# Handbook of Meteorology/Fogs and Clouds

THE MOISTURE OF THE AIR: FOG AND CLOUD
Fog
In his "Floating Matter of the Air" Tyndall demonstrated that, when the air pressure under the receiver of an air pump was reduced, the cooling of the air by expansion produced a perceptible fog. He demonstrated also that, if the air admitted to the receiver were filtered, a second exhaustion would produce a fog only to an extent scarcely observable, or not at all. In other words, the dust motes and molecules of hygroscopic gases are necessary for condensation. When there were no longer any dust motes, there was no condensation.
Fog and cloud are the most striking examples of condensation on a large scale; in weather science it is commonly called volume condensation. One cannot readily make a distinction between fog and cloud; in general, fog is cloud on the ground, while cloud is fog high in the air. When the blue sky becomes white, the change in color is due to condensation—perhaps water dust, perhaps ice dust. If the condensation thickens, distant objects become blurred by the accumulated condensation; a moisture-haze, quite distinct in color from the blue dust-haze, occurs. Perhaps the white sky might not be called cloud; but if the condensation increases until the color becomes a dark gray, by common consent it is "cloud" in the air, or "fog" if it extends to the ground. The distinction is merely one of degree. Fog and cloud are examples of condensation; but until the droplets coalesce into drops that fall to the ground they are not precipitation.
It is likely that fog and cloud droplets vary much in size; but definite knowledge of the extent of this variation is wanting. Wells found that fog droplets were approximately 0.0002 inch (0.005 mm) in dimension, and that the fog droplets in a cubic yard were not far from 7 grains in weight.
​Fog Types.—A common illustration of fog formation may be observed when a cake of ice is at the doorstep. Almost immediately it begins to “steam.” The ice chills the air in contact below the dew-point, and condensation is at once apparent in the form of fog. Condensation liberates enough latent heat to give the moisture a certain amount of updraught, and therefore a steaming effect. It is an instructive illustration of contact cooling, and the fog produced is the radiation fog of weather science.
On still nights during spring and fall, fog is frequent over rivers and ponds, especially in relatively low places. If the air is still over such bodies of water during the day, it is apt to be moist. Therefore the normal lowering of temperature soon reaches the dew-point and, as a result, a radiation fog forms. Sometimes its depth is only a few feet; occasionally it overtops buildings and trees.
In various instances fogs hover over manufacturing districts when nearby rural areas are free from them. It is pretty certain that the products of combustion are the “favorable nuclei” in such cases. Dr. Owen of the British Meteorological Office found that many such floating particles were extremely hygroscopic, and that they tended to produce condensation when it did not occur in air free from them. At all events, the city fog has become a factor in meteorology as well as in city traffic.
Advection fog is the name given to fogs that result when warm moist air invades a surface so cold that dew-point temperature is reached. The sea fogs of the North Atlantic are an example. Warm, moist winds of a southerly origin invade the region of cold Arctic currents, and condensation of the moisture brought to the region occurs. “Skin friction” between wind and water causes the eddying movements of the air known as turbulence, and the fog blanket extends higher and higher as
* ↑ In the fog over a manufacturing district Dr. Owen also found moisture droplets coated with liquid hydrocarbon, derived evidently from coal smoke. In other words, the fog droplet itself was a nucleus upon which the smoke-hydrocarbon condensed. The author failed to find this condensation in the manufacturing districts near New York City; but it is highly probable that it occurs in such atmospheres as those of Pittsburgh and South Chicago.
* ↑ From the Latin ad, “to,” and vehere, to “carry”—that is, fog produced by conditions carried to a locality from an external source. Although the name is comparatively recent, it is very aptly formed.
Cloud Photography
According to Arthur J. Weed, Chief Instrument Maker, U. S. Weather Bureau, the first requisite for cloud photography is a good camera with a rigid support. To this equipment a ray filter to shut out the excess of actinic rays from the blue sky is added. The filter, consisting of colored screens
Ellerman, photo.
