Basic Facts Everyone Should Know About Clouds

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Clouds may look like big, fluffy marshmallows in the sky, but in reality, they are visible collections of tiny water droplets (or ice crystals, if it's cold enough) that live high in the atmosphere above the Earth's surface. Here, we discuss the science of clouds: how they form, move, and change color. 

Formation

Clouds form when a parcel of air rises from the surface up into the atmosphere. As the parcel ascends, it passes through lower and lower pressure levels (pressure decreases with height).

Recall that air tends to move from higher to lower pressure areas, so as the parcel travels into lower pressure areas, the air inside of it pushes outward, causing it to expand. This expansion uses heat energy, and therefore cools the air parcel. The farther upward it travels, the more it cools. When its temperature cools to that of its dew point temperature, the water vapor inside of the parcel condenses into droplets of liquid water. These droplets then collect on the surfaces of dust, pollen, smoke, dirt, and sea salt particles called nuclei. (These nuclei are hygroscopic, meaning they attract water molecules.) It is at this point—when water vapor condenses and settles onto condensation nuclei—that clouds form and become visible.

Shape

Have you ever watched a cloud long enough to see it expanding outward, or looked away for a moment only to find that when you look back its shape has changed?

If so, you'll be glad to know it isn't your imagination. The shapes of clouds are ever-changing thanks to the processes of condensation and evaporation.

After a cloud forms, condensation doesn't stop. This is why we sometimes notice clouds expanding into the neighboring sky. But as currents of warm, moist air continue to rise and feed condensation, drier air from the surrounding environment eventually infiltrates the buoyant column of air in a process called entrainment.

When this drier air is introduced into the cloud body, it evaporates the cloud's droplets and causes parts of the cloud to dissipate.

Movement 

Clouds start out high up in the atmosphere because that's where they're created, but they remain suspended thanks to the tiny particles they contain.

A cloud's water droplets or ice crystals are very small, less than a micron (that's less than one-millionth of a meter). Because of this, they respond very slowly to gravity. To help visualize this concept, consider a rock and a feather. Gravity affects each, however the rock falls quickly whereas the feather gradually drifts to the ground because of its lighter weight. Now compare a feather and an individual cloud droplet particle; the particle will take even longer than the feather to fall, and because of the particle's tiny size, the slightest movement of air will keep it aloft. Because this applies to each cloud droplet, it applies to the entire cloud itself.

Clouds travel with the upper-level winds. They move at the same speed and in the same direction as the prevailing wind at the cloud's level (low, middle, or high).

High-level clouds are among the fastest moving because they form near the top of the troposphere and are pushed by the jet stream.

Color

A cloud's color is determined by the light it receives from the Sun. (Recall that the Sun emits white light; that white light is made up of all the colors in the visible spectrum: red, orange, yellow, green, blue, indigo, violet; and that each color in the visible spectrum represents an electromagnetic wave of a different length.)

The process works like this: As the Sun's lightwaves pass through the atmosphere and clouds, they meet the individual water droplets that make up a cloud. Because the water droplets have a similar size as the wavelength of sunlight, the droplets scatter the Sun's light in a type of scattering known as Mie scattering in which all wavelengths of light are scattered. Because all wavelengths are scattered, and together all colors in the spectrum make up white light, we see white clouds.

In the case of thicker clouds, such as stratus, sunlight passes through but is blocked. This gives the cloud a grayish appearance.