A Guide to the Sky

(Note: A very special thanks to the publishers of the poster "A Guide to the Sky" from which the following material and the cloud pictures were taken. The webpage was generated as a class project for Atmospheric Sciences 301, an introduction to meteorology taught at the South Dakota School of Mines and Technology. However, the information is of value to anyone who has an interest in the weather, so please browse the pages.  An index of the cloud photos appears at the bottom of this text.)

    When compared to the earth's size, the layer of atmosphere in which all clouds form is no thicker than the leather cover on a softball.  Within this fragile layer is a myriad of continuously evolving forms of that most ubiquitous and amazing of all substances, water. Water, unlike most substances on earth, exists in all three phases-gaseous, liquid, and solid-within the tiny range of atmospheric conditions found here on earth. Due to this "versatility", clouds can suddenly appear and disappear, and "precipitate", pelting us with rain and snow. While these events are commonplace to us, in a cosmic sense, they are nothing less than miraculous. And there is still considerable mystery. For example, it is not known how ice crystals form in clouds, and in many clouds there are far more of them than scientists had expected. The sky itself, where we can most easily appreciate these miracles, can be a joy, revealing something new every day if we would but look.

     This chart explores some of the most frequently seen shapes and forms of clouds in the sky, including the ten types (or genera), from Cirrus to Stratus, and their most frequently observed species or varieties (e.g., fibratus, translucidus).

     Cirriform clouds, Cirrus, Cirrostratus, and Cirrocumulus are called "high clouds" by meteorologists and are, except for some cases of Cirrocumulus clouds (see 1A), composed of ice crystals. These clouds, except for Cirrocumulus clouds, are rarely less than 1 km (3,300 feet) thick even when they show no shading. Their bases range from 5 to 13 km (about 16,000 to 45,000 feet) above ground level (AGL). Widespread cirriform clouds (e.g., 2A) are often the forerunners of storms, particularly when they advance from a direction between southwest and northwest. They are the smoke from the fire. This is because they often represent "blow-off" from the tops of deep, precipitating clouds upwind caused by the stronger winds at the highest levels of the storm.

    Middle-level clouds, Altocumulus and Altostratus, are shown in figures 3A-E and 4A-E. These clouds have bases that range from 2 to 7 km (about 6,600 to 24,000 feet) above ground level (AGL). Altocumulus clouds (e.g., 3C) are relatively thin, rarely more than 1 km (3,300 feet) thick, and are usually composed of tiny droplets only about one-third the diameter of a human hair or smaller. They often spread over the sky soon after Cirrus or Cirrostratus clouds have appeared. When solid banks of Altocumulus clouds appear and move rapidly from a direction between south and west, it is a good sign that precipitation will occur in less than 12 hours. Altostratus clouds almost always cover much or all of the sky when they are present and usually have gray shading throughout. They are primarily composed of ice crystals and snowflakes. Often, however, Altocumulus clouds may be embedded in them or comprise the top of the cloud (see 4B). The dominance of ice crystals and snowflakes causes most Altostratus clouds to appear diffuse, with little detail. This is because the concentrations of ice crystals and snowflakes in clouds are only about a hundredth to a thousandth of that of cloud droplets. Hence, droplet clouds look very sharp edged while ice crystal clouds, like Cirrus and most Altostratus, appear more fibrous or diffuse. Altostratus clouds are nearly always more than 2 km thick (6,600 feet) even when the sun can be seen through them (see 4B). They are perhaps the surest sign in the sky that precipitation is imminent. They show that the atmosphere is in gentle ascent over a great depth.

     When it has been raining or snowing steadily from widespread dark and gray clouds, you have been experiencing the underside of Nimbostratus clouds (see 4C). These low-based layer clouds are generally greater than 3 km (10,000 feet) thick, and their tops often reach as high as Cirrus clouds. The tops of Nimbostratus, if you could strip away the bottom 90% of the cloud, resemble the clouds shown in 1C or 3E. Altocumulus-like clouds are often embedded in them, and Stratocumulus clouds (see 4D) are usually present at the bottom. These embedded clouds help to increase the rain or snow that falls from Nimbostratus clouds considerably because the cloud droplets in them are partially swept out by the rain and snow of the Nimbostratus cloud.

    Stratus, Stratocumulus, Cumulus, and Cumulonimbus clouds, also termed "low clouds", are shown in 4D through 6E. The heights of their bases can range from near ground level in Stratus clouds (see 4E) to as much as 4 km (13,500 feet) AGL for Cumulus clouds forming over deserts.

    Stratus clouds are nearly featureless, low hanging clouds that usually cover the entire sky. Storms are usually far away. They are featureless because there is little upward or downward motion in them. Stratocumulus clouds may look dark, but they are usually less than 1 km (3,300 feet) thick. They are lumpy looking because of embedded small cumulus-like clouds within them. They tend to be darker looking at inland locales because there the droplets in them are generally smaller and this causes more sunlight to be reflected back into space from their tops.

