Classification of cloud type is based primarily on the shape or form of the cloud. The basic forms of cloud are stratiform, cumuliform and cirriform.
Stratiform cloud, see Figure 10.7, is a layered type of cloud of considerable horizontal extent, but little vertical extent.
Figure 10.6 The cloud tops may be predicted from graphs showing the ELR and the SALR.
Figure 10.7 Stratiform Cloud.
Cumuliform cloud is heaped cloud, displaying a marked vertical extent, of greater or lesser degree. (See Figure 10.8.)
Cirriform cloud is a cloud which is fibrous, wispy or hair-like in appearance. This type of cloud is found only at high levels in the Troposphere.
Clouds are also identified by reference to the height at which they occur. There are 3 distinct cloud levels within the troposphere.
Low-level clouds are those which are found between the surface and 6 500 ft.
These clouds may be stratus, stratocumulus, cumulus and cumulonimbus. (The suffix nimbus implies “rain bearing”.) However, cumulus and cumulonimbus will have significant vertical development and will extend from low level to higher levels.
Cumulonimbus clouds may even extend up to the Tropopause and, in the tropics, possibly above it if they have enough vertical energy.
Medium-level clouds are found between 6 500 feet and 23 000 feet.
Figure 10.8 Cumuliform Cloud.
Figure 10.9 Cirriform Cloud.
Clouds are classified initially by their shape or form.
Stratiform cloud is layered cloud with little vertical extent.
Cumuliform cloud is heaped cloud with a large vertical extent. Cirriform cloud is wispy and fibrous, being present only at high level.
CHAPTER 10: CLOUDS AND PRECIPITATION
The names of medium-level clouds are characterised by the prefix “alto-”: such as altostratus and altocumulus. Nimbostratus is also a medium-level cloud, but it may also extend into both the lower and upper levels of the atmosphere.
High-level clouds are generally found between 16 500 feet and the Tropopause. The names of high-level clouds are prefixed by “cirro-”: cirrostratus, cirrocumulus, and cirrus. (Latin cirrus means curl.)
Stratus.
Stratus (from Latin stratum, meaning layer) is generally a grey, layered cloud with a fairly uniform base, which may produce drizzle, or light snow. Stratus cloud is generally no more than 1 000 - 1 500 ft thick, and is often much thinner. Stratus is usually the lowest of all cloud types. The main hazard associated with stratus is that it often covers high ground, concealing hill tops from pilots. When stratus is at its thinnest, the sun can be clearly seen through the stratus layer.
Figure 10.10 Low-Level Cloud:- Stratus (St), Stratocumulus (Sc), Cumulus (Cu), Cumulonimbus (Cb).
Medium-Level Cloud :- Altostratus (As), Altocumulus (Ac), Nimbostratus (Ns).
High-Level Cloud:- Cirrostratus (Cs), Cirrocumulus (Cc), Cirrus (Ci).
Clouds are also classified by the level at which they
occur. Low-level cloud occurs between the surface and 6 500 ft, medium-level cloud between 6 500 ft and 23 000 ft, and high-level cloud from 16 500 ft to the Tropopause.
Figure 10.11 Stratus (St).
Nimbostratus.
Nimbostratus, see Figure 10.12, is a dense, dark-grey, rain-bearing, stratiform cloud, producing extensive and long-lasting precipitation.
Cumulus.
Cumulus cloud, see Figure 10.13, is the most common form of convective cloud, being classified as heaped cloud, from Latin cumulare meaning to heap up. For glider pilots, a developing cumulus is regarded as a reliable indication of the presence of thermal upcurrents which, if skilfully exploited, can enable the glider to gain height.
Pilots of light aircraft, on the other hand, will note that, on a day when the sky is peppered with fine-weather cumulus, as depicted in Figure 10.13, flight below cloud base is turbulent, whereas, above the cloud tops, the air is likely to be very smooth.
A developed cumulus cloud is generally dense, with sharp outlines. As it continues to develop vertically, a cumulus cloud forms mounds, domes or towers, of which the upper parts often resemble the head of a cauliflower. The sunlit parts of cumulus clouds are brilliant white, but their bases are relatively dark.
Cumulus clouds of small vertical development can appear benign, but they can grow rapidly when the atmosphere is unstable, with no upper-air inversion, and may develop into cumulonimbus clouds, with their tops reaching the Tropopause.
Figure 10.13 Fine-weather cumulus (Cu).
Figure 10.12 Nimbostratus (NS).
Flight below the base of cumulus cloud is often turbulent, while above the cloud tops, conditions are smooth.
CHAPTER 10: CLOUDS AND PRECIPITATION
Stratocumulus.
