Basic Meteorology
Weather is the single biggest theory gap between your Sail Canada delivery and the IYT Bareboat Skipper standard — and a likely focus in Jon's knowledge review. This module covers the full IYT Module 16 syllabus to Working depth, with interactive visuals. Work top to bottom, then take the knowledge check.
Sources of Weather Information
Meteorology is the study of weather — driven by the movement and transfer of energy as air moves through the atmosphere. As a skipper you don't forecast from scratch; you gather, cross-check and interpret several sources, then watch the sky to confirm or contradict them.
- National met office / Environment Canada marine forecast
- NAVTEX & VHF continuous marine broadcast (WX channels)
- GRIB / model data via app (Windy, PredictWind)
- Surface pressure (synoptic) charts — isobars & fronts
- Barometer trend (falling = deteriorating)
- Cloud sequence & sky changes
- Wind shift in direction or strength
- Sea state and swell building
Pressure Systems & the Coriolis Effect
Weather forms in the troposphere (the lowest ~11 miles), driven by the sun heating the surface unevenly and the rotation of the Earth. Warm air rises, cooler air sinks, and pressure differences set the air in motion. Meteorologists measure pressure in millibars (hPa) and join points of equal pressure with lines called isobars — the contours you see on a synoptic chart.
The Earth's rotation bends that moving air — the Coriolis effect — so the wind spirals around pressure centres rather than flowing straight from high to low:
LOW (a "depression")
Air spirals counter-clockwise in the Northern Hemisphere (clockwise in the South), rising in the centre → cloud, rain and stronger, shifting winds. Tightly packed isobars = strong wind.
HIGH (an "anticyclone")
Air spirals clockwise in the North (counter-clockwise in the South), sinking in the centre → settled, often clear weather and lighter winds.
Air Masses & Fronts
An air mass is a large body of air with roughly uniform temperature and humidity, taking its character from where it formed (maritime/continental, tropical/polar). A front is the boundary where two air masses meet — and most of our changeable weather lives along fronts.
Warm air rides up over cold. Cloud lowers & thickens over hours (cirrus → stratus), steady rain, then warmer, often murky/foggy air. Gentle slope, slow approach.
Dense cold air wedges under warm, forcing it up sharply. Towering cumulonimbus, squally heavy rain, gusty wind shift — then clearing, cooler, brighter.
A faster cold front catches the warm front and lifts the warm air clear of the surface — mixed cloud and rain, the system maturing and decaying.
Sea Breeze & Land Breeze
These are thermal (local) winds caused by land and water heating and cooling at different rates. They're hugely relevant in PEC and Caribbean charter alike — a sea breeze can fill in on an otherwise calm afternoon, or reinforce a gradient wind into something stronger.
Land heats faster than water; warm air rises over the land, cooler sea air flows in to replace it. Begins ~mid-morning, peaks ~14:00, dies by evening. Can reach Force 4, extends up to ~30 NM offshore. Won't develop if the gradient wind is ≳25 kn.
Land cools faster than the sea; cool dense air drains off the land out to sea. Weaker than the sea breeze and felt a shorter distance offshore. Strongest in the pre-dawn hours.
Katabatic & Anabatic Winds
Cold, dense air flows down from high ground into valleys and bays, often at night. Can funnel and accelerate violently — the Mistral down the Rhône valley reaches 80 mph. Anchoring hazard: in a bay ringed by high mountains, a katabatic gust can hit hard and drag your anchor with no warning from the open-water forecast.
Air warmed by the sun rises up mountain slopes during the day. Generally light and "more useful to hang-gliders than sailors," but worth naming for the exam as the opposite of katabatic.
The Beaufort Wind Scale
Marine forecasts are issued in Beaufort force, not knots. The scale maps wind speed to the observed sea state — learn to read the water and you can estimate wind without an instrument. Drag the dial and watch the sea build.
Clouds — Reading the Sky
Clouds are rising columns of moist air that have cooled to their dew point, condensing moisture into visible droplets. Three base types combine by height into the cloud family. The sequence matters more than any single cloud: high wispy cirrus thickening and lowering is the calling-card of an approaching warm front.
Hurricanes, Typhoons & Cyclones
Same storm, three regional names: hurricane (Atlantic / E. Pacific), typhoon (W. Pacific), cyclone (Indian Ocean / Australia). All are intense tropical low-pressure systems that grow from a tropical depression → tropical storm → hurricane as sustained winds increase.
Warm ocean (≥ ~26.5°C), moist rising air releasing latent heat, and low wind shear. The system organises around a calm eye, ringed by the eyewall — the most violent winds and rain.
Far beyond a Bareboat certificate's "fair weather, ≤20 NM offshore" limits. The competency is avoidance: know the season (Atlantic ~Jun–Nov), watch the National Hurricane Center, and never sail into the forecast track. Charter bases will require a hurricane plan.
Fog & Restricted Visibility
Fog is simply cloud at the surface — air cooled to its dew point so moisture condenses into water droplets. Meteorologists define fog as visibility under 1 km; below ~1 NM it triggers your COLREGs restricted-visibility obligations (Rule 19): proceed at a safe speed, sound signals, post a lookout, and use radar/AIS if fitted. IYT recognises four types:
Warm, moist air flows over a cold sea surface and is chilled to condensation. Most likely in late spring / early summer before the sea has warmed. The offshore sailor's main fog — can persist for days until the air mass changes.
Forms over land on clear, calm nights as the ground radiates away its daytime heat; a light breeze deepens it. Common in winter anticyclones; can drift out to sea at dawn and burns off as the sun warms the ground.
Forms along a front where two air masses of different temperature meet. If both are moist, fog develops at the boundary — usually a band less than ~50 miles wide.
Extremely cold air passing over warmer water — the water appears to "steam." Seen in high latitudes and cold snaps.
Thunderstorms, Lightning & After the Storm
Thunderstorms form when moist air rises into cold air and the water vapour condenses into towering cumulonimbus. IYT splits them into two:
Last about an hour; typical of warm summer afternoons. Localised but still pack gusts and lightning.
Can produce tennis-ball hail, flash floods, squall lines, strong gust fronts and microbursts. The squall line can build a dangerous sea state in minutes and swamp a boat if you're caught unprepared.
Lightning at sea
Lightning is an electric current. Inside the cloud, colliding ice particles build a charge — light positive charges at the top, heavy negative charges at the base. When the difference grows large enough a giant spark jumps. On the ground (or sea) the positive charge gathers around anything that "sticks up" — a tree, a person, or the mast of a sailboat.
Cumulus going hard and vertical, an anvil top, darkening base, and a sudden cool gust = squall imminent. Watch the radar / Windy lightning layer.
Reef early or drop sail, secure hatches, crew below or clipped in & in PFDs, note your position, stand off any lee shore.
Stay low, avoid touching metal/shrouds, keep crew out of the water and clear of the mast base.
After a storm
Calm often follows — but a large sudden rainfall washes debris, branches and vegetation down rivers and into estuaries: a real hazard to navigation. Creeks rise fast, the water turns muddy and murky (a person overboard without a lifejacket may be impossible to see), and the downstream flow accelerates. Leave navigating up-river until the flood has died down.
Meteorology — Knowledge Check
13 questions at IYT exam standard (75% to pass). Pick an answer to see the explanation instantly.