Separated clouds, generally dense and with sharp outlines, developing vertically in the form of rising mounds, domes or towers. The upper parts of larger Cumulus can resemble a cauliflower. The sunlit parts of these clouds are mostly brilliant white; their bases are relatively darker and nearly horizontal.
Small Cumulus clouds with very ragged edges and with outlines that are continuously undergoing rapid changes are known as Cumulus fractus. Cumulus fractus sometimes forms in or near precipitation from other cloud types. It is distinguished from Stratus fractus by its generally greater vertical extent and its usually whiter and less transparent appearance.
Very small, rather flattened and isolated Cumulus is the species Cumulus humilis.
Cumulus with a moderate vertical development is the species Cumulus mediocris.
Larger Cumulus is known as Cumulus congestus. If this species reaches a ‘great vertical extent’ it becomes known as Towering Cumulus for aviation reporting purposes.
Distinguishing Cu from other genera
Cumulus differs from Altocumulus and Stratocumulus in that Cumulus tops are dome-shaped and the bases are not merged; caution must be exercised when viewing Cumulus from a distance as the bases may appear merged due to the effect of perspective.
Cumulus humilis clouds never give precipitation.
Cumulus mediocris clouds generally give no precipitation.
Showers of rain or snow are possible with Cumulus congestus; precipitation is more common though when the cloud is of great vertical extent (Towering Cumulus).