Nimbus, with fog or stratus hovering in the valleys, Mount Wilson, Cal.
varying from yellow to red, is usually necessary, inasmuch as the exposure required for the cloud results in an over exposed sky. Very dense clouds may be photographed without the use of a ray filter. Cirrus clouds, however, require a strongly-colored ray filter. A black mirror answers the purpose of a ray filter and, in certain cases, gives a better negative. The details of cloud photography are described in the Monthly Weather Review, August, 1920.
the chilling of the air progresses. Advection fogs are more apt to follow gentle movements of the air; gale winds may create mixing to the extent that dew-point temperature is not reached. The advance of a cold wave moving gently into a region of warm, moist air—the fog in front of a high—is an example of advection fog.
Velo cloud is a name now commonly given to fog drifting in from the sea and hovering over a coast a few hundred feet from the ground. It is of frequent occurrence along the coast of southern California during summer months, and is occasional along the Atlantic coast. The velo is an example of advection condensation. Perhaps, strictly speaking, it should be classed as cloud rather than as fog; nevertheless it is advective condensation. Inasmuch as the term “high fog” is sometimes popularly used to denote a very thick fog meteorologists have generally adopted the term “velo.” The velo is rarely more than 1000 feet high.
Clouds
Cooperative observers are not required to report information concerning cloudiness, except the extent of cloud-covered sky during the daylight period. At the regular Weather Bureau stations the character, movement and height of clouds are recorded and at some stations nephoscopes are provided. With the aid of these instruments, the velocity of the clouds, and therefore that of the upper winds, may be determined.
The photogrammeter is one of the most practical instruments for measuring cloud heights. It consists of a pair of cameras mounted in the same manner as a surveyor’s transit. Two instruments set at different positions are employed. The sensitive plates are ruled with intersecting horizontal and vertical lines. By the aid of these, the photographs of the cloud indicate its comparative position, and from this both its altitude and its velocity may be determined. Air navigation now demands definite knowledge of wind at different elevations, and this knowledge is best obtained by a study of the clouds.
Formation of Clouds.—A cloud consists of an aggregation
Ellerman photo.
Wind ripples in upper surface of fog, or stratus cloud. Cirro-stratus clouds above, Mount Wilson, Cal.
​of visible particles of condensed water vapor. As in the formation of fog, each particle of cloud matter has condensed upon a dust mote. One cannot say why cloud matter floats in the air, apparently contrary to the laws of gravity. A theory that the cloud particle is repelled from the earth because it is charged with the same kind of electricity has been advanced; but it is not certain that this theory satisfies all conditions. That clouds form and disappear in accordance with the laws of temperature and dew-point is the fact that is important in weather science.
For convenience, cloud matter may be considered to be in a stage of condensation intermediate between vapor and liquid—a condition which may be brought about by several means:
    Local ascending currents, or updraughts, which are vertical or nearly vertical;
    Very slow obliquely ascending currents;
    The rapid chilling of the lower air by the radiation of earth warmth;
    The contact of high air layers which differ in temperature and humidity.
Any one of the foregoing conditions will produce cloud if the temperature falls below the dewpoint; nevertheless it is probable that cloud condensation is more complex in fact than the foregoing paragraphs indicate.
Classification.—Various schemes of cloud classification have appeared from time to time. Some of them have possessed great merit, but have been too complicated for practical use. More than a century ago, Luke Howard, of London, devised the classification upon which the scheme now in use was elaborated by the Cloud Committee of the International Meteorological Congress in 1891. The four fundamental forms are cirrus, cumulus, stratus, and nimbus. Other forms are designated by the combination of the foregoing terms. Two
* ↑ From the Latin cirrus (pl. cirri), a curl or wisp; cumulus (pl. cumuli), a heap, or pile; stratus (pl. strati, rarely used), a layer; nimbus (pl. not used), a rain cloud. The adjective derivatives are: cirro-, cumulo-, and strato-. Other definitive adjectives are alto-, high, and fracto-, broken.
* ↑ Some observers still employ the abbreviations of cloud names employed when the Weather Bureau was a part of the Signal Corps: Cirrus, C; Cumulus, K; Stratus, S; Nimbus, N. These symbols are used in Army and Navy practice.
Weed photo.
Cirrus plumes and “catail” streamers, upper half; cirro-stratus, lower half, Mount Weather, Va.
