KNOWING How to read the clouds in the sky to tell what the weather will be like


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Air can contain a maximum amount of water vapor, which is the greater the higher its temperature. Since relative humidity is a measure of how close you are to this maximum amount, if the air contains (at a certain temperature), for example, 50 percent of the possible amount of vapor and the temperature begins to decrease, the relative humidity increases.

In other words, the more the temperature drops, the closer we get to the point where condensation, that is, the formation of water droplets or ice, occurs. As mentioned above, air that is warmer than the surrounding air tends to rise (as in a so-called “thermal,” used by gliding enthusiasts, or seagulls), expanding and taking on gradually lower pressure values than the initial values. During that expansion phase, the air cools, precisely causing the moisture to condense into droplets a few thousandths of a millimeter in diameter.

If condensation occurs at high altitudes, where temperatures can reach -60°C, small, off-white ice crystals form instead of droplets. This collection of ice and/or droplets forms clouds. They are empirically classified into ten kinds based on their heights each type gives a rough indication about the state of the weather.


Cirrus (in this case associated with thick cirrus)

Cirri: are the highest clouds (above 8000 meters) and take on the appearance of feathery white filaments or wisps being formed by thin ice crystals; if cirrus clouds are followed by denser cloud formations located at lower levels, they may indicate the arrival of bad weather (warm front), especially if accompanied by southerly winds (naturally in our seas).

Rolling cirrocumulus. The famous sheepskin sky. They indicate variable weather with warm front that may bring rain

Cirrocumulus: is the famous “sheep-like sky” formed by clouds in groups or in rows, indicating variable weather with a high probability that a warm front bringing rain is approaching.

Cirrostrates: form a thin veil that gives the sky a dazzling light gray color, tending to create a halo around the sun; they are indicative of weather deterioration.

Altostrata: these are also detectable by the gray veil similar to that created by cirrostrata, but more homogeneous and without the halo phenomenon.

Altocumulus with lenticular shape as flattened disks with grayish hues) indicate probable arrival of wind blows

Altocumulus: roundish expanses sometimes with distinct contours and shadows, other times like frayed cotton balls; if they fade during the hottest hours of the day the weather remains good stationary, if they take on a lenticular shape they are of particular importance to sailors because they indicate that a gust of wind is coming.

Stratocumulus: appear as broad, grayish, pebble- or roll-shaped masses; normally preceding and following a thunderstorm.

These cumulus clouds indicate a generally good weather condition, if they tend to dissolve as the sun sets, as the temperature decreases.

Cumulus: thick whitish clouds with mostly vertical development; if they turn dark at the bottom they announce the strong possibility that a thunderstorm is approaching; if, on the other hand, there are small isolated cumulus or in sparse clusters (cumulus humilis) generated by local convective motions of weak intensity, they usually indicate good weather. Cumulus is perhaps the most frequent cloud; it can occur in many guises, either solitària or associated with large systems.

Nembostratus: very dark rain clouds with jagged edges; if they appear frayed underneath and seem to touch the ground in a homogeneous flow, heavy rain falls there.

Cirrus and cirrostratus overlying altocumulus and stratocumulus formations

Layers: these are the lowest (about 450-500 meters) formed by a gray, thick homogeneous mantle accompanied by heavy rain.

Cumulonimbuses: clouds with strong vertical development (starting at 1,000 meters they can reach 13,000); often accompanied by violent thunderstorms, they are imposing in size with a characteristic anvil shape at the top, where they are formed mainly of ice and therefore take on the appearance of false cirrus. They are by far the clouds the navigator should be most afraid of, because of the gusts of strong wind they are often accompanied by. Finally, note that cumulonimbus precipitation is often associated with considerable electrical activity.



This article is excerpted from the valuable handbook “Sailing with a Small Crew”: how many times have you given up going out on your boat because you couldn’t find enough crew? From now on, the problem will no longer exist. Chapter by chapter, we will reveal the tricks to carry your boat practically on your own. From entering port to anchoring in the roadstead, from setting sails to preparing for the most challenging crossings, each topic is illustrated with photos and drawings, and we also tell you how to cope with common emergencies on board. Get ready to cast off your moorings!




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