Detached clouds, generally dense and with sharp outlines, developing vertically in the form of rising mounds, domes or towers, of which the bulging upper part often resembles a cauliflower. The sunlit parts of these clouds are mostly brilliant white; their base is relatively dark and nearly horizontal. Sometimes Cumulus is ragged.
MAIN DIFFERENCES BETWEEN CUMULUS AND SIMILAR CLOUDS OF OTHER GENERA
Cumulus is distinguished from most Altocumulus and Stratocumulus by the fact that Cumulus clouds are detached and dome-shaped. When viewed from a distance, Cumulus clouds may appear merged, owing to the effect of perspective; in this case, they should not be confused with Altocumulus or Stratocumulus.
Cumulus tops may spread and form Altocumulus cumulogenitus or Stratocumulus cumulogenitus. They may also enter or transpierce pre-existing layers of Altocumulus or Stratocumulus or they may merge with Altostratus or Nimbostratus. In all such cases, the appellation Cumulus should be used as long as the cumuliform clouds remain detached from one another or as long as they show a relatively considerable vertical extent. When a very large precipitating Cumulus cloud is directly above the observer, it may be confused with Altostratus or Nimbostratus. The character of the precipitation may then help in distinguishing Cumulus from the latter clouds; if the precipitation is of the showery type the cloud is Cumulus. Since Cumulonimbus generally results from the development and transformation of Cumulus, it is sometimes difficult to distinguish Cumulus with a great vertical extent from Cumulonimbus. The cloud should be named Cumulus as long as the sprouting upper parts are everywhere sharply defined and no fibrous or striated texture is apparent. If it is not possible to decide on the basis of other criteria whether a cloud is to be named Cumulus or Cumulonimbus, it should by convention be called Cumulus if it is not accompanied by lightning, thunder or hail. Cumulus fractus is distinguished from Stratus fractus by its generally greater vertical extent and its usually whiter and less transparent appearance. Cumulus fractus, furthermore, sometimes has rounded or dome-shaped tops, which are always lacking in Stratus fractus.
CLOUDS FROM WHICH CUMULUS MAY FORM
The formation of Cumulus is often preceded by the occurrence of hazy spots out of which the clouds develop. Cumulus may originate from Altocumulus (Cu altocumulogenitus) or Stratocumulus (Cu stratocumulogenitus). It may also form as a result of the transformation of Stratocumulus or Stratus (Cu stratocumulomutatus or Cu stratomutatus); the latter case frequently occurs in the morning over land. Cumulus fractus of bad weather is formed under Altostratus, Nimbostratus, Cumulonimbus or precipitating Cumulus (Cu fra altostratogenitus, nimbostratogenitus, cumulonimbogenitus or cumulogenitus).
Cumulus is composed mainly of water droplets. When of great vertical extent, Cumulus may release precipitation in the form of rain showers. Ice crystals may form in those parts of a Cumulus in which the temperature is well below 0˚ C; they grow at the expense of evaporating supercooled water droplets, thereby transforming the cloud into Cumulonimbus. In cold weather, when the temperature in the entire cloud is well below 0˚ C (32˚ F), this process leads to the degeneration of the cloud into diffuse trails of snow.
Cumulus develops in convection currents which occur when the lapse rate in the lower layers is sufficiently steep. Such steep lapse rates result from heating of the air near the earth’s surface; they also result from cooling or advection of cold air in the higher layers or, finally, from lifting of air layers with vertical expansion.
Over land, the diurnal variation in Cumulus activity is generally. pronounced. On clear mornings with the sun rapidly heating the surface of the ground, conditions are favourable for the formation of Cumulus. This formation may begin early when the lapse rate is steep and the relative humidity is high; it begins late if it occurs at all, when the lapse rate is small and the relative humidity is low. After having reached a maximum, usually in mid-afternoon, the Cumulus activity decreases and finally the clouds disappear in the late afternoon or early evening.
Over the open oceans, the diurnal variation of Cumulus is so small that its existence is sometimes doubtful, but when it exists, maximum Cumulus activity appears to oceur in the late hours of the night.
Near coasts, Cumulus may form over the land by day in connection with the sea breeze and over the sea by night in connection with the land breeze. The ascending motion of convection currents is slowed down or even stopped when these currents reach stable layers, particularly inversions. The characteristics of Cumulus clouds depend in the main on the vertical distance between the condensation level and the base of the stable layer, and on the degree of stability and the thickness of the stable itself. When the stable layer is very stable, it may cause the tops of Cumulus clouds which reach its level to spread out. When the layer is not very thick, the spreading out of the tops of Cumulus clouds may be only partial and some may penetrate it.
A low level of condensation and a high stable layer are favourable to considerable vertical development and, therefore to the formation of Cumulus mediocris or Cumulus congestus. When the level of condensation and the stable layer are close together, any Cumulus clouds which may form have a flattened appearance (Cumulus humilis); they may spread out, becoming either Altocumulus or Stratocumulus, both of which are often very persistent. It may happen that the level of condensation rises gradually in the course of the day until its height exceeds, sometimes that of the base of the stable layer; the Cumulus clouds then dissipate. Nevertheless, even when the height of the base of the stable layer is less than that of the condensation level, upwards convection currents may be able to enter the stable layer so that the ascending air may reach its level of condensation. This is one of the cases when the Cumulus formed belongs to the species humilis or, rarely, to the species mediocris.
Since the condensation level and the stable layer are usually much farther apart in the tropical regions than in other regions, the vertical extent of Cumulus in the tropical regions is generally much greater than elsewhere.
When a well-developed Cumulus is observed opposite the sun, the diffuse reflection of the sunlight falling on the surface of the cloud reveals the relief of the protuberances by very pronounced strongly differences in luminance. When illuminated from the side, Cumulus shows contrasted shading. When lighted from behind, the Cumulus appears relatively dark, with an extremely brilliant border. Against a background of ice clouds, not too near the horizon, Cumulus appears a little less white than these clouds and its margins appear grey, even when the Cumulus is directly illuminated by the sun. Whatever the illumination of the Cumulus may be, its base is generally grey.
SUPPLEMENTARY FEATURES AND ACCFSSORY CLOUDS
One or more of the following supplementary features and accessory clouds may be associated with Cumulus: pileus, velum, virga, praecipitatio (the precipitation occurs generally in the form of showers), arcus (rarely), pannus (rarely) and tuba (very rarely).