​factors, appearance and altitude, aid the observer in determining the name and character of a cloud. For all practical purposes the physical form and appearance must always be the chief feature in cloud determination; experience will teach the observer to determine whether the cloud in question is to be classed as “upper,” “intermediate,” or “lower”; a distinction which is sometimes essential. The following is the classification elaborated by Abercromby and Hildebrandsson and adopted by the International Meteorological Congress:
(a) Detached clouds with rounded upper outlines.
(b) Clouds of great horizontal extent suggesting a layer or sheet.
The first (a) are most frequent in fair weather; the second (b) are wet-weather clouds.
| Upper clouds, 30,000 feet (9000 meters) | Upper clouds, 30,000 feet (9000 meters) | Upper clouds, 30,000 feet (9000 meters) | $\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$ | a. 1. Cirrus |
| Upper clouds, 30,000 feet (9000 meters) | Upper clouds, 30,000 feet (9000 meters) | Upper clouds, 30,000 feet (9000 meters) | $\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$ | b. 2. Cirro-stratus |
| Intermediate clouds, 10,000 to 23,000 feet (3000 to 7000 meters) | Intermediate clouds, 10,000 to 23,000 feet (3000 to 7000 meters) | Intermediate clouds, 10,000 to 23,000 feet (3000 to 7000 meters) | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | a. 3. Cirro-cumulus |
| Intermediate clouds, 10,000 to 23,000 feet (3000 to 7000 meters) | Intermediate clouds, 10,000 to 23,000 feet (3000 to 7000 meters) | Intermediate clouds, 10,000 to 23,000 feet (3000 to 7000 meters) | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | a. 4. Alto-cumulus |
| Intermediate clouds, 10,000 to 23,000 feet (3000 to 7000 meters) | Intermediate clouds, 10,000 to 23,000 feet (3000 to 7000 meters) | Intermediate clouds, 10,000 to 23,000 feet (3000 to 7000 meters) | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | b. 5. Alto-stratus |
| Lower clouds, less than 6500 feet (2000 meters) | Lower clouds, less than 6500 feet (2000 meters) | Lower clouds, less than 6500 feet (2000 meters) | $\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$ | a. 6. Strato-cumulus |
| Lower clouds, less than 6500 feet (2000 meters) | Lower clouds, less than 6500 feet (2000 meters) | Lower clouds, less than 6500 feet (2000 meters) | $\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$ | b. 7. Nimbus |
| Clouds of diurnal ascending currents | $\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$ | top 6000 feet (1800 meters); base 4500 feet (1400 meters) | $\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$ | a. 8. Cumulus |
| Clouds of diurnal ascending currents | $\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$ | top 10,000 to 260,00 feet (3000 to 8000 meters); base 4500 feet (1400 meters) | $\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$ | b. 9. Cumulo-nimbus |
| High fogs, less than 3500 feet (1000 meters) | High fogs, less than 3500 feet (1000 meters) | High fogs, less than 3500 feet (1000 meters) | High fogs, less than 3500 feet (1000 meters) | 10. Stratus |
1. Cirrus (Ci). —Detached clouds of delicate and fibrous appearance, often showing a featherlike structure, generally of a whitish color. Cirrus clouds take the most varied shapes, such as isolated tufts, thin filaments on a blue sky, threads spreading out in the form of feathers, curved filaments ending in tufts, sometimes called cirrus uncinus, etc.; they are sometimes arranged in parallel belts which cross a portion of the sky in a great circle and, by an effect of perspective, appear to converge toward a point on the horizon, or, if sufficiently extended, towards the opposite point also (Ci-St and Cu-Ci, etc., are also sometimes arranged in similar bands).
Cirrus clouds moving from the southwest indicate falling temperature; moving from the northwest they indicate the probability of rising temperature. They are the mares’ tails and cattails of sailors’ cant. Near the horizon, cirrus clouds may have a stratiform appearance.
Weed photo.
Tufted cirrus (top); cirro-cumulus (bottom) Mount Weather, Va.
​2. Cirro-stratus (Ci-St).—A thin, whitish sheet of cloud, sometimes covering the sky and giving it only a milky appearance; it is then called cirro-nebula—at other times presenting more or less distinctly a formation like a tangled web. This sheet often produces halos around the sun and the moon.
