Predicting the Weather With Clouds

23 November 2023 4 min

Sailors can use clouds to stay high, dry and ahead of the weather. Know your mare’s tail from your thunderhead?

Here’s a little poem about clouds that will serve you well:

“Cumulo cumulus, heaps & piles,

Strato Stratus , are all in a layer,

Nimbus are wet, Cirrus are dry,

These are the clouds that float in the sky.”

In the modern age, it’s all too easy to rely on technology taking the hard work out of sailing for us.

It is now common practice to have electric winches, self- furling sails, and all kinds of bells and whistles to help plot your route, check your depths and pinpoint the wind direction and speed for you.

The problem is that sometimes, technology fails, and when it does, the age old sailors’ instinct on passage planning and weather forecasting don’t just help make your charter more enjoyable, but will also keep you safe.

Have you ever tuned in to the latest weather broadcast, checked your smart phone, given the sky a cursory glance, shrugged your shoulders and said that everything looks fine, only to later be caught in torrential rain, stuck at the helm in your shorts and t-shirt with bimini still down while your first mate curses you for the lies you told them about the idyllic and predictable weather in your chosen cruising ground?

Clouds are one of the best indicators of what type of front is coming your way. As the sailor’s adage above suggests, by keeping a few things in mind you can quickly assess when the weather might be about to turn on you, and plan your next move, whether to head for a bolt hole or carry on cruising.

Here is Sunsail’s simple and useful guide to clouds. It should be useful to newer sailors and old salts alike.

What you need to remember:

High level clouds (20,000ft)

Cirrus clouds

So you’re sailing along under clear blue skies. You glance to port, and see no clouds; you swivel to starboard, nada. Then, you look over the bow once more, and in the far distance you see long, wispy streaks of cloud high in the sky. These, my friend, are Cirrus clouds, sometimes known as mare‘s tails.

Though they won’t leak on you, they indicate the arrival of a potentially wet warm front, so take note of their speed and direction. If they are heading to the east, you’re in luck, and fair weather is on its way. If not, you may be in for some rain and strong wind before things brighten up.

Cirrocumulus

These are rare, honeycomb-shaped clouds made up of tiny cloudlets grouped together. This type of cloud often produces what is known as mackerel sky. Beware of the old saying; “mackerel sky and mare’s tails make tall ships carry low sails”. If they look a little low and have lots of ripples, you could be in for some stormy weather.

Cirrostratus

A long, thin blanket of cloud, composed mostly of ice crystals, they often drape the sun or moon in a hazy halo of refracted light. They are hard to detect, but if you’re sailing around and the sky is cloaked in cirrostratus, there should be a warm front on its way.

Mid-level clouds (7-20,000ft)

Altocumulus Altostratus

Generally a bad omen for sailors, these bluish-grey formations may be fluffy, streaky or flat, but they’re not very friendly! They reduce the sun to a light grey area in the sky, like a photographer’s diffuser box. If you wake up feeling a little muggy and see the fluffy kind, you can expect heavy weather to force you into port by lunchtime. Flatter patterns mark the imminent arrival of a warm, and likely wet, front.

Low-Level Clouds, (below 7,000 feet)

Cumulus

Everyone’s favourite. Cute and fluffy, these are clouds you can stare at and spot shapes in. Their flat bottoms and fluffy tops are a sign of fair weather, and any rain they drop will be short, sweet and light. If they form over land near the coast, especially on a warm day, they may herald the arrival of a sea breeze in the afternoon.

Cumulonimbus

If you see our nice white, fluffy friends begin to get taller and taller; you need to move fast, as your fair weather ally could be turning into a ‘thunderhead’ (surely the coolest of all cloud nicknames). If the bottom starts to go dark, you’ve been staring at it too long – these clouds precede some of the world’s most extreme weather, including hurricanes, tornadoes, snow and hail.

Stratus Clouds

Huge grey blankets that hang in the sky, the multi-layered stratus may look ominous, but they’re pretty harmless, producing little if any rain. They are essentially fog in its rightful place, but sometimes they settle down to ground level to cause a bit of mischievous pea soup – expect dry, steady temperatures with these depressives at the helm.

Strato-Cumulus

A confused mass of lumpy cumulus, packed into tight groups, these clouds can’t decide whether they want to be fluffy fair weather champions or mundane stratus .Watch out for them, because they have a habit of giving way to Nimbostratus, the ominous, sky-dominating, sun-blocking rollers that bring heavy, lasting rain.

What you need to do

We always advise you to keep a weather eye out, make use of local information, and to plan your passages carefully to include contingencies. Forecasters aren’t always right, and any self-respecting sailor knows that the sea and weather go hand in hand, and knows how quickly conditions can change in any sailing ground.

So it could be well worth your time to pay attention to the type of clouds you can see from time to time. At the very least, it might cut down the amount of times in your sailing life that you get hit by torrential rain with the bimini down and your shorts and t-shirt on.

For anyone wishing to explore weather forecasting in greater detail visit the Royal Meteorological Society website https://www.rmets.org/resource/how-can-you-make-high-quality-weather-observations

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