The 10 Basic Types of Clouds

How to Recognize Clouds in the Sky

cumulus and sun rays

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According to the World Meteorological Organization's International Cloud Atlas, over 100 types of clouds exist. But although there are so many variations, each one can be divided into one of ten basic types depending on its general shape and height in the sky. Divided by their height in the sky the ten types of clouds are:

  • low level clouds (cumulusstratusstratocumulus) that lie below 6,500 feet;
  • middle clouds (altocumulusnimbostratusaltostratus) that form between 6,500 and 20,000 feet;
  • high level clouds (cirruscirrocumuluscirrostratus) that form above 20,000 feet;
  • and cumulonimbus which towers across the low, middle, and upper atmosphere.

Whether you're interested in cloud watching or are just curious to know what clouds are overhead, read on to find out how to recognize them and what type of weather you can expect from each.

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cumulus sky open road

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Cumulus clouds are the clouds you learned to draw at an early age and that serve as the symbol of all clouds (much like the snowflake symbolizes winter). Their tops are rounded, puffy, and a brilliant white when sunlit, while their bottoms are flat and relatively dark.

When You'll See It

Cumulus develops on clear, sunny days when the sun heats the ground directly below (diurnal convection). This is where it gets its nickname "fair weather" cumulus. It appears in late morning, grows, then disappears towards evening.

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stratus clouds

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Stratus hang low in the sky as a flat, featureless, uniform layer of grayish cloud. It resembles fog that hugs the horizon (instead of the ground).

When You'll See It

Stratus are seen on dreary overcast days and are associated with light mist or drizzle.

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stratocumulus sky desert

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If you took an imaginary knife and spread cumulus clouds together across the sky, but not into a smooth layer (like stratus) you'd get stratocumulus—low, puffy, grayish or whitish clouds that occur in patches with blue sky visible in-between. When viewed from underneath, stratocumulus have a dark honeycomb appearance. 

When You'll See It

You're likely to see stratocumulus on mostly cloudy days. They form when there's weak convection in the atmosphere.

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altocumulus and the moon

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Altocumulus clouds are the most common clouds of the middle atmosphere. You'll recognize them as white or gray patches that dot the sky in large rounded masses or are aligned in parallel bands. They look like the wool of sheep or scales of mackerel fish—hence their nicknames "sheep backs" and "mackerel skies."

Telling Altocumulus and Stratocumulus Apart

Altocumulus and stratocumulus are often mistaken. Besides altocumulus being higher up in the sky, another way to tell them apart is by the size of their individual cloud mounds. Place your hand up to the sky and in the direction of the cloud; if the mound is the size of your thumb, it's altocumulus. (If it's closer to fist-size, it's probably stratocumulus.)

When You'll See It

Altocumulus are often spotted on warm and humid mornings, especially during summer. They can signal thunderstorms to come later in the day. You may also see them out ahead of cold fronts, in which case they signal the onset of cooler temperatures.

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nimbostratus rain clouds

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Nimbostratus clouds cover the sky in a dark gray layer. They can extend from the low and middle layers of the atmosphere and are thick enough to blot out the sun.

When You'll See It

Nimbostratus is the quintessential rain cloud. You'll see it whenever steady rain or snow is falling (or is forecast to fall) over a widespread area.

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altostratus Boeri Lake

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Altostratus appear as gray or bluish-gray sheets of cloud that partially or totally cover the sky at mid-levels. Even though they cover the sky, you can typically still see the sun as a dimly lit disk behind them, but not enough light shines through to cast shadows on the ground.

When You'll See It

Altostratus tend to form ahead of a warm or occluded front. It can also occur together with cumulus at a cold front. 

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cirrocumulus over mountains

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Cirrocumulus clouds are small, white patches of clouds often arranged in rows that live at high altitudes and are made of ice crystals. Called "cloudlets," the individual cloud mounds of cirrocumulus are much smaller than that of altocumulus and stratocumulus, and often look like grains.

When You'll See It

Cirrocumulus clouds are rare and relatively short-lived, but you'll see them convection.

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cirrostratus sky

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Cirrostratus clouds are transparent, whitish clouds that veil or cover nearly the entire sky. A dead giveaway to distinguishing cirrostratus is to look for a "halo" (a ring or circle of light) around the sun or moon.

When You'll See It

Cirrostratus indicate a large amount of moisture is present in the upper atmosphere. They're also generally associated with approaching warm fronts.

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cirrus sky
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Like their name (which is Latin for "curl of hair") suggests, cirrus are thin, white, wispy strands of clouds that streaks across the sky. Because cirrus clouds above 20,000 ft (6000 m)—an altitude where low temperatures and low water vapor exist—it is made up of tiny ice crystals rather than water droplets. mare's tails

When You'll See It

Cirrus typically occur in fair weather. They can also form out ahead of warm fronts and large-scale storms like nor'easters, tropical seeing them can also indicate storms may be coming soon!

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Cumulonimbus clouds are one of the few clouds that span the low, middle, and high layers. They resemble the cumulus clouds from which they grow, except they rise into towers with bulging upper portions that look like cauliflower. Cumulonimbus cloud tops are usually always flattened in the shape of an anvil or plume. Their bottoms are often hazy and dark. 

When You'll See It

Cumulonimbus clouds are thunderstorm clouds, so if you see one you can be sure there's a nearby threat of severe weather (short but heavy periods of rainfall, hail, and possibly even tornadoes).