     Both Stratus and Stratocumulus clouds, except when their tops are colder than about -8 degrees C (18 deg F), are composed solely of droplets. Heavy precipitation does not fall from them, though drizzle (mist-like rain) or light snow can fall. Stratocumulus and Stratus clouds cover more of the earth's surface than any other type of cloud.

     The remaining photos (5A through 6E) show Cumulus and Cumulonimbus clouds. These clouds are distinguished from other clouds by the appreciable updrafts in their building regions, and appreciable downdrafts in their decaying regions. The updrafts in these clouds range from brief puffs of a few meters per second (5-10 mph) in small Cumulus and Cumulonimbus clouds (see 5A, 5B, 6A, and 6C) to sustained updrafts of 30 m/s (65 mph) or more in the most powerful thunderstorms (see 6B, 6D and 6E). Cumuliform clouds are avoided by pilots not only because of the turbulence, but also because of the heavy icing they can produce. Icing is a situation where cloud droplets, and even raindrops, are encountered at below freezing temperatures and impact and freeze on an airframe, thus disrupting the lift of the wing.

     Small and medium-sized Cumulus clouds (see 5A and 5B) are less than 2 km thick and generally composed of droplets. When their tops become colder than about -10 deg C (14 deg F), some ice crystals may form and some flurries are possible. Cumulus congestus clouds (see 5C and 5D) are the largest Cumulus clouds. They are greater than 2 km (6,600 feet) thick and often appear as wide as they are tall. Occasionally these clouds undergo a transition from a highly detailed, cauliflower look (e.g., 5C) at cloud top to a smoother, fibrous appearance (see 6B and 6C). This change, which can occur within 10 minutes, is associated with the development of ice crystals or raindrops, and can precede the fall of a sharp shower from the cloud by a few minutes. Thus, a watchful eye on the tops of Cumulus congestus clouds can lead to the prediction of a shower just before it falls out of the bottom of the cloud!

     Cumulonimbus clouds are the Tyrannosaurus rexes of the cloud world. The transition in the appearance of the tops of the cumulus clouds to a fibrous appearance, and in the development of a rain shaft which obscures the cloud's base also marks the moment when the cloud changes from a Cumulus congestus cloud to a Cumulonimbus cloud. The very tallest Cumulonimbus clouds, those that form in tropical air (e.g., 6C), can have tops as high as 20 km (67,000 feet) ASL. Their "overshooting" tops penetrate into the stratosphere where they can inject aerosols drawn from near the earth's surface. They frequently combine into clusters of several to dozens of huge clouds that produce such destructive weather as lightning, flash floods, hail, damaging winds, and tornadoes. However, the destructive regions are relatively small and beneficial rains often widespread. Most of the rain that falls during the growing season in the United States is due to these kinds of cloud systems.

     The smallest Cumulonimbus clouds form in polar air (see 6A and 6C) and can be less than 2 km thick. These kinds of cumulonimbus clouds frequent the West Coast in the cooler half of the year following the passage of fronts. In these kinds of situations, clouds similar to those shown in 6A and 6C can be spawned almost continuously over and upwind of mountain ranges bringing surprisingly heavy snow and soft hail (called graupel). Polar Cumulonimbus clouds rarely produce lightning or severe weather.

Published by Sky Guide, P.O. Box 30027, Greenwood Station, Seattle, Washington 98103-0027. Tel. 206 782-8485. Acknowledgments. Thanks to Judy Rossman, Art and Marge. E. Rangno for support and advice, to Candace Gudmundson and Melanie Richter-Bernburg for editorial suggestions, and to Jessica Klein for the graphic design. Sixth edition © 1995 by A.L. Rangno. Text and photos by A.L. Rangno, a meteorologist in airborne cloud studies at the University of Washington. Printed in the U.S.A.

The following index will allow you to choose cloud types at random:

1A. Cirrocumulus
1B. Cirrus (floccus)
1C. Cirrus (uncinus)
1D. Cirrus (spissatus)
1E. Cirrus (fibratus)

2A. Cirrostratus (fibratus)
2B. Cirrostratus (fibratus)
2C. Three species of cirrus and a cirrocumulus cloud
2D. Cirrostratus (fibratus)
2E. Cirrostratus (nebulosus)

3A. Altocumulus (lenticularis)
3B. Altocumulus (lenticularis)
3C. Altocumulus (perlucidus)
3D. Altocumulus (floccus and castellanus)
3E. Altocumulus (castellanus)

4A. Altocumulus (opacus)
4B. Altostratus
4C. Nimbostratus
4D. Stratocumulus (stratiformis)
4E. Stratus (nebulosus)

5A. Cumulus (humilis)
5B. Cumulus (mediocris)
5C. Cumulus (congestus)
5D. Cumulus (congestus)
5E. Cumulonimbus (calvus)

6A. Cumulonimbus (calvus)
6B. Cumulonimbus (capillatus)
6C. Cumulonimbus (capillatus)
6D. Cumulonimbus
6E. Cumulonimbus (mammatus)

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Test your knowledge

(for Atmospheric Sciences 301 class)
Check out this chart - list the cloud types represented by the photo and its corresponding number.