Clouds are classified according to how they are formed. There are two basic types:

  1. Clouds formed by rising air currents, named cumulus clouds. These clouds are, typically, piled up and fluffy. Air is rising; as if it is bubbling up into something. Sometimes these clouds will be flat on the bottom and look like a bubble on the top. The looks are not deceiving. That is precisely what is happening with their formation. Imagine a bubble of air coming up through a swimming pool. When it gets near the top, it leaves a bubble or foam on the surface of the water. It is flat on the bottom where it contacts the water. Similarly, air rises up through the fluid of the atmosphere when it may form a cloud. Also where it can no longer keep going up further because of the temperature and moisture relations of the atmosphere, a fluffy, rounded cloud is formed (Fig. 5.17).

    Fig. 5.17 Cumulus cloud
  2. The second type of cloud is formed when a layer of air is cooled to the saturation point without localized vertical movement. In this case a widespread area is lifted slowly, causing a large sheet of clouds. These clouds are called stratus (Fig. 5.18), meaning "sheet-like". Stratus clouds are formed under conditions where the air is stratified, and they exist something like a sheet across a large area. That is why the stratosphere is called the stratosphere because the air does not mix all around and churn. The atmosphere is layered in stratosphere.

    Fig. 5.18 Stratus clouds

Cloud tops, even on thunder clouds can rise up to 70,000-75,000 ft (22,500-24,200 m). Cirrus clouds and some other types of clouds reach that height quite often.

A second designation is given a cloud according to height or if a cloud is precipitating.

Clouds' names are descriptive. "Nimbus" in a cloud name designation means rain. There are "fracto-" (meaning "fragment"ed clouds), where the wind breaks clouds into pieces, "alto-" meaning middle. But all of the names have meanings. They do not seem to have changed very much from the original cloud classification. The designation is applied to the cumulus or stratus types to further describe clouds. First we will apply the height designation.

The high clouds, clouds that are above 20,000 ft. (6000 m) , are almost always ice clouds. There are three basic types:

  1. The cirrus clouds, sometimes called mares tails, are wispy, fibrous clouds that occur around 25.000 ft (Fig 5. 19). There is something to be learned from those clouds called mares tails. When there are mares tails in the sky, we quite often see trails behind a jet flying overhead, condensation trails. Cirrus clouds are high level clouds.

    Fig. 5.19 Cirrus clouds
  2. "Cirrocumulus" clouds, occurring (Fig. 5.20) between 20,000 to 25,000 ft. (6000-8000 m), are patchy clouds that often make patterns referred to as a mackerel sky. They are always thin; they never really make shadows on the ground, although they may cut down the intensity of the sunshine. The cumulus elements are very small, resembling the scales of a fish, hence, the term "mackerel sky".

    Fig. 5.20 Cirrocumulus clouds
  3. "Cirrostratus" clouds (Fig. 5.21), a great sheet of ice particles above the surface, may look like wind-blown gauze. The sun or moon is always visible through this thin layer of ice crystals. Some photos show a halo around the sun or around the moon. Sometimes there is a double halo when the temperature is 40° F (4° C). These halos are the product of refracting and reflecting light through the crystals. It has been said that when there is a halo around the moon or around the sun, there is going to be a storm. It is the same thing as watching the condensation trails in the sky. A warm front is probably on the way, the moisture is building up in the air, and the clouds form in the upper atmosphere creating the halos.

    Fig 5.21. Cirrostratus clouds

In the middle of the atmosphere, clouds are basically stratus or cumulus types. The bases of these clouds average about 10,000 ft. (3200 m) above the earth.

  1. The "altocumulus" clouds (Fig. 5.22) are patches or layers of puffy clouds or sometimes rolled clouds at middle levels of the atmosphere. They again do not consist of ice crystals. The cloud elements are larger than cirrocumulus and can form in larger arrangements, also, up to 6000 m.

    Fig. 5.22 Altocumulus cloud
  2. "Altostratus" clouds (Fig. 5.23) are dense veils of gray or blue which often appear to be striped. The sun and the moon do not form a halo in a middle cloud because these clouds do not have ice particles in them.

    Fig. 5.23 Altostratus clouds

Low clouds are clouds near the ground. There is a difference between fog and a cloud that is just at ground level. The difference depends on how they are formed. Fog is due to local cooling of a cloud that settles down to the ground. Low clouds are formed by lifting processes in the atmosphere which lift already very moist air slightly to cause condensation. We will find a lot of interesting things happen on this. When you are driving in foggy weather in Iowa, sometimes the fog is in the valley bottoms. Other times the fog is on the hilltops, and the valley bottoms are clear. So this really does differentiate the conditions under which fog and clouds were formed. When they were formed, was it because of surface cooling, or was it because of a layer of air that was below the dew point?

  1. Cumulus clouds (Fig. 5.24) are the ones often associated with fair weather. They are regularly seen as puffy clouds which have the shapes that we love to lay on our back on a sunny summer day and watch the clouds move and change. On the days when a cold front has come through and brought dry air and comfortable conditions, the fair weather cumulus clouds appear.

    Fig. 5.24 Cumulus clouds
  2. "Stratocumulus" (Fig. 5.25) are the regular masses of clouds, gray, dark shading and do not produce rain unless they start to thicken and change into a rain cloud. As the name indicates, they are a layered cloud with cumuliform elements.

    Fig. 5.25 Stratocumulus clouds

Two forms of clouds produce precipitation.

  1. "Nimbostratus" (Fig. 5. 26) are the true rain clouds. They are a dark layered cloud (low-based cloud) with a shaft of rain seen coming down toward the ground. They have a wet look to them. They are often accompanied by some low clouds that are moving along beneath them, usually quite quickly as the wind blows under these clouds. These little clouds are called "scud" and move along quickly underneath the rain clouds. "Nimbo" is an added prefix meaning "rain".

    Fig. 5.26 Nimbostratus clouds
  2. A special form of a cumulus (Fig. 5.27) cloud is a cumulonimbus cloud (cumuliform with rain). This type differs from regular cumulus clouds because of its huge vertical development. They form in very unstable conditions transporting huge amounts of moisture high into the atmosphere. The height of these can range from near the surface to 70,000 ft (22,500 m) in the most extreme cases. The great storm that produced the huge floods with 15" of rain in 1993 had its top at 60,000 ft (19,350 m). They are well known for their anvil shape caused by spreading of the cloud by the jet stream.

    Fig. 5.27 Cumulonimbus, a cumulus cloud with great vertical development and rain.