This name is apt to be misleading to observers who have followed the old nomenclature. It applies not so much to the striated or banded cirri as to the whitish, or creamy, haze with banded or feathery edges. Frequently it appears as a whitish bank, with here and there a web of tangled fibers; at times it covers the whole visible sky. The halo produced when a cirro-stratus film is in front of the moon is varied in form. Occasionally mock moons, paraselenae, are formed; so also are the light pillar and the “heavenly cross.”
Cirro-stratus clouds have long been associated with approaching stormy weather, and tradition seems to be borne out by investigation. The name cirrus haze is sometimes applied to cirro-nebula.
3. Cirro-cumulus (Ci-Cu).—Mackerel Sky.—Small globular masses or white flakes without shadows, or showing very light shadows, arranged in groups and often in lines.
Cirro-cumulus clouds are not always distinguishable from alto-cumulus clouds. They are much higher, however, and the arrangement usually possesses a geometric regularity. C. F. Brooks describes them as “small white flakes or tenuous globular masses which produce no diffraction colors when covering the sun or the moon.”
4. Alto-stratus (A-St).—A thick sheet of gray or bluish color, sometimes forming a compact mass of dark gray color and fibrous structure. At other times the sheet is thin, resembling thick Ci-St; and through it the sun or the moon may be seen dimly, gleaming as through ground-glass. This form exhibits all the changes peculiar to Ci-St, but it is about one-half as high.
It is not always easy to distinguish alto-stratus from cirro-stratus clouds. One cannot always estimate its altitude and, if the cloud is thin, it may be about as white as a cirro-stratus formation. The lower edge may be undulate, but it is hardly
Ellerman, photo.
Turreted alto-cumulus below developing into thunder-heads, curro-cumulus above. Cirro-stratus at right center. Mount Wilson, Cal.
​mammillate in the manner of mammato-cumulus clouds. The fibrous alto-stratus is composed of snow crystals. It does not cause halos. The compact form is composed of water droplets and may cause coronas. Alto-stratus clouds indicate varying conditions of moisture and quiet air, rather than definite weather conditions. Nevertheless rain and snow may fall from them.
5. Alto-cumulus (A-Cu).—Large globular masses, white or grayish, partly shaded, in groups or lines, and often so closely packed that their edges appear confused. The detached masses are generally larger and more compact (resembling St-Cu) at the center of the group, but the thickness of the layer varies. At times the masses spread themselves out and assume the appearance of small waves, or thin, slightly curved plates. At the margin they form into finer flakes (resembling Ci-Cu). They often spread themselves out in lines in one or two directions.
It is evident that the observer will record alto-cumulus as cirro-cumulus and vice versa; many times a description of either will fit the other. Fortunately such an error is harmless.
6. Strato-cumulus (St-Cu).—Large globular masses or rolls of dark clouds often covering the whole sky, especially in winter. Generally St-Cu presents the appearance of a gray layer irregularly broken up into masses of which the edge is often formed of smaller masses, often of wavy appearance resembling A-Cu. Sometimes this cloud-form presents the characteristic appearance of great rolls arranged in parallel lines, and pressed close against one another. In their centers these rolls are dark in color. Blue sky may be seen through the intervening spaces, which are much lighter in color. (Roll-cumulus in England, Wulst-cumulus in Germany.) Strato-cumulus clouds may be distinguished from Nb by their globular, or their roll appearance and by the fact that they are not generally associated with rain.
Strato-cumulus clouds usually follow a winter storm, covering the sky during the filling of a low barometer. The foregoing description is sufficiently plain and clear to indicate the character and appearance of strato-cumulus clouds. If they are high enough, however, they may be mistaken for alto-cumulus. In such a case it might be correct to call them alto-cumuli. Close to the horizon, strato-cumulus clouds resemble the normal stratus clouds at times, but they are much higher. Pretty nearly every transition between strato-cumulus and alto-cumulus clouds may be observed.
7. Nimbus (Nb).—Rain Clouds.— A thick layer of dark clouds, without shape and with ragged edges, from which steady rain or snow usually falls.
