?
Measure the spacing on the latitude scale and enter the geostrophic scale; take less round a low, more round a high, then about 0.8 of that at the sea, backed 15°.
Measure the spacing in miles on the latitude scale and enter the geostrophic scale, drawn for one interval and latitude: the same spacing gives less wind further north. Round a low the real (gradient) wind is less than the scale shows, round a high more; at the sea about 0.8 of that, backed 15°. A prognosis is valid at data time plus step.
Forecast and Beaufort wind is the 10 m mean; a masthead sensor reads more. Without one, read the sea.
| Force | Knots | Sea |
|---|---|---|
| 6 | 22–27 | Large waves, white crests, spray |
| 8 | 34–40 | Spindrift, foam in streaks |
| 10 | 48–55 | Storm; sea 'high' 6–9 m |
| 12 | 64 up | 'Hurricane force', any deep low |
How far can I trust a GRIB file against the gale warning?
Less than the bulletin: a GRIB is one model run, hours old, giving the mean wind. Believe a gale that the bulletin and the ensemble show but your GRIB lacks.
| Source | What it gives |
|---|---|
| NAVTEX | A few hundred miles; add the station ahead |
| SafetyNET | High seas bulletins by satellite |
| HF radiofax | Analysis and prognosis charts |
| GRIB | One model run, hours old; mean wind |
Read your own area, your route's and those to windward. The synopsis says where a low will be and how deep; 'losing its identity' means fading. An ensemble runs the model many times: the share of members showing a gale is its chance; a wide spread means low confidence.
Believe a gale the bulletin and ensemble show but your GRIB lacks. A coarse grid misses mountain gaps; a measured satellite wind beats the model; laden, divide a router's time by 0.8. Your own log outranks any forecast: allow for the barometer's daily swing in the tropics and your own run across the isobars; a slow fall means a long blow.
How much bigger than the significant wave height is the biggest wave?
Nearly twice: significant height (Hs) is the mean of the highest third of the waves, and the biggest single wave is nearly twice it.
Significant height (Hs) is the mean of the highest third; the biggest wave is nearly twice it. The sea is limited by the smaller of fetch and duration. Swell runs on from its storm, whatever the local wind. Steepness, not height, breaks crests: a short wind sea beats a higher long swell, worst against a current.
Offshore, sea room is safety: drift, hove-to or running, times the gale's hours must fit inside the distance to the lee shore. A port of refuge counts only if you reach it with hours in hand and its entrance is safe in the gale. Give a window a margin; after a gale let the sea go down. A motor yacht's limit is the head sea, not the wind.
What causes sea fog, and when does it clear?
Sea fog is advection fog: moist air cooled below its dew point by colder water, as over upwelling. It lasts until a cold front brings dry air.
Sea fog is advection fog: moist air cooled below its dew point by colder water, as over upwelling. It lasts until a cold front brings dry air: pick the wide, buoyed entrance. Sea smoke is steam where very cold air crosses warmer water.
Land on your right looking downwind (northern hemisphere): the wind converges along the coast and freshens; land on your left, it diverges and eases. Swell bends round a point into the bay behind it and reflects off cliffs: a confused sea. An east wind off the Sahara brings the calima, dust haze.
Over a warm sea thunderstorms peak at night; high CAPE in a GRIB warns of them. Plot a cell on radar like a ship: you cannot outrun it, so reef and pass astern; leave a waterspout's track at right angles. The gust front brings a sharp shift and double the wind. St Elmo's fire warns of a strike: crew off the rig, spare electronics in the oven; afterwards check the compass.
Questions from this topic, as the app asks them. Open one to see the answer.
You are two days out on an offshore passage, where the boat is drawn, south of the centre of this high. Where does the wind come from?
A. From about ENE
Round a high in the north air turns clockwise and slightly outwards: on its south side it blows from about ENE.
You are deciding whether to start a 3-day passage south across Biscay, and the forecaster's notes mention the jet stream. What does it tell you?
D. Lows will run fast along it, one after another
A strong straight jet steers and feeds surface lows: they form, deepen and race along beneath it in a quick series.
You are one day out on a passage south-west of Ireland and read the latest analysis. Where do you watch for a new low that could reach you?
B. At the triple point, where the fronts meet
A new (secondary) low often forms at the triple point of an old occluded system — here 150 M from your route.
You are measuring the isobar spacing over your track on this chart. What geostrophic wind do you read?
A. About 25 kn
The scale is for 4 hPa: two 2-hPa gaps make 120 M, and 120 M on the scale gives 25 kn.
You are planning a 3-day passage whose middle lies out of NAVTEX range. What do you rely on for the met warnings there?
A. SafetyNET — the METAREA warnings by satellite
SafetyNET (Inmarsat EGC) broadcasts the GMDSS high seas warnings and forecasts for each METAREA, far beyond NAVTEX range.
You will spend three hours of tonight's passage in this sea. What is the biggest wave you plan for?
D. About 6.5 m
3 h of 8-s waves is about 1350 waves, so the 1-in-1000 wave will come: 1.86 × 3.5 = 6.5 m.
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.