Appearance of clouds

(Section 2.5.1.2)

The effect of perspective

Due to the effect of perspective, an observer flying at or near cloud level may see clouds as a fairly continuous layer, even if they are, in fact, detached.

Apparent width of cloud elements

While ground-based observers can distinguish between cloud genera (Cirrocumulus, Altocumulus, Stratocumulus) partly based on the apparent width of cloud elements, this criteria is of little use to an airborne observer. In some cases, the cloud altitude may be the only useful criterion to determine the genus.

Outline of clouds

As the airborne observer approaches clouds, their outlines appear less distinct and more ragged.

Base of clouds

The appearance of the cloud base changes with distance, becoming more diffuse and more ragged as the observer approaches. At close range, relief is difficult to distinguish; for example, the base of an opaque Altocumulus layer can appear very similar to that of an Altostratus.

Upper surface of clouds

Observations of the upper surface of clouds are very useful, because they provide indirect information about the degree of instability of the atmosphere.

Clouds of different genera can appear similar when viewed from above. This may make it difficult for an airborne observer to identify clouds from their upper surface.

The upper surface of clouds is usually better defined than their base. It may appear smooth or rough, clear cut or diffuse. It is also brighter and has greater variation in luminance.

The upper surface of a cloud layer may be flat or may have well-defined undulations of varying width. Undulations may have scales of 10 – 1 000 m (33 – 3 300 ft), suggestive of ocean waves. It may also have shallow rounded projections, bulges or domes, sometimes arranged in rows and with a fleecy appearance. Well-developed domes or towers may be visible, coming from within the layer or penetrating from below it. If there are many of these, it can become difficult to detect the surface from which they emerge. A cloud veil (velum) may cover the shallow domes or the sides of the well-developed towers. Occasionally, such veils are extensive enough and thick enough that the underlying clouds are partially or totally masked.