​A bank of cirro-stratus clouds in the west is apt to be the advance of a cyclonic storm. By the time the advancing clouds have reached the eastern sky, the storm is close at hand. Undulated alto-stratus clouds form under the
Ellerman, photo.
Thin, undulated alto-stratus forming above a fog, or stratus, Mount Wilson, Cal.
cirrus haze and these very shortly develop into rain clouds, or else are followed by them. A winter cyclonic storm may be likened to a cone with its apex tipped one hundred miles or more beyond its base.
​Through the openings in these clouds an upper layer of Ci-St or A-St may almost invariably be seen. If a layer of Nb separates into shreds in a strong wind, or if small loose clouds are visible floating under a large Nb, the cloud may be described as fracto-nimbus (Fr-Nb), the “scud” of sailors.
Inasmuch as the sky is almost always wholly overcast during a steady rain or snow, the ragged edges are rarely visible. The foregoing description is hardly true of tropical rain clouds with their sharp, greasy-appearing edges. The observer will not be in serious error in designating any low cloud from which rain is falling as nimbus. The flying scud, its top pointing with the wind, drops no rain. The breaking of a nimbus usually denotes the clearing of a storm; and although the scud is rainless, it is properly nimbus cloud matter though not “rain clouds.”
8. Cumulus (Cu), Wool-pack Clouds.—Thick clouds of which the upper surface is dome-shaped, and exhibits protuberances while the base is horizontal. These clouds appear to be formed by a diurnal ascensional movement which is almost always noticeable. When the cloud is opposite the sun the surfaces facing the observer have a greater brilliance than the margins of the protuberances. When the light falls aslant, as is usually the case, these clouds throw deep shadows; when, on the contrary, the clouds are on the same side of the observer as the sun, they appear with bright edges.
True cumulus has well-defined upper and lower limits, but in strong winds a broken cloud resembling cumulus is often seen, in which the detached portions undergo continual change. This form may be distinguished by the name fracto-cumulus (Fr-Cu).
The cumulus cloud with its flat base and rounded dome is so full of character that the foregoing description is ample. It is the summer cloud of the temperate zones and the shower cloud of the tropics. To the unaided eye the constant motion of the cloud matter is apparent; with a field glass the convectional motion is plainly visible in the larger clouds. The cumulus is an “ascensional” cloud, because the water vapor is carried upward until cooling brings about condensation. The condensed vapor sinks until it is again warmed to the temperature of vaporization.
9. Cumulo-nimbus (Cu-Nb), Thunder-cloud, Shower-cloud.—Heavy masses of cloud rising in the form of mountains, turrets, or anvils, generally surmounted by a sheet or screen of fibrous appearance (false cirrus) and having at its base a mass of cloud similar to nimbus. From the base local showers of rain or snow (occasionally of hail or soft hail) usually fall. Sometimes the upper edges assume the compact form of cumulus, and form massive peaks
Cumulo-nimbus cloud resulting from eruption of Vesuvius, Note the heavy rain falling from lower part of cloud.
​round which delicate “false cirrus” floats. At other times the edges themselves separate into a fringe of filaments similar to cirrus clouds. This last form is particularly common in spring showers.
The front of thunder-clouds of wide extent frequently presents the form of a large arc spread over a portion of a uniformly brighter sky.
The difference between the ordinary cumulus cloud and the cumulo-nimbus is mainly one of depth and intensity of motion within its mass. If condensation is so intense that its water content reaches the ground, the cloud is cumulo-nimbus. But Humphreys points out the fact that, in arid regions, where the ground is very warm, a well-developed thunder-head sinks until the excessive warmth vaporizes and scatters it. The aborted cumulo-nimbus has been observed by the author. On the other hand, a torrential shower may fall for a few minutes from a tropical cumulus cloud—shallow as to extent and without the angry-appearing cauliflower head of the ordinary cumulo-nimbus.
The fibrous mantle that hovers over the top of the cumulo-nimbus is a cloud of snow flakes. The cloud itself is usually, but not always, a thunder-storm. The observer may disregard all theoretical matters and record it as a cumulo-nimbus if rain is falling from its base. The marvelous photograph of a thunder-storm obtained by Lieutenant W. F. Reed, Jr., U.S.N. (p. 106), surpasses any verbal description of a thunder-storm.
