Weather decides whether you sail, where to and under how much canvas. Read the pressure pattern, the sky and the forecast, know the local winds, and you are in harbour before the trouble starts.
Which way does the wind blow around a low-pressure system in the northern hemisphere?
Anticlockwise round a low and clockwise round a high, along the isobars and angled 10–15° in towards the low over the sea; the closer the isobars, the stronger the wind. With your back to the wind, the low is on your left.
Wind follows the isobars: in the northern hemisphere anticlockwise round a low and clockwise round a high, angled 10–15° in towards the low over the sea. The closer the isobars, the stronger the wind.
Buys Ballot's law: stand with your back to the wind and the low is on your left.
One barometer reading says little; watch the trend. A fall of 6 hPa in three hours warns of strong wind; so does a fast rise after a front.
How does the wind change as a depression passes north of me?
It veers, clockwise from SW to NW, as a warm front, a mild warm sector and then a cold front go through; the cold front brings squalls, heavy rain and a sharp veer to NW. Passing south, the wind backs.
As a low passes north of you the wind veers, clockwise from SW to NW; passing south, it backs, anticlockwise. It brings a warm front, a mild warm sector, then a cold front; an occlusion is both in one.
| Stage | Signs |
|---|---|
| Warm front coming | High cloud thickening, a halo, falling glass, rain |
| Warm sector | Wind veers, drizzle, milder, glass steadies |
| Cold front | Dark towering cloud, squalls, heavy rain, sharp veer to NW |
| Behind it | Gusty NW, showers, clear air, glass rising fast |
Small flat cumulus that vanish by evening: settled weather. Cumulus towering by mid-morning: thunderstorms by afternoon. Lens-shaped clouds over the peaks: strong wind aloft, hard gusts in their lee.
Should I reef for the forecast mean wind or for the gusts?
Reef for the gusts, and early: the force is named from the mean wind, but gusts overpower the boat, and under high land they can be half as strong again. Double the wind is four times the load.
| Force | Knots | Sea |
|---|---|---|
| 4 | 11–16 | Fairly frequent white horses |
| 5 | 17–21 | Many white horses |
| 6 | 22–27 | Large waves, spray |
| 7 | 28–33 | Sea heaps up, foam in streaks |
| 8 | 34–40 | Gale |
| 9 | 41–47 | Strong gale |
The force is named from the mean wind, but gusts overpower the boat: reef for them, and early. Under high land they can be half as strong again. Double the wind is four times the load. In an app, 1 m/s is about 2 knots.
You feel the apparent wind. Running, boat speed is taken off the true wind; beating, most of it is added. Reef before you turn upwind.
Can I trust a GRIB file or weather app, or do I need an official forecast?
Go by the official forecast: Navtex, coast radio, the harbour office or a national service. A GRIB or app is raw model output that misses funnelling, gusts and thunderstorms; an official warning outranks any app.
Official forecasts come by Navtex, by coast radio (announced on VHF channel 16, read on a working channel), from the harbour office and from national services such as Poseidon in Greece or DHMZ in Croatia. Check the time of issue.
A GRIB or an app is raw model output: it misses funnelling, gusts and thunderstorms, and days ahead it is only a trend. If models disagree, plan for the strongest. An official warning outranks any app; a port-police departure ban is binding.
| Term | Means |
|---|---|
| Imminent · soon · later | Within 6 h · in 6–12 h · after 12 h |
| Veering · backing | Shifting clockwise · anticlockwise |
| Visibility good · moderate · poor · fog | Over 5 M · 2–5 M · 1,000 m–2 M · under 1,000 m |
| Sea slight · moderate · rough | 0.5–1.25 m · 1.25–2.5 m · 2.5–4 m |
| Locally | In channels and off capes |
Why does the wind get up every afternoon near the coast on a hot day?
That is a sea breeze: on a hot sunny day the land draws wind in from the sea, often 15 knots by afternoon, more where the gradient wind blows the same way. It is gone at sunset.
On a hot sunny day the land draws in a sea breeze: often 15 knots by afternoon, more where a gradient wind blows the same way, gone at sunset. After a clear night a light land breeze blows offshore until sunrise.
High land bends the wind: it accelerates through straits and round capes, falls in gusts in the lee of a high island, and on clear nights pours out of valleys as a cold katabatic wind.
Waves grow with fetch, the open water the wind has crossed: the sea is smallest under the windward shore. Wind against current makes short, steep seas. Swell outruns its weather: it warns of a distant system and can ruin a calm anchorage.
When does the Meltemi blow in the Aegean, and how strong does it get?
The Meltemi is the Aegean's summer northerly, dry and clear, force 5 to 7. It is lightest at dawn and strongest in the afternoon, so sail the upwind legs early in the day.
The Meltemi is the Aegean's summer northerly: dry, clear, force 5 to 7, between a high over the Balkans and a low over Turkey. It blows NE in the north, N in the Cyclades and NW in the Dodecanese, lightest at dawn, strongest in the afternoon.
| Wind | From | Where and what |
|---|---|---|
| Bora | NE | Adriatic: cold, violent gusts; cloud cap on the mountains |
| Jugo, sirocco | SE | Slow build, rain, heavy sea |
| Maestral | NW | Adriatic afternoon breeze, fair weather |
| Mistral | NW | Gulf of Lion: clear-sky gale |
| Tramontana | N | Balearics, Gulf of Lion |
| Libeccio | SW | Corsica's west coast: heavy sea |
| Gregale | NE | Malta |
| Levante · Poniente | E · W | Gibraltar |
Plan round it: upwind legs early in the day and early in the week; anchor for tonight's wind and swell; with a gale warning or a green crew, wait.
What should I do on a sailboat when a thunderstorm squall is coming?
Reef hard or drop the sails, close the hatches and put lifejackets on: ahead of a thunderstorm the wind drops, then 40 knots can hit. In lightning, send the crew below, clear of the mast.
A thunderstorm cloud, towering with an anvil top, pushes a cold gust front ahead: the wind drops, then 40 knots can hit from the storm. A low roll of cloud over white water is minutes away.
Radiation fog forms over land on clear, calm nights and burns off by mid-morning. Sea fog (advection fog) forms when warm moist air meets a colder sea, mostly in spring; wind does not clear it.
Questions from this topic, as the app asks them. Open one to see the answer.
The forecast chart shows isobars packed tightly over your area tomorrow. What do you expect?
A. Strong wind — stay in port or reef early
Close isobars = steep pressure gradient = strong wind.
You have watched the weather for six hours. What has just happened?
B. A warm front has passed — warm sector now
Warm front passing: wind veers, rain eases to drizzle, temperature rises, glass steadies.
You read the wind instruments before leaving the anchorage. What force is the mean wind?
A. Force 6
F6 is 22–27 kn; the force is named from the mean wind, not the gusts.
You read this forecast before a passage. What does it tell you to expect?
D. A clockwise shift to west, F5, after 12 h
Veering = clockwise; "later" = more than 12 hours from the time of issue.
A hot sunny morning, calm. By 14:00 a steady 15-knot onshore wind has set in. What is it?
A. A sea breeze — it will die at sunset
Warm land draws air in from the sea on sunny afternoons.
Aegean, July. The forecast shows this pattern for a week. What wind?
B. The Meltemi — strong dry northerly, F5–7, strongest in the afternoon
High over the Balkans, low over Turkey: the Meltemi.
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.