Explanatory remarks and special clouds

(Section 2.3.9.8)

Cumulus develops:

• In convection currents resulting from heating near the Earth’s surface
• Owing to cooling or advection of cold air (instability) in higher layers
• From lifting of air layers where vertical expansion results in cooling

The characteristics of Cumulus clouds depend essentially on their vertical extent; that is, the vertical distance between their base and the stable layer that has inhibited vertical development. 

The degree of stability and thickness of the stable layer determine how effective this layer is at inhibiting vertical development. When it is very stable, for example, a strong inversion, the tops of Cumulus clouds will spread out, forming Stratocumulus cumulogenitus or Altocumulus cumulogenitus. When it is stable but not very thick, the spreading out of the tops of Cumulus clouds may be only in parts or momentary, and then some tops may penetrate it.

Comments on vertical extent:

• When Cumulus have great vertical extent (a low base and a high stable layer), the Cumulus is of the species congestus; the vertical extent of Cumulus in tropical regions (when not under the influence of a trade wind inversion) is generally much greater than elsewhere
• When Cumulus have moderate vertical extent (the base and stable layer are reasonably close together), Cumulus mediocris tops may spread out, forming either Stratocumulus or Altocumulus
• When Cumulus have small vertical extent (the base and stable layer are very close together), Cumulus have a flattened appearance (Cumulus humilis); they may even spread out in their entirety into Stratocumulus or Altocumulus
• Cumulus may dissipate during the day as the surface air temperature rises and the Cumulus base rises until its height exceeds, sometimes considerably, that of the stable layer
• When there is no vertical extent (the stable layer is lower than the level where sufficient cooling had occurred for condensation to take place), Cumulus may be present, but only if there is a mechanism forcing air to rise to reach the level where condensation could have occurred; orographic lift is an example of a forcing mechanism.

Diurnal variation in Cumulus activity:

• Is generally pronounced over land. 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 midafternoon, the Cumulus activity decreases, and finally the clouds disappear in the late afternoon or early evening.
• Is so small over the open oceans that its existence is sometimes doubtful. When it exists, maximum Cumulus activity appears to occur in the late hours of the night.
• Has Cumulus forming over the land by day in connection with the sea breeze and over the sea by night in connection with the land breeze, near coasts.

With regard to the illumination of well-developed Cumulus:

• When the 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 differences in luminance
• When the Cumulus is illuminated from the side, it shows strongly contrasted shading
• When the Cumulus is illuminated from behind, it appears relatively dark, with an extremely brilliant border (every cloud has a silver lining)
• When the Cumulus is against a background of cirriform (ice) clouds and away from the horizon, it appears a little less white than the cirriform clouds and its margins appear grey, even when it is directly illuminated by the Sun  

Whatever the illumination of the Cumulus may be, its base is generally grey.

Cumulus may develop in convection initiated by heat from forest fires, wildfires or volcanic eruption activity. It will be classified by any appropriate species, variety and supplementary feature, followed by “flammagenitus” (for example, Cumulus congestus flammagenitus).

Cumulus may develop as a consequence of human activity such as forming in convection initiated above power station cooling towers. When it is clearly observed to have originated as a consequence of human activity, it will be classified by any appropriate species, variety and supplementary features followed by “homogenitus” (for example, Cumulus mediocris homogenitus).

Cumulus may develop locally in the vicinity of large waterfalls as a consequence of water broken up into spray by the falls. It will be classified by any appropriate species, variety and supplementary feature, followed by “cataractagenitus” (for example, Cumulus mediocris cataractagenitus).