The strong updraught caused by forest fires and burning strawstacks has resulted in the formation of cumulus clouds that still later developed into typical cumulo-nimbus shower clouds. The eruption of Vesuvius in 1872 created a series of cumulo-nimbus clouds with mammoth cauliflower heads. Torrential rains fell on the leeward side of the cinder-cone during a considerable time.
Experience has taught the airman that the cumulo-nimbus cloud is an object to be avoided; its beautiful exterior hides a generous accumulation of holes and bumps. The turbulent cumulus cloud has been called “the most treacherous wild beast of the air.”
10. Stratus (St). — A uniform layer of cloud resembling a fog, but not resting on the ground. When this sheet is broken into irregular shreds by the wind, or by the summits of mountains, it may be distinguished by the name fracto-stratus (Fr-St).
Weed, photo.
Thin alto-stratus (lower middle) merging into alto-cumulus. Dense alto-stratus (right), Mount Weather, Va.
​Tradition has made the long, flat cloud-streak near the horizon the type form of stratus cloud. But if that same cloud-streak were overhead it would appear merely as a low cloud covering more or less of the sky. When a fog lifts, it forms a stratus cloud; and if it floats away toward the observer’s horizon it becomes a long gray cloud streak. In the first case one is looking at the under side of the surface; in the second, at the edge. The components of a stratus cloud may be fog, smoke or dust—or even all three.
Qualifying Descriptive Terms.—Usage in the matter of descriptive terms is not uniform. The following have been suggested.
Fibrous, characteristic of streaks of falling rain or snow seen from a distance.
Smooth, characteristic of sheet-like clouds.
Flocculent, scaly, flaky, in small tufts (floccus, a tuft of wool).
Waved, or in rolls, characteristic of waves and windrows observable In billow clouds.
Round-topped, characteristic of the summits of clouds produced by rising currents.
Down-bulged, or round-holed, characteristic of the lower sides of clouds produced by down-draughts.
Ragged, characteristic of forming and of evaporating clouds in turbulent wind.
Recording Cloud Conditions.—The International Cloud Committee recommends the following instructions for the guidance of observers:
Kind or character.—Clouds may be designated by name, or by symbol, as Ci-St, for cirro-stratus. Where doubt exists, the number of the picture in the classification scheme should be designated.
Direction.—If the clouds are high the motion may be observed best by noting their position relative to a fixed object—a tree, or a flag-pole. Where the movement of the cloud is very slow, a rest for the head and shoulders may be necessary. The direction is best observed when clouds are near the zenith. The movement and direction of horizon clouds are apt to be decep- tive, giving to the observer an imperfect perspective. When possible, a nephoscope should be used if the direction is doubtful.
Atlas Photographique des nuages, Loisel.
Typical cumulus clouds of moist summer weather. The large cloud in the foreground is developing into a thunder-head. Note the wisps of false cirri.
Radiant Point of Upper Clouds.—Though apparently in radial position, streamers of cirrus clouds are actually parallel. The radial form is merely a perspective. The apparent point of convergence should be noted in the same manner as wind direction; as, se, or nw.
Undulatory clouds.—If the clouds show parallel and equidistant striations, such as suggest a succession of water waves, the direction of the striæ should be noted; and if more than one system of striæ appear, this fact should be noted.
Density and position of cirrus forms.—The cirro-stratus haze may become a dense bank of gray in its lower part. It is desirable that its density be recorded by a scale of intensity, 0 to 4; and also that the cardinal direction of the point of greatest density be noted. The gathering of cirrus clouds and the formation of a cirro-stratus bank is closely connected with cyclonic storms.
Unusual Cloud Forms.—Various cloud forms that are not readily classified are noted by every observer:
Billow clouds, or windrow clouds, are the same as the undulatory clouds noted in a previous paragraph. The name is derived from their wave-like form. Sometimes they are at cirrus height, and should be classed as cirrus clouds. For the greater part they form at lower altitudes. They are due to cross-winds in plane contact, the two differing in temperature and humidity.
