The Ultimate Guide to Clouds

Clouds are an essential part of the Earth's water cycle. Their appearance can change as they take different forms and create weather phenomena.

Clouds are formed when water on the Earth's surface evaporates. The resulting water vapor rises and meets cool air in the troposphere (the lowest level of the Earth's atmosphere). The cool air causes the vapor to condense and form water droplets on condensation nuclei - which are small particles of salt, dust, pollen, and other debris - floating in the environment.

The sun plays a part in how the human eye sees the color of clouds. Waves of every color in the rainbow make up the sun's white light. The sky appears blue because the human eye sees the blue light that reflects off nitrogen and oxygen molecules in the air, while clouds appear white because they are made up of water or ice, which do not reflect light waves. When the sun's light passes through clouds, the water molecules scatter the light's rainbow of colors in all directions, making clouds appear white. If a cloud becomes so thick with water droplets that the sun's light cannot penetrate it, the cloud will appear dark to someone standing underneath it. If someone were in an airplane, looking down on that cloud, it would still appear white.

Stratus, cumulus, and cirrus are the three general types of clouds, and they can be identified by their appearance and height in the atmosphere.

Stratus clouds are the lowest clouds in the atmosphere. They may stay on the ground as fog or go up to 9,800 feet and spread across the sky like a blanket. Nimbostratus clouds are the thick, dark blanket of clouds that are responsible for overcast skies and constant precipitation.

Cumulus clouds have their base at a low level, between 1,600 to 9,800 feet, but rise vertically into the atmosphere. These are tall white, puffy clouds. The cumulonimbus is the tallest of all clouds. It may look like a tower or mountain, and rise up to over 60,000 feet.

Altostratus and altocumulus are middle base level clouds. They are found between 9,800 to 23,000 feet. The altostratus clouds give the sky a gray appearance, but it is difficult to distinguish any details. The altocumulus clouds may often look like a sheet of puffy, white ripples.

Cirrus clouds form in the highest levels of the troposphere at over 23,000 feet. These clouds are composed of ice crystals and have a wispy appearance. Sometimes a combination of cloud types will form. Cirrostratus clouds are ice clouds that blanket the sky, but have distinguishing wispy or fibrous strands. Cirrocumulus clouds are high, ice clouds that resemble light ripples.

There are other cloud phenomena that form under unique circumstances. When water droplets become too dense and heavy for the cloud to support them, they drop in the form of precipitation (like rain or snow). When there is low humidity and high temperatures, the water may evaporate before the precipitation hits the ground. This creates virga, the fibrous strands that hang under a rain cloud. 

Funnel clouds and waterspouts have the potential to become destructive. A funnel cloud is a funnel-shaped, cumuliform cloud caught in a rotating column of wind. If the funnel cloud touches the ground, it becomes a tornado. If it touches down on water, it becomes a "water tornado", called a waterspout, which can pose a serious risk to boats and marine life.

Some cloud phenomena are optical illusions. Crepuscular rays are often seen as bright shafts of sunlight, coming from a central point in the sky. They usually appear at dusk or dawn, when the sunlight breaks through thin areas of a cloud, but is shadowed in other areas. A glowing, ring-shaped, halo cloud can be created when different air temperatures combine and the cloud reflects a setting sun.

People may admire a cloud's changing shape, or watch for dark clouds as predictors of rain, but clouds in all their forms are key elements of our weather.

For more information about clouds, please refer to the following links:

• Classifications of clouds

• A glossary of cloud terms

• Pictures and descriptions of common cloud types

• Diagrams of different clouds and how they are formed

• Make a cloud in a jar

• Types of clouds

• Pictures and descriptions of clouds and how they are formed

• Clouds and the water cycle

• Descriptions of clouds and how they form