Cumulus cloud

“ When the cloud and the sun are on opposite sides of the observer, the surfaces facing the observer are more brilliant than the margins of the protuberances. When, on the contrary, it is on the same side of the observer as the sun, it appears dark with bright edges. When the light falls sideways, as is usually the case, cumulus clouds show deep shadows. True cumulus has well-defined upper and lower margins; but one may sometimes see ragged clouds, like cumulus torn by strong wind, of which the detached portions are continually changing ”

“ 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 cumulus clouds therefore indicate, as shown in Chart 1, that the air is rising vertically in certain layers, while drifting eastward, and the stratus that it is moving horizontally with a different velocity in adjacent strata. The lowest stratus clouds are elevated fog; the strato-cumulus is about two miles high; the altostratus averages four miles and the cirro-stratus six miles above the earth.
There is furthermore a stratification of the vapor contents of the atmosphere within every high cumulus cloud, which is interesting. ”

“ The vapour of the lower atmosphere, at this elevation, is condensed, or rendered visible. In fog the dew point is at the surface of the earth; in summer it may be several thousands of feet above. The Cumulus cloud forms from below. The invisible vapour of the lower atmosphere is condensed, parts with its thousand degrees of latent heat, which rush upwards, forcing the vapour into the vast hemispherical heaps of snowy, glittering clouds, which, seen in midday, appear huge mountains of clouds; the "cloud-land" of the poet, floating in liquid air. The Cumulus cloud is ever changing in form. ”

“ Each cumulus must be looked upon as simply the visible top of an ascending pillar of damp air. The vapour which makes its appearance in the cloud is present in the transparent air beneath, and the base of the cloud is simply the level at which that vapour begins to condense into visible liquid particles. Since cumulus clouds are caused by ascending currents, these currents must be brought about either by the general disturbance of the air due to a cyclonic movement, or by the local irregularities of temperature on the ground produced by the sun’s heat. ”

“ The rapid changes of temperature in summer and the sudden rising of vast masses of heated air produce cloud-structures of the most imposing description, especially huge, irregular cumulus clouds that float in equilibrium above us like colossal icebergs, airy mountain-ranges or tottering battlemented towers and "looming bastions fringed with fire." ”

“ More recent meteorological research has established that in many cumulus forms the entire mass is in constant rotation, although seen from outside, it appears as a stable, unvarying shape. Nowhere in nature may the supremacy of form over matter be so vividly observed as in the cumulus cloud. And the forms of the cumuli themselves tell us in manifold metamorphoses of a state of equilibrium between expansive and contractive tendencies within the atmosphere. ”

“ The appearance of the cumulo-stratus, among ranges of hills, presents some interesting phenomena. It appears like a curtain dropping among them and enveloping their summits; the hills reminding the spectator of the massy Egyptian columns which support the flat-roofed temples of Thebes. But when a whole sky is crowded with these clouds, and the cumulus rises behind them, and is seen through the interstices, the whole, as it passes off in the distant horizon, presents to the fancy mountains covered with snow, intersected with darker ridges, lakes of water, rocks and towers. ”

“ Saturated air cools as it rises more slowly than dry air, consequently the upward motion is maintained through the cloud mass, causing the swelling up of the tops of the cumulus clouds, which reach their highest development in the thunder clouds, or [Pg_40] cumulo-nimbus, as they are called. The lower limit of the cloud region is determined therefore by the height at which the rising currents reach their dew-point, and the altitude of the cloud formation depends upon the humidity of the ascending current, the drier it is, so much the higher must it rise to have its vapour condensed. ”

“ Soon patches of lower cloud appear, at first hazy and indistinct, but gradually shaping themselves into cumulus with hazy base and rounded summits. These rapidly assume the typical pyramidal shape, with level base and sharply contoured top, and so far there is little to distinguish them from an ordinary cumulus (see Plate 48) . But watch them carefully. Here and there some will be growing taller than their fellows, and as they grow their rate of growth increases until the top begins to show signs of spreading outwards. ”

“ The true cumulus has clear superior and inferior limits. It is often broken up by strong winds, and the detached portions undergo continual changes. These altered forms may be distinguished by the name of Fracto-cumulus.
9. Cumulo-nimbus (Cu.-N.) ; The Thunder-cloud; Shower-cloud.—Heavy masses of clouds, rising in the form of mountains, turrets or anvils, generally having a sheet or screen of fibrous appearance above (false cirrus) and underneath, a mass of cloud similar to nimbus. From the base there generally fall local showers of rain or snow (occasionally hail or soft hail) . ”

“ All the cumulus clouds, by which we mean to include alto-cumulus and cirro-cumulus, are most frequent when the levels of condensation are rising, while the stratiform clouds are an indication of no vertical movement or of active descent. Pure cirrus is indicative rather of movement in a horizontal direction, and may occur when the condensation levels are stationary, or when they are rapidly changing either way. ”

“ CUMULO-NIMBUS Grandest of all clouds are the huge mountains of vapour which are the parents of summer thunder-storms. They are at once distinguished from ordinary cumulus by their upper parts, which sometimes reach beyond the region of the alto clouds high into the realm of cirrus, and extend outwards as a broad disc, which is occasionally indistinguishable from the cirro-nebula and cirro-stratus which form the van of a cyclone cloud canopy. ”

“ It may also be a contributing cause in producing the sharply defined upper surfaces of some cumulus or strato-cumulus clouds, but these are in the main most certainly due to the chief cause of cloud production—namely, what is known as dynamic cooling.
If a quantity of air exists under a certain pressure and at a certain temperature, on reducing the pressure it will expand, and in the act of expanding it will become cooler. ”

