Smooth sailing: how to predict the weather by reading the clouds

Clouds are not only beautiful to look at, but can be used as a means of predicting the weather. We notice them almost every time we gaze up at the sky, and they have been a source of fascination since the dawn of time. Artists have drawn inspiration from them, and provided others with the opportunity to gauge how the weather will develop. However, you don't need to be a meteorologist to read the clouds and learn what weather to expect. In fact, this knowledge is practically essential for sailors. So, how do you interpret the clouds?

10 basic cloud types and what they mean

Cloud formation is influenced by water and moisture levels in the atmosphere, condensation of water vapour, and air movement, all of which are studied in atmospheric physics and meteorology. But you don't need to be a graduate researcher to understand clouds. All you really need for sailing is some basic knowledge, the first thing being the 10 main types of clouds. Of course, if you'd really like to impress your crew, you can learn all about the 14 cloud species, 9 varieties of cloud and some 6 anomalies.

However, for routine weather forecasting, understanding the 10 basic types is sufficient enough. These types are categorised according to their altitude and defined in terms of how they can be identified by the naked eye. In this respect, there are 4 categories:

• High-level clouds (5–13 km): Cirrus (Ci), Cirrocumulus (Cc) and Cirrostratus (Cs)
• Mid-level clouds (2–7 km): Altocumulus (Ac) and Altostratus (As)
• Low-level clouds (max. 2 km): Cumulus (Cu), Stratocumulus (Sc) and Stratus (St)
• Clouds with vertical development: Cumulonimbus (Cb) and Nimbostratus (Ns)

Each of these clouds forms under distinct conditions, indicating the current state of the atmosphere as well as how the weather will develop. Every sailor should have at least a rudimentary understanding of meteorology, in case they ever find themselves unable to access various electronic weather forecasting tools. So, what do the clouds indicate?

Cirrus (Ci) — predict weather changes

Cirrus clouds are translucent, delicate clouds that are wispy, feathery and can look like tufts of hair (from the Latin word cirrus, meaning 'curling lock of hair'). They are usually narrow bands or patches in the uppermost levels of the sky, casting no shadow on the ground. 

Because they occur in regions of high altitude where temperatures are as low as -50 °C, they are largely made up of ice crystals. They can also be recognised by the fact that they produce beautiful halos (light phenomena in the atmosphere created by the sun or moon) due to the refraction and reflection of light.

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In the vast majority of cases, cirrus clouds do not bring precipitation with them. They are, nevertheless, an excellent way to gauge wind direction and wind changes. Cirrus clouds are made of ice crystals and when these crystals are large enough, they begin to fall to lower altitudes where they are carried by the wind. The resultant streaking of the cirrus clouds (cirrus uncinus) makes it possible to predict the direction of the wind and any changes to it.

However, reading cirrus clouds is a bit tricky as the way they develop can forecast both stable fine weather and a major deterioration. Generally speaking, as long as the cirrus remains fine and sparse, you can rely on the fact that the current weather will remain stable. If the cirrus clouds start to thicken, the weather is likely to worsen.

Cirrocumulus (Cc) — a sign of changeable weather

Despite being relatively uncommon, a cirrocumulus cloud is very easy to spot in the sky. It resembles a cirrus cloud, but unlike the thin long whisps, it takes the form of tiny regular patches or ribbed strips. This type of cloud also casts no shadow.

Cirrocumulus clouds can easily be confused with cirrus, cirrostratus and altocumulus clouds. Since it is a high altitude cloud, it is made up of ice crystals and has similar characteristics to cirrus clouds. If it occurs, you don't have to worry about possible precipitation, and can enjoy halo effects. 

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Cirrocumulus clouds can form in completely clear skies or develop from cirrus and cirrostratus clouds, or altocumulus clouds. Such a development suggests that something is happening in the atmosphere and you need to prepare for a change in the weather. Cirrocumulus clouds especially appear in areas where high terrain pushes moist air upwards and are a precursor to the coming of a cold front.

Cirrostratus (Cs) — the weather may worsen

Cirrostratus cloud is the third type of high-level cloud. Usually appearing as a white, transparent layer covering a large area of the sky, it lacks definition and can give a hazy look to the sky.

Similar to the other types of high-level cloud, as it is made up of ice crystals there is no risk of precipitation. Often covering the entire sky, it produces beautiful halo effects, perhaps the most beautiful you can observe from a sailboat. 

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Cirrostratus clouds are formed when lighter warmer air slides on top of heavier colder air. Cirrostratus usually precedes massive cloud cover and predicts worsening weather. Even if the skies appear relatively clear but you see halo phenomena, this is likely the result of cirrostratus and you'll need to prepare for deteriorating weather conditions.

