Tuesday Weather Topic in Greater Depth

From: Subject: Tuesday Weather Topic in Greater Depth Date: Fri, 23 Oct 2009 10:13:54 -0700 MIME-Version: 1.0 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.ametsoc.org/amsedu/dstreme/learn/t_sup.html X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 Tuesday Weather Topic in Greater Depth

SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

To complement the Daily Summary for Tuesday, 20 = October 2009=20

CLOUD IDENTIFICATION AIDS

Clouds are an indicator of atmospheric processes and they can be used = at=20 times to forecast future weather events. While many cloud shapes can be=20 observed, only a few basic cloud forms are associated with most = atmospheric=20 processes. A cloud classification scheme that was initially developed in = the=20 early 19th century recognizes several fundamental cloud types that are = based=20 according to both appearance and altitude. This uniform cloud = classification=20 scheme, which ultimately has 10 fundamental cloud types, is vital to our = understanding of the cloud formation processes. You are encouraged to = look at=20 the clouds that may appear overhead and try to identify those formation=20 processes by the cloud appearance. You may want to consult and verify = those=20 cloud photographs that have been identified by cloud type, such as those = appearing in the NOAA/NASA = Cloud=20 Chart (a pdf file that can be accessed from the DataStreme = Atmosphere=20 website), the Plymouth = State=20 (NH) University Meteorology Program Cloud Boutique and the U of=20 Illinois Cloud Catalog.

We can begin our cloud observations by attempting to classify the = clouds by=20 whether they appear to be layered in the horizontal direction or puffy, = with=20 more vertical development than horizontal extent. Those clouds that = appear as=20 layers usually are formed by large scale, gentle lifting processes that = usually=20 involve non-turbulent condensation in a relatively stable layer. These = clouds,=20 sometimes called "stratiform clouds", typically occur in association = with warm=20 fronts. On the other hand, clouds that are vertically developed with a = billowing=20 appearance typically indicate smaller scale convection with vigorous = vertical=20 motions in a less stable atmosphere. These clouds, which are often = grouped as=20 "cumuliform clouds", may be found where warmer air would be located near = the=20 surface with colder air aloft - the result of intense surface heating or = from=20 cold air advection, such as the advance of a cold front.

We can refine our initial cloud observations, aiming toward the = conventional=20 classification scheme used by meteorologists, by estimating the altitude = of the=20 bases of the stratiform clouds and the vertical extent of the cumuliform = clouds=20 by further examination of their appearance. With practice, we could = ultimately=20 classify the clouds that we may see by the 10 cloud types.

The identification of individual cloud types requires experience = gained from=20 continuous cloud watching. This determination will not necessarily be = easy at=20 first. Several telltale signs can be used to aid in the identification = of=20 apparently similar cloud types.

Precipitating clouds

If you observe significant precipitation (more than just a few = raindrops=20 or snowflakes) falling from a cloud, you can safely assume that the = cloud is=20 either a nimbostratus cloud or a cumulonimbus cloud. Which type of = cloud=20 depends upon the overall appearance or the situation (nimbostratus = clouds=20 typically form ahead of approaching warm fronts, while cumulonimbus = clouds are=20 thunderstorm clouds that often accompany cold fronts). The prefix = "nimbo" and=20 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=20 known landmarks, such as television towers, mountains or aircraft at = known=20 flight levels. If a cloud would conceal the top of even the tallest = television=20 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=20 may be high clouds.

Size of features

The apparent size of a cloud feature indicates the distance from = the=20 observer, and provides a clue as to cloud altitude. Grading from = largest to=20 smallest features, stratocumulus would often appear as fist sized with = your=20 arm outstretched, altocumulus would be thumb sized, while cirrocumulus = would=20 be fingernail sized.

Shades of gray

Generally, nimbostratus or the undersides of cumulonimbus clouds = are dark=20 gray with bases diffused by falling precipitation ; the dark color = results=20 from a large concentration of raindrops that attenuate the sunlight.=20 Altostratus would often be less gray, while cirrostratus are usually = pale gray=20 or semi-transparent.

Texture

A filmy or wispy appearing cloud indicates that the cloud is = composed=20 mainly of ice crystals. If the cloud appears to have more distinct or = "harder"=20 boundaries, then the cloud would be composed primarily of liquid water = droplets. Texture helps to distinguish and correctly identify cumulus=20 congestus and cumulonimbus clouds.

Existence of optical phenomena

Luminous and sometimes colored thick rings called "coronas" that=20 immediately surround the moon or sun indicate the existence of liquid = droplets=20 in warm clouds such as altostratus. On the other hand, thin bright = rings or=20 portions of rings that are at approximately a hand width away (using = your=20 outstretched arm) from the sun or moon imply the presence of ice = crystals in=20 high level clouds such as cirrostratus. These latter optical effects = fall into=20 the "halo" phenomena category and include those bright spots called = "sun=20 dogs", "mock suns" or "parhelia".

Existence of thunder

Essentially any time that you hear thunder, a cumulonimbus cloud = is=20 nearby.

As you attempt to identify clouds, do not hesitate to check with = weather maps=20 to locate the major weather systems, such as low and high-pressure = centers, as=20 well as warm and cold fronts. Armed with this knowledge, you can then = see that=20 the cirrus clouds that you see may be the forerunners of an approaching = warm=20 front or the cumulus clouds that are beginning to swell are developing = along a=20 cold front.

NOTE: In October 2004, the United States Postal = Service=20 issued a set of 15 commemorative "Cloudscapes" = postage=20 stamps that featured nine cloud formations.

Return to the Tuesday = Daily=20 Weather Summary

Return to DataStreme = Atmosphere=20 website

Prepared by Edward J. Hopkins, Ph.D., email mailto:hopkins@meteor.wisc.edu
=A9=20 Copyright, 2009, The American Meteorological Society. =