“ The strato-cumulus clouds that follow an anticyclone also indicate a lid. The high fog that completely covers the sky at heights varying from 7000 to 10,000 feet—practically a stratus or an alto-stratus cloud—is a lid. Cross-winds at a very considerable height likewise may indicate a lid. ”

“ Air compressed into a machine and permitted to escape will form ice.
Fig. 717.—Cumulus cloud.
Water is present in clouds which assume very fantastic and beautiful forms. We know nothing more enjoyable than to sit watching the masses of cumuli on a fine afternoon. The grand masses built up like the Alps appear to be actual mountains, and yet we know they are but vapour floating in the air, and presently to meet with clouds of an opposite disposition, and produce a thunderstorm with torrents of rain. ”

“ The whole of the moisture-laden layer of the air would be snugly concealed under the cloud-layer, and would exercise no absorptive action whatever on the light which the remote observer would examine. If from the upper surface of the layer of cumulus clouds aqueous vapour rose still higher, and were converted in the cold upper regions of the atmosphere into clouds of ice-crystals, the distant observer would have still less chance of recognizing the presence of moisture in our atmosphere. ”

“ While the former condition in itself does not indicate an approaching storm, it is instructive in connection with other local indications.
A sky overcast with high clouds does not indicate stormy conditions if the clouds remain high. If the pressure falls and the clouds lower, stormy weather may be expected.
Rapidly increasing cumulus clouds indicate thunder-storms. A thunder-head or high cauliflower top to a cumulus cloud denotes a rapid updraught, which in itself is the beginning of a thunder-storm. ”

“ The place of fog is in winter taken by the fine snow crystals, which often darken the air like fog when strong winds raise the dry snow from the surfaces on which it is lying. Cumulus cloud forms are rare, even in summer, and it is doubtful whether the cloud occurs at all in its typical development. Stratus is probably the commonest cloud of high latitudes, often covering the sky for days without a break. Cirrus cloud forms probably decrease polewards.
Cyclones and Weather.—The prevailing westerlies continue up into the margins of the polar zones. ”

“ It is, whether there be, or not, an eastern progression of the body of the atmosphere above the machinery of distribution. I have thought there was, and that in set fair weather I had seen a peculiar kind of cirro-cumulus cloud, in patches, the small cumuli very distinct and rounded, moving due east, which indicated such a current. But I am not satisfied, from my own observation, that it is so, nor is it easy to determine the question. The moisture of evaporation rarely, if ever, ascends to any considerable [Pg 203] elevation, and the upper strata must be very dry. ”

“ Cumulus clouds develop strongly below the one-mile level, and above them it is possible to get the cloud motions in the higher levels only through the more or less occasional rifts in the lower cloud sheets. It is therefore very desirable that an extensive campaign of theodolite measurements of cloud motions in the upper levels of cyclones be instituted, in order to carry out much more fully the details of the discussion which have been suggested in this fundamental research. ”

“ They produce some very striking sky effects just before sunrise as they catch the sun’s rays aloft. An hour or two after sunrise they disappear and small cumulus clouds begin to form. These grow rapidly as the winds begin and by afternoon become bulky and numerous. In the wet season they grow into the nimbus and stratus types that precede a sudden downpour of water or a furious hailstorm. This is best seen from the base of a mountain range looking towards the crest, where the cloud-and rain-making processes of this type are most active.
UNUSUAL WEATHER PHENOMENA, SANTA LUCIA, 1913-14
Fig. ”

“ There is a peculiarly warm sensation in the centre of a cumulus cloud, especially when it is not dense. A glow of heat seems to radiate from all points. Yet the face and hands are quite dry, and exposed objects are not wetted ”

“ As the Cumulus belongs to the day, so does the Stratus to the night. It is the lowest of all the clouds, and actually rests upon the earth, or the surface of water. It is of variable extent and thickness, and is called Stratus, a bed or covering. It is generally formed by the sinking of vapour in the atmosphere, and on this account has been p. 85called Fall-cloud. It comprehends all those level, creeping mists, which, in calm evenings, spread like an inundation from the valleys, lakes, and rivers, to the higher ground. ”

“ The Cumulus is another form of cloud, which floats along in fleecy masses, in the days of summer, but dissolves at night. Sometimes it resembles a great stack or pile of snow, sometimes it has a silvery or a golden edge, as if we saw a little of the lining. Sometimes they lie motionless in the distance, and are mistaken by mariners for land. They rest upon a large base, and are borne along by surface-winds. Their greatest height is not more than two miles. They carry large quantities of moisture with them, and, when preceding rain, fall rapidly into other shapes. ”

“ Between the flocks of small countless clouds which occupy the highest heavens, and the gray undivided film of the true rain-cloud, form the fixed masses or torn fleeces, sometimes collected and calm, sometimes fiercely drifting, which are, nevertheless, known under one general name of cumulus, or heaped cloud. ”

“ The cyclone cumulus may come at any hour of the day or night, though comparatively rare between midnight and the morning. Heat cumulus is generally formed during the afternoon, and it is only under relatively uncommon conditions that it [86] persists during the night. If the cloud has not grown to very great size it usually begins to break up and disappear about sunset, but if it has grown to the enormous dimensions of a summer thunder-cloud it may go on growing, piling mass on to mass, until it generates a thunderstorm, even in the hours of early morning. ”

“ The sea of cumulus, as in the case of similar cloud-formations of most other isolated mountains, when viewed from above, as from the mountain-top, presents a cloud-field of dazzling whiteness, sparkling in the sun. ”