What Are Clouds Made Of?


When we think of clouds we think of those white cotton ball masses in the air. What we don’t really think about is what are clouds made of. We all know about the water cycle in some form. We know that clouds are created from the water that evaporates from various lakes, rivers, and oceans. We also know that at some time this evaporated water becomes rain and starts the cycle all over again.

However there are important questions about clouds we overlook. First, how are clouds visible if water vapor is normally supposed to be invisible like air or at least dissipate quickly after the first gush of steam? Second, why do clouds last so long in their different forms? Finally, what gives clouds their white or grey colors? As you can see there is a lot we take for granted in our understanding of clouds and how they are formed.

We know that clouds are made of water vapor, what we don’t know or at least forget is the important role that condensation plays in making clouds visible. For the most part water vapor is invisible. This is proven by the fact that the air we breathe regularly has some water vapor as part of its composition. However we don’t see it since its apart of the air. Condensation is what makes water vapor visible.

Basically high temperatures excite water molecules until they change from a liquid state to a gaseous one. However lower temperatures can cause enough water vapor to condense back into liquid form. This small amount stays as very small droplets that can stay suspended in the air mostly thanks to small dust particles that they attach themselves to.

It is pretty much the same way you see small bits of glitter suspended in clear glue. The drops are small enough to stay trapped in the air until condensation reaches a point of no return making rain. One result of this is that light becomes reflected and refracted. This is what makes clouds visible.

Now if you think about it we also just answered the second question about why clouds last so long. You may understand the first explanation because you can see your breath on a cold day. However after a while depending on the weather you notice that later in the day you can no longer see your breath. Clouds are visible because of colder temperatures in the upper atmosphere.

You have to remember that in the upper reaches of the atmosphere that the temperatures are much colder. This means that water vapor once condensed can no longer return fully to its gas state. Since temperatures don’t change in this region clouds are able to keep shape longer.

Finally, clouds have color. Some are white, some are grey, and in special circumstances such as major storms can have weird colors like green or red. This goes back to refraction. Most color that we can see is visible because are eyes perceive how objects absorb or reflect certain wavelengths of light. The white colors of clouds come from the condensed water vapor having a high reflective quality.

When all wavelengths of light are reflected back you see white. The grey color comes from seeing clouds from beneath. White clouds are white if you notice, on sunny days. This is because you can see the sunlight directly hitting them and see that light almost completely reflected back. On cloudy days most sunlight is blocked by the translucent and refractive quality of cloud cover. This makes clouds appear darker in color as part of the light has been uniformly absorbed.

We have written many articles about clouds for Universe Today. Here’s an article about the types of clouds, and here’s an article about cirrocumulus clouds.

If you’d like more info on clouds, check out an article aboutClouds. And here’s a link to NASA Spaceplace Page about Clouds.

We’ve also recorded an episode of Astronomy Cast all about the Atmosphere. Listen here, Episode 151: Atmospheres.

Types of Clouds

Clouds around Olympus Mons


There are numerous types of clouds, but they are generally classified differently. Some organizations classify them into two main groups while others organize them into three or four groups or even more. The National Weather Service divides clouds into three groups – low, medium, and high clouds.  In meteorology, there are 27 types of clouds with nine in each of the three categories – low, medium, and high.

The lowest level is between the surface and up to two kilometers in the atmosphere. Low level clouds include cumulus, stratocumulus, stratus, and cumulonimbus clouds. Cumulus clouds are one of the most well known types. They are the puffy clouds that look like sheep or clumps of cotton balls. They usually occur where warm air rises and forms condensation when it hits cool air. Stratocumulus clouds are also rounded clouds, but they are darker than cumulus clouds. Stratus clouds are flatter and more horizontal. They are the type of clouds that makes a day seem hazy and cloudy.

The medium level is measured at different elevations depending on the region. This depends on a number of factors including elevation and weather. In the polar region, the middle clouds are between two and four kilometers high; in the temperate regions, these clouds are between two and seven kilometers. They are between two and eight kilometers high in the tropical regions. The mid level clouds are altocumulus, altostratus, and nimbostratus. Altocumulus clouds are somewhat patchy, round forms that are white or grey. The often come before a cold front and also predict thunderstorms. Because these clouds look darker they can seem intimidating.

Altostratus clouds are part of the stratus family of clouds. They are like a sheet of clouds somewhere in between the nimbostratus and cirrostratus in color and often turn the whole sky grey. The nimbostratus clouds are very dark grey sheets of clouds. They look similar to other stratus types, but are much darker.

High altitude clouds are also located at different heights depending on region. They can be found between three and 18 kilometers depending on the region. They are found at a much higher altitude in the tropical regions. The clouds at high altitudes are different cirrus clouds and include cirrus, cirrocumulus, and cirrostratus clouds. Cirrus clouds are the thin, wispy clouds found high in the atmosphere. Because of their thin appearance, they are sometimes called mare’s tail; these clouds form when ice vapor freezes high in the sky. Cirrocumulus clouds appear to be a sheet of tiny cumulus clouds, so they almost look as though they are ripples on a pond. Cirrostratus clouds are a mix between cirrus and stratus clouds. They are thin, but resemble a sheet like stratus clouds. Often, they appear to form a halo around the Sun because they are so thin.

Universe Today has articles on stratus clouds and cloud types.

For more information, check out cloud classifications and types of clouds.

Astronomy Cast has an episode on Earth you will want to check out.