Intro to Clouds

Clouds, how do they work?

They’re all around us, a daily companion, perhaps relegated to the background. As the National Weather Service states, “They can weigh tens of millions of tons yet float in the atmosphere.” Do you make time for miracles? If, as I do, you make time for miracles, let’s discern a thing or two about these ephemeral denizens.

The NWS defines a cloud thusly: “They are the visible aggregate of minute particles of water and/or ice which form when water vapor condenses.”

But clouds are not comprised only of water. They actually require two parts for formation: water and “cloud condensation nuclei.” Water molecules in the atmosphere are too small to bond together to form droplets. They need a “nucleus” around which to glob. These nuclei are usually things such as soil, dust, salt crystals, or particles from volcanoes and fires. Despite the two ingredients, a cloud is still nearly all water. The nuclei end up being about 1/100th the size of the overall droplet.

Musing on clouds - Peanuts by Charles Schulz

The presence of water and a nucleus does not, however, automatically equal a cloud. The temperature needs to be below what is called the saturation point. The point of saturation is the magical spot where evaporation equals condensation. If the temperature is below this threshold, also called the dew point, clouds form; if the temperature is above it, too much water evaporates to coalesce around the nucleus. The most common spot for this condensation to occur is up in the sky, as higher elevations are cooler.

The process of condensation and evaporation is constantly occurring, however, so droplets are always being formed and de-formed, causing the shifting nature of clouds.

Now that we know how these miracles become, let’s take a look at the basic types of clouds.


Cirrus – From the Latin word cirro, which means “curl of hair.” High, wispy, made of ice crystals. Looks like…curly hair.

Cumulus – Your cotton-ball or pillow type of cloud; usually detached with sharp outlines. The Latin word means “heap” or “accumulation.”

Stratus – From the Latin word for “layer”; They stretch across the sky like a blanket. 

Nimbus – From the Latin word for “rain,” this category kind of combines the other three and is now generally considered an outdated term. Most precipitation comes from nimbo-form clouds. 

Of course, as we look into the sky we see more than just four core types of clouds. They can morph between the categories or have features that combine two or more of the types. These days we recognize 10 basic cloud types, according to the World Meteorological Organization’s International Cloud Atlas. Generally, they are divided by height.

The following images and definitions are all from the National Weather Service:


Cirrus – Detached clouds in the form of white, delicate filaments, mostly white patches or narrow bands. They may have a fibrous (hair-like) and/or silky sheen appearance.




Cirrocumulus – Thin, white patch, sheet, or layers of clouds without shading. They are composed of very small elements in the form of more or less regularly arranged grains or ripples.



Cirrostratus  Transparent, whitish veil clouds with a fibrous (hair-like) or smooth appearance. A sheet of cirrostratus which is very extensive, nearly always ends by covering the whole sky.



Altocumulus – White and/or gray patch, sheet or layered clouds, generally composed of laminae (plates), rounded masses or rolls. They may be partly fibrous or diffuse and may or may not be merged.



Altostratus – Gray or bluish cloud sheets or layers of striated or fibrous clouds that totally or partially covers the sky. They are thin enough to regularly reveal the sun as if seen through ground glass.



Nimbostratus – Resulting from thickening Altostratus, This is a dark gray cloud layer diffused by falling rain or snow. It is thick enough throughout to blot out the sun.



Cumulus – Detached, generally dense clouds and with sharp outlines that develop vertically in the form of rising mounds, domes or towers with bulging upper parts often resembling a cauliflower.



Cumulonimbus – The thunderstorm cloud, this is a heavy and dense cloud in the form of a mountain or huge tower. The upper portion is usually smoothed, fibrous or striated and nearly always flattened in the shape of an anvil or vast plume.



Stratocumulus – Gray or whitish patch, sheet, or layered clouds which almost always have dark tessellations (honeycomb appearance), rounded masses or rolls. 




Stratus – A generally gray cloud layer with a uniform base which may, if thick enough, produce drizzle, ice prisms, or snow grains. When the sun is visible through this cloud, its outline is clearly discernible.

The above information feels like just the tip of the cloud when it comes to this part of our world’s atmosphere, but the article is starting to get lengthy, so we’ll have to leave it here and pick it up in our next class (figuratively). Even this brief introduction to the world of clouds has me agreeing with a quote from The Cloudspotter’s Guide by Gavin Pretor-Pinney:

“We pledge to fight ‘blue-sky thinking’ wherever we find it. Life would be dull if we had to look up at cloudless monotony day after day.”

Check out the links below for some really cool cloud maps.

Further Reading and Exploration

International Cloud Atlas – World Meteorological Organization

Current Global Map of Total Cloud Water

Monthly Map of Global Cloud Cover – NASA’s Earth Observatory

Cloud Appreciation Society

The Clouds Outside My Window – by John “Dr. Lightning” Jensenius (free pdf book)

The Cloudspotter’s Guide: The Science, History, and Culture of Clouds – by Gavin Pretor-Pinney (e-book version)

Cloud Atlas – by David Mitchell (e-book version)

Cloud Atlas Bluray – directed by The Wachowskis, starring Halle Berry and Tom Hanks

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