Clouds are closing in, ominous and black, while a line squall races towards your boat.
Might not be the best time to bring it up, but with better observational skills and a bit of background you could have been watching the spectacle from the safety of your berth.
But there is a silver lining: clouds offer a wealth of information.
As John Rousmaniere writes in the Annapolis Book of Seamanship: Clouds are the faces of weather.
Lesson one: clouds are condensed water droplets. As sun-heated air ascends, it takes water molecules (in gas form) along for the ride. High enough to achieve a relative humidity of 100% and you get clouds, ice crystals or fog.
When those clouds grow, the water molecules bond. Bond enough together and gravity takes over. Translation: rain.
Lesson two: clouds formed by air movement across surfaces of differing temperatures are called convection clouds. Maybe it’s a warm air mass passing over cold water, maybe cold air above sun-warmed land.
Frontal clouds are formed when the leading edge of a new air mass – whether hot or cold – forces air upward. Clouds formed this way have the greatest impact on boaters.
Orographic clouds – those formed over land or hanging out on mountain peaks (think Nevis and Saba) – can even be a navigational aid for blue water cruisers, for piled cumulus clouds on a clear day surely shelter land beneath.
“Understanding clouds can be one of your most important tools on a boat,” says WeatherNetwork meteorologist and erstwhile sailing instructor Ron Bianchi. “They’re often your first signal of changing weather. And it’s those weather changes that are going to impact on you.”
Lesson three: cloud names can be confusing. They can be identified by shape or height – sometimes by both at the same time.
Cirrus clouds are curly and wispy. They are thin and may have mares’ tails. They generally signal fair weather, though a whole series looking like scales on the bellies of mackerel is early warning of impending storm.
Remember this one? “Mackerel skies and mares’ tails make tall ships carry low sails.”
Stratus clouds are flat and sometimes layered. “Stratus clouds are normally associated with a warm front,” says Bianchi, “so they aren’t as ominous as cold front clouds.”
Cumulus clouds look like gigantic cotton balls.
Lesson four: more confusion.
Clouds higher than 20,000 feet are identified with the prefix ‘cirro’, as in cirrostratus or cirrocumulus. Alto clouds hang out between 6000 and 20,000 feet. No prefix? Then they’re low-lying.
Cirrostratus clouds are also high in the sky but they tend to lie in sheets or layers. They suggest a closer warm front than do cirrus. Altostratus are thicker, darker and lower but have similar shapes. They promise rain.
Fair weather cumuli appear relatively high so they get called altocumulus. They’ll either dissipate or transform into cumulonimbus.
Dark, tightly-packed puffy structures, cumulonimbus can tower to 20,000 feet. They signal an approaching cold front.
Stratus – or layered – clouds aren’t particularly significant but they can disguise or hide approaching storms. Nimbostratus – the ‘nimbo’ attached to clouds means that it’s raining – are flat, heavy and low-lying.
Thoroughly confused? Not to worry.
It’s more important to take Bianchi’s advice to heart. “Cloud changes are the most important indicator of changing weather patterns,” he says. “And the rate of change is crucial.”
Bianchi also suggests that cumulus clouds are the most significant – and dangerous – when it comes to mariners.
“Watch for any vertical growth in cumulus clouds,” says Bianchi. “They can actually look like they are boiling over in a pot. And an anvil top – flat and wide – brings nothing but evil.”
Cloud color is a good indicator of deteriorating weather. “Dark green or black bases in clouds are never a good sign,” says Bianchi.
Rousmaniere points out that high, isolated cloud formations signal fair weather, while growing formations and lowering ceilings indicate bad weather. “The weather will change,” writes Rousmaniere, “if cloud color, shape and size change.” Vertical growth is your biggest concern.
High-level cirrus clouds signal fair stable weather, but are also an early precursor of deteriorating weather.
Similarly, stratus clouds signal stable weather, but lowering stratus are a bad sign because they can cause reduced visibility in the form of drizzle or fog.
Cumulonimbus clouds provide a wealth of news to the observant boater, most of it bad. A smattering of cumulus can change to a full-blown storm in fifteen minutes. Watch the anvil at the top of thunderheads. It leads ahead of the storm like a wedge and offers clues as to size, growth rate, organization and structure.
While it’s tough to beat weather on a sailboat you can be prepared. “Watch the clouds and read them as carefully as you’d read your charts,” advises Bianchi.
And remember that every cloud has a silver lining – a pirate’s ransom of information.
Information worth its weight in gold.
Mark Stevens is an award-winning travel writer whose specialties include Canada, the Caribbean and boating.