Altocumulus (Ac) — get ready for a "blow dry"

Altocumulus clouds are some of the most visually interesting clouds, especially altocumulus lenticularis. They are easily recognisable as they are white, grey to bluish clouds or lens-shaped clouds tinged with red.

Since these are mid-level clouds, the clouds are no longer made up of just ice crystals, but also supercooled water droplets. They are denser and generally more compact than high-level clouds. As a result, these clouds do not cause halo phenomena, but they are still among the most beautiful clouds in the sky.

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Altocumulus clouds, especially altocumulus lenticularis and altocumulus stratiformis, are formed when the wind flows over a mountain barrier with sufficient humidity in the air. So if you are sailing around land (on the leeward side) where there are mountains, you can expect altocumulus to form. They are associated with a descending dry warm wind, sometimes referred to as "hairdryer winds". Be prepared for fairly strong gusts of wind.

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Altostratus (As) — rain or snow is coming

Altostratus does not look like a cloud, rather a grey, sometimes blue-coloured, translucent sheet covering the entire sky. The sun's rays can still penetrate it, but the sun looks like it's behind a translucent curtain or frosted glass.

Altostratus cloud is uniform, without structure, and at most may have a slightly fibrous or banded appearance. It is a mixed cloud composed of water droplets and ice crystals, making halo phenomena almost non-existent. The sun or moon appears as a disk of light, in some cases giving rise to what is called a corona.

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If the altostratus cloud is uniform and unchanging, you can expect stable weather. Wind direction can be gauged from any streaking that appears, as it is caused by the wind currents. Altostratus is typical of an approaching warm or occluded front, bringing precipitation. However, if cumulus or cumulonimbus clouds appear along with altostratus, prepare for a cold front that will bring cooler temperatures, precipitation and gusty winds.

Cumulus (Cu) — mostly clear with a chance of showers

Cumulus clouds are very easy to spot, standing out like sheep against a bright blue sky. They have a flat base and a cauliflower shape on top, which glow white, brightly illuminated by the sun. The individual clouds are clearly separated and float across the sky together in one direction.

Cumulus clouds are formed from the sun's rays heating the Earth's surface, with rising currents causing moisture to condense in the atmosphere. In some cases, cumulus clouds can even form when the wind rises over a mountain, when a mass of moist air reaches a greater height, expands and then cools down.

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Cumulus clouds are typical of stable sunny weather and there is no risk of precipitation or wind changes when they occur. However, as the wind can drop between clouds, if you are racing a yacht, you can expect nice gusts to give you an edge in these places. The moment to take notice is when the cumulus begins to grow. Cumulus congestus signals approaching precipitation and cumulonimbus signals a thunderstorm approaching.

Stratocumulus (Sc) — nothing to worry about

Stratocumulus clouds are very easily confused with cumulus clouds. But they can be distinguished by the fact that stratocumulus clouds have a very noticeable vertical development and have a slightly different shape, more resembling tiles, boulders or pebbles. 

Being low-level clouds, they are formed, similarly to cumulus clouds, by the rise of warm air into the atmosphere, where the moist air then condenses. They are less brilliant white than cumulus clouds and may appear grey, and have darker spots.

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If stratocumulus cloud begins to stretch across the sky, you can expect light precipitation and slight changes in wind strength. However, this type of cloud does not bring with it any significant changes that you would have to worry about at sea.

Stratus (St) — fog

Stratus clouds, similarly to altostratus clouds, are not clearly defined, rather a uniform layer of cloud. They are the lowest-lying cloud type and can appear like a fog, being normally found only a few metres above the ground.

These fairly uniform grey clouds lack any significant details and if the sun's rays do penetrate them, the sun appears as a brightly defined circle without a corona. Stratus cloud brings with it a light drizzle or very light snow.

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Stratus clouds are more common on land, coastlines and mountains. If this type of weather inversion weather catches you in port and you aren't experienced in navigating using instruments and markers, it is better to postpone your sailing trip until the afternoon hours when the stratus begins to dissolve and visibility improves substantially.

Multi-level clouds

High, medium and low-level clouds occur only within their particular bands. However, there are clouds that vertically cross these levels. Two of them are included in the basic meteorological classification:

• Cumulonimbus (Cb): a cloud that expands from low-level through mid-level to high — a sign of incoming precipitation and part of a cold front.

• Nimbostratus (Ns): a cloud that can grow from the mid-level to the low and high levels — it causes persistent and intense precipitation and is associated with all kinds of frontal systems.