Crest clouds frequently gather about the summits of snow-clad peaks. They are frequently observable shrouding the summits of Mounts Hood and Rainier. On even a grander scale they envelop the summits of Popocatepetl and Ixtaccihuatl, during periods of still air. When a moist wind blows against snow-clad peaks, a stream of condensed moisture flows from the leeward side, forming a banner cloud. The so-called “smoking” of Mounts Hood and Rainier is a cloud banner of this sort.
Mammillated, or mammato-cumulus clouds, are globular projections from the under side of thunder-heads. They usually accompany thunder-storms, hailstorms and tornadoes. A similar waviness, very strong in character, is sometimes observable in the bands of cirro-stratus clouds near to the horizon.
Scarf clouds are the feathery wisps that sometimes form at the summits of cumulus clouds, especially those of the storm type. They seem to increase in size as the turbulence within the cumulus cloud increases, and sometimes appear like a coverlet over its top.
Observatorio del Ebro, Spain.
Heavy strato-cumulus broken by a strong wind. Note the wisps and curls on lower edges formed by wind eddies.
​Various other terms such as lenticulate, maculate, flocculent, and castellate, are used by observers. Any descriptive term which conveys a definite meaning is permissible in recording cloud observation.
Cloud Heights.—Bigelow’s measurements of cloud heights are somewhat greater than those determined by the International Cloud Committee, due to the fact that the measurements were made in a lower latitude. The table (p. 286), taken from Hahn’s Lehrbuch der Meteorologie, shows that the altitudes of the various cloud levels increase from polar to equatorial regions.
The level of each type of cloud is a level of maximum cloudiness; between cloud levels are levels of minimum cloudiness. The airman may find that neither Dr. Bigelow’s figures nor those of the International Committee apply to the locality in which his flights are made; but the altitudes of maximum and of minimum cloudiness for any locality are not far from the figures noted and are roughly proportioned. The airman will find also that the various cloud regions are thicker as one approaches equatorial latitudes. The lowest level of minimum cloudiness is that between “scud” cloud height and the base height of stratus clouds—from 300 feet to 1200 feet.
The thunder-head excepted, the lower clouds are shallow; but they vary greatly in depth. A mean of 10,000 feet (3000 meters) may be approximately correct for their depth, but it is unsafe as an estimated depth at any one time. The fact that the highest mountain peaks of the United States are snow-capped shows that precipitation occurs at an altitude of about 15,000 feet; and the fact that observers in mountain regions are frequently above storm clouds is evidence that the cloud blanket may be materially less than 10,000 feet in thickness.
The Distribution of Cloudiness.—In the Pacific Coast region of the United States, cloudiness is more or less seasonal. Practically all the lower clouds are prevalent during the winter months—that is, during the rainy season. During the summer the lower clouds may be absent for weeks at a time. From a
Atlas Photographique des nuages, Loisel.
Mammato alto-stratus in the wake of a thunder-storm. Strato-cumulus in lower left side.
​camp in the Sierra Nevada Mountains overlooking the Sacramento Valley, clouds formed by dust, smoke and fog are sometimes the only ones visible for a considerable period.
Along the Atlantic Coast and the Mississippi Valley a cloudless sky for more than one or two days is unusual; for a period of three days it is very rare. During the moist periods of midsummer a very thin cloud veil may prevail for a week or more at a time. The cloud veil, technically a “haze,” is too thin to obscure the sun visibly, but it is dense enough to prevent radiation to a considerable extent. During the period when the cloud veil prevails, the night temperature is from 5 to 10 degrees higher than at other times, and the amount of insolation is almost always lowered.
Arizona, southern California and southern Nevada constitute the region of minimum cloudiness in the United States. In this region cloudless skies may persist for a month or more. The observer who watches the hygrometer closely will acquire not a little useful information on clouds and their relation to atmospheric moisture. No part of meteorology is more fascinating than the study of clouds, and none is more important in forecasting weather changes.
Ellerman, photo.
Alto-cumulus clouds. Note the shadows in the center of each mass. At a lower altitude these clouds would be strata-cumulus near the horizon, and fracto-cumulus nearer the zenith. Mount Wilson, Cal.