Stratocumulus cloud, see Figure 10.14, is probably the most common form of cloud in the skies of the United Kingdom. It appears grey, or whitish, but usually always has distinct dark parts. Stratocumulus can be seen as patches, or in a continuous layer. Stratocumulus is usually no more than 2 000 to 3 000 feet thick, but may become 5 000 to 6 000 ft deep in certain conditions.
Cumulonimbus.
Cumulonimbus clouds, see Figure 10.15, are clouds that the aviator should avoid.
Cumulonimbus clouds consist of vigorous convective cloud cells of great vertical extent. In the later stages of their development, cumulonimbus clouds display a characteristic anvil top, as the upper part of the cloud hits the Tropopause. The upper parts of a cumulonimbus cloud often consist of supercooled water droplets and ice crystals. The base of cumulonimbus clouds is often very dark, with ragged cloud appearing beneath the main cloud cell.
The risk of icing and turbulence associated with cumulonimbus is always severe.
Within cumulonimbus, very strong upcurrents and downdraughts are continually at play, producing severe precipitation in the form of heavy showers of rain and hail.
Other hazards associated with cumulonimbus are lightning and static discharge, which may lead to airframe damage and erroneous instrument readings.
Figure 10.14 Stratocumulus (SC).
Figure 10.15 Cumulonimbus (CB).
The risk of icing and turbulence is always severe
in, and in the proximity of, cumulonimbus.
Moist unstable air throughout a deep layer of the atmosphere is necessary for the formation of cumulonimbus cloud. A ‘trigger’ to set off the lifting process is also required, and could be convection, frontal or orographic lifting or convergence.
Altocumulus.
Altocumulus, see Figure 10.16, takes the form of speckled white or grey cloud.
The patches of cloud appear as rounded masses of fibrous or diffuse aspect.
Altocumulus usually occur between 8 000 and 15 000 ft with tops on some occasions as high as 23 000 ft.
There are two forms of altocumulus which are of particular significance, namely: altocumulus lenticularis and altocumulus castellanus; both types are illustrated in Figure 10.17.
Altocumulus lenticularis, also known as lenticular cloud, is found downwind of mountainous or hilly areas, and is indicative of the presence of mountain wave activity.
(See Chapter 9.) Because of its position downwind of high ground, moderate or even severe turbulence may be found beneath altocumulus lenticularis. However, the air in the lenticular clouds, themselves, is always smooth.
Altocumulus castellanus is a “bubbly” form of normal altocumulus. The “towers”
that form in altocumulus castellanus are like battlements on castles, hence the name. These clouds are significant because they often herald a change to showery, thundery weather and are a feature of summer weather in temperate latitudes.
Cumulonimbus clouds sometimes develop from altocumulus castellanus, when instability is present at medium levels of the Troposphere.
Figure 10.17 Altocumulus Lenticularis and Altocumulus Castellanus.
Figure 10.16 Altocumulus (AC).
Altocumulus lenticularis are signposts of the presence of mountain wave.
In the lee of mountains, below altocumulus lenticularis, severe turbulence may be encountered, sometimes marked by rotor cloud.
CHAPTER 10: CLOUDS AND PRECIPITATION
Altostratus.
Altostratus (Figure 10.18) is a grey or bluish sheet, or layer of cloud, which can be fibrous or uniform in appearance.
Sometimes, altostratus covers the whole sky, giving a “ground glass” effect around the Sun or Moon.
Altostratus is usually found between 8 000 and 15 000 feet and can be from around 2 000 to 8 000 feet thick. But despite its thickness, altostratus is not a dense cloud, and the sun is usually perceptible through the cloud layer.
Cirrus.
Cirrus (from Latin cirrus, meaning curl) is the highest of all the cloud types and is composed entirely of ice crystals. Cirrus clouds take the form of white delicate filaments, in patches or narrow bands. They may also be described as fibrous or hair-like. Cirrus clouds can be found between 16 500 and 45 000 ft. They often herald the approach of a warm front. (See Figure 10.19.)
Cirrostratus.
Cirrostratus (see Figure 10.20) is a transparent, whitish cloud-veil of fibrous or smooth appearance, totally or partially covering the sky.
Cirrostratus is made up of ice crystals, and lies between 18 000 and 45 000 ft.
Cirrostratus is a further warning of an approaching frontal system, and, like altostratus, may cause the Sun and Moon to appear with a halo.
Figure 10.18 Altostratus (As).
The Sun or Moon may be visible through altostratus,
appearing with a halo round it.
Figure 10.19 Cirrus (CI).
“nimbo” or “nimbus” meaning rain-bearing, “alto” which indicates “middle level” and, lastly, “cirro” which indicates
“high level”.
Cirrocumulus.
Cirrocumulus (see Figure 10.21) is probably the cloud which is least often seen in the sky. Cirrocumulus is a thin, white and patchy layer of cloud, with ripples, more or less regularly arranged.
Cirrocumulus consists of ice crystals and is generally found between 20 000 and 30 000 feet.