4 Common Misconceptions About Clouds Clarified

There are teachable moments everywhere. During the intermission of a concert Sunday night, I stumbled upon a conversation on social media about clouds. As I read through the comments, I thought it would be useful as a NASA scientist and Atmospheric Sciences Professor at the University of Georgia for me to float this article out there to clarify four common misconceptions I’ve heard over the years.

There Are More Than Four Types Of Clouds

One misconception is that there are only four types of clouds: cumulus, cirrus, stratus, and thunderstorm clouds. This misconception has its birth in elementary school curricula and usually carries into our adult years. It has driven me crazy for years.

Though those are basic types of clouds, there are numerous others, which are determined by form, altitude or other characteristics. For example, altocumulus are clumps of puffy-type clouds found at mid-levels of the atmosphere. Nimbostratus are stratus or stratocumulus clouds that are producing snow or rainfall. If you see a root of the word “nimbus,” think precipitation. There are also an array of interesting clouds like mammatus, lenticular, and asperitas. The UK Met Office has an excellent website for learning about the various cloud types.

Clouds Are Not Gases

This is misconception that I see often in social media. I think part of the reason is because clouds appear to “float” (more on that later). I will turn to the Glossary of the American Meteorological Society (AMS) for a definition of clouds. It defines a cloud as, “A visible aggregate of minute water droplets and/or ice particles in the atmosphere above the earth’s surface.” However, at some point, the gas phase of water (water vapor) is involved. The Glossary definition goes on to say, “Clouds form in the free atmosphere as a result of condensation of water vapor in rising currents of air, or by the evaporation of the lowest stratum of fog.”

Cloud water or ice crystals generally require a “seed” (called a condensation or ice nucleus, respectively) to form. The process is actually a bit more complicated and also depends on things like temperature throughout cloud depth, at what point saturation happens (which means the atmosphere cannot hold more water vapor at that temperature), and the distribution of nuclei (dust, clay, and so on).

Clouds Are Floating? Sort of.

There are two factors that explain why clouds do not fall. The first factor is that cloud particles (water droplets or ice crystals) are very small. Because they are so small, they do not have significant fall velocity.

OK, I am about to get a bit technical, but I promise to explain it all.

Writing in Scientific American, Doug Wesley said, “the distance from the center of a typical water droplet to its edge—its radius—ranges from a few microns (thousandths of a millimeter) to a few tens of microns (ice crystals are often a bit larger).” That’s pretty tiny.

The speed that an object falls is related to its mass and surface area. Wesley notes in that article, “which is why a feather falls more slowly than a pebble of the same weight.” Even the largest cloud droplets do not attain significant fall velocities, and because of their shape, such velocities are even smaller with ice crystals.

OK, so why do the clouds appear to float? The answer is rather simple. There is generally some type of upward motion associated with clouds. In a cumulonimbus cloud with significant hail, these “updrafts” can be quite strong. Even upward motions with more benign stratus clouds are significant enough to offset the fall velocities of the droplets or crystals.

How the water vapor is distributed and density are also important. The U.S. Geological Survey (USGS) has a neat website that answers the question, “How much does a cloud weigh?” For their analysis, they assumed a 1 cubic kilometer cloud contains 1 billion cubic meters. If 0.5 grams per cubic meter is assumed for cloud density, a little simple math yields roughly 1.1 million pounds or about 551 tons. Here’s the science magic though. That mass is distributed over a large distance, and the air beneath the cloud is denser.

The USGS website summarizes it this way, “The key to why clouds float is that the density of the same volume of cloud material is less than the density of the same amount of dry air.” I demonstrated such density differences for my family one time by mixing a diet soda with a very sugary, yellow soda at a dinner buffet.

Clouds Are White Just Because

Clouds appear white because the droplet sizes are about the same size as the wavelength of white light. When this is the case, something called “Mie scattering” happens. A National Weather Service website explains, “Mie scattering does not differentiate individual wavelength colors and therefore scatters ALL wave length colors the same….therefore we see white clouds.”

Sometimes the clouds will appear grey or black. That’s because some of the drops in the cloud have grown to the size of rain drops, which scatter even more light and prevents white light from reaching our eyes.