ONLINE WEATHER SUPPLEMENTAL INFORMATION

To complement the Daily Summary for Tuesday, 23 March 1999

CLOUD IDENTIFICATION AIDS

Clouds are an indicator of atmospheric processes and they can be used at times to forecast future weather events. While many cloud shapes can be observed, only a few basic cloud forms are associated with most atmospheric processes. A cloud classification scheme that was initially developed in the early 19th century recognizes several fundamental cloud types that are based according to both appearance and altitude. This uniform cloud classification scheme, which ultimately has 10 fundamental cloud types, is vital to our understanding of the cloud formation processes. You are encouraged to look at the clouds that may appear overhead and try to identify those formation processes by the cloud appearance. You may want to consult and verify those cloud photographs that have been identified by cloud type, such as those appearing in the PSC Meteorology Program Cloud Boutique .

We can begin our cloud observations by attempting to classify the clouds by whether they appear to be layered in the horizontal direction or puffy, with more vertical development than horizontal extent. Those clouds that appear as layers usually are formed by large scale, gentle lifting processes that usually involve non-turbulent condensation in a relatively stable layer. These clouds, sometimes called "stratiform clouds", typically occur in association with warm fronts. On the other hand, clouds that are vertically developed with a billowing appearance typically indicate smaller scale convection with vigorous vertical motions in a less stable atmosphere. These clouds, which are often grouped as "cumuliform clouds", may be found where warmer air would be located near the surface with colder air aloft - the result of intense surface heating or from cold air advection, such as the advance of a cold front.

We can refine our initial cloud observations, aiming toward the conventional classification scheme used by meteorologists, by estimating the altitude of the bases of the stratiform clouds and the vertical extent of the cumuliform clouds by further examination of their appearance. With practice, we could ultimately classify the clouds that we may see by the 10 cloud types.

The identification of individual cloud types requires experience gained from continuous cloud watching. This determination will not necessarily be easy at first. Several tell-tale signs can be used to aid in the identification of apparently similar cloud types.

Precipitating clouds

• If you observe significant precipitation (more than just a few raindrops or snowflakes) falling from a cloud, you can safely assume that the cloud is either a nimbostratus cloud or a cumulonimbus cloud. Which type of cloud depends upon the overall appearance or the situation (nimbostratus clouds typically form ahead of approaching warm fronts, while cumulonimbus clouds are thunderstorm clouds that often accompany cold fronts). The prefix "nimbo" and the suffix "nimbus" are derived from the term meaning rain.

Cloud bases

• If possible, compare the height of the cloud bases with the altitudes of known landmarks, such as television towers, mountains or aircraft at known flight levels. If a cloud would conceal the top of even the tallest television tower, the cloud would be considered a low cloud, such as a stratus or nimbostratus cloud. Clouds that appear above the height of aircraft in flight may be high clouds.

Size of features

• The apparent size of a cloud feature indicates the distance from the observer, and provides a clue as to cloud altitude. Grading from largest to smallest features, stratocumulus would often appear as fist sized with your arm outstretched, altocumulus would be thumb sized, while cirrocumulus would be finger-nail sized.

Shades of gray

• Generally, nimbostratus or the undersides of cumulonimbus clouds are dark gray with bases diffused by falling precipitation; the dark color results from a large concentration of rain drops which attenuate the sunlight. Altostratus would often be less gray, while cirrostratus are usually pale gray or semi-transparent.

Texture

• A filmy or wispy appearing cloud indicates that the cloud is composed mainly of ice crystals. If the cloud appears to have more distinct, or "harder" boundaries, then the cloud would be composed primarily of liquid water droplets. Texture helps to distinguish and correctly identify cumulus congestus and cumulonimbus clouds.

Existence of optical phenomena

• Luminous and sometimes colored thick rings called "coronas" that immediately surround the moon or sun indicate the existence of liquid droplets in warm clouds such as altostratus. On the other hand, thin bright rings or portions of rings that are at approximately a hand width away (using your outstretched arm) from the sun or moon imply the presence of ice crystals in high level clouds such as cirrostratus. These latter optical effects fall into the "halo" phenomena category and include those bright spots called "sun dogs", "mock suns" or "parhelia".

Existence of thunder

• Essentially any time that you hear thunder, a cumulonimbus cloud is nearby.

As you attempt to identify clouds, do not hesitate to check with weather maps to locate the major weather systems, such as low and high pressure centers, as well as warm and cold fronts. Armed with this knowledge, you can then see that the cirrus clouds that you see may be the forerunners of an approaching warm front, or the cumulus clouds that are beginning to swell are developing along a cold front.

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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 1999, The American Meteorological Society.