Observing clouds
A cloud is defined as a visible aggregate of minute particles of water or ice or both having its base above the surface. For observation clouds are classified by height and by the WMO ten cloud types or genera.
Classification by Height
1. High Clouds (5500m to 14000m)
Cirrus Ci
Cirrocumulus Cc
Cirrostratus Cs
These clouds are mainly composed of ice-crystals.
2. Middle Clouds (2000m to 7000m)
Altocumulus Ac
Altostratus As
These clouds are mainly composed of water droplets. Sometimes nimbostratus (Ns) is included in this group.
3. Low Clouds (less than 2000m)
Nimbostratus Ns
Stratus St
Stratocumulus Sc
These clouds are usually made up entirely of water drops. Sometimes cumulus (Cu) and cumulonimbus (Cb) are included with low cloud.
4. Low Clouds of Marked Vertical Extent
Cumulus Cu
Cumulonimbus Cb
These clouds have low cloud bases, but their tops grow high into the troposphere. They are mainly composed of water drops at their base, but change to ice crystals higher in the sky.
Origin of cloud names
Cirrus comes from the latin cirrus which means a lock of hair or a tuft of horsehair
Cumulus comes from the latin word cumulus which means an accumulation or a pile.
Stratus comes from the latin word stratus which comes from the verb sternere which means to spread out, to flatten out, to cover a layer.
Nimbus comes from the latin word nimbus which means rainy cloud
Alto comes from the latin word altum which means height
The World Meteorological Organisation (WMO) Classification
1. Cirrus (Ci)
"Detached clouds in the form of white, delicate filaments or white or mostly white patches or narrow bands. These clouds have a fibrous (hair-like) appearance, or a silky sheen, or both".
Cirrus clouds are composed of ice crystals. It appears in varied forms, isolated tufts, feather-like plumes or bands across the sky.
Cirrus clouds often evolve from the upper part of a Cumulonimbus.
2. Cirrocumulus (Cc)
"Thin, white patch, sheet of layer of cloud without shading, composed of very small elements in the form of grains, ripples, etc. merged or separate, and more or less regularly arranged, most of the elements have an apparent width of less than one degree".
(One degree is approximately the width of the little finger at arm's length). Cirrocumulus is composed, almost exclusively, of ice crystals. If any supercooled water droplets are present they are usually rapidly replaced by ice crystals. Cirrocumulus is not very common. It can form in clear air or result from a change in Cirrus, Cirrostratus or Attocumulus. Cirrocumulus lenticularis may be produced by orographic lifting.
3. Cirrostratus (Cs)
"Transparent, whitish cloud veil of fibrous (hair-like) or smooth appearance, totally or partly covering the sky, and generally producing halo phenomena".
Cirrostratus is composed mainly of ice crystals. It is never thick enough to prevent objects on the ground casting shadows, except when the sun is low. Sometimes the could is so thin that the halo is the only indication that cloud is present. Cirrostratus is formed as a result of the slow ascent of extensive layers of air to high levels such as at a warm front. It may also form by the spreading out of the anvil of a Cumulonimbus.
4. Altocumulus (Ac)
"White or grey, or both white and grey, patch, sheet or layer of cloud, generally with shading, composed of laminae, rounded masses, rolls etc. which are sometimes partly fibrous or diffuse and which may or may not be merged; most of the regularly arranged small elements usually have an apparent width between one and five degrees".
(Five degrees is approximately the width of three fingers at arms length). Altocumulus is composed of water droplets except at very low temperatures where ice crystals may form. Altocumulus may form as a result of the spreading out of cumulus or cumulonimbus clouds. Altocumulus lenticularis is usually produced by local orographic lifting. Altocumulus can also form due to convection at middle levels in the troposphere.
5. Altostratus (As)
"Greyish or bluish cloud sheet or layer of striated, fibrous or uniform appearance, totally or partly covering the sky, and having parts thin enough to reveal the sun at least vaguely, as through ground glass. Altostratus does not show halo phenomena".
Altostratus is composed of water droplets and ice crystals, the cloud also contains raindrops and snowflakes. Altostratus is usually so dense that the sun is hidden completely or can only be seen vaguely, as through ground glass. Altostratus is a precipitating cloud. The precipitation may be seen trailing from the base. Precipitation is usually of the "continuous" type and in the form of rain, snow or ice pellets. Altostratus is usually formed as a result of slow ascent of extensive layers or air.
6. Nimbostratus (Ns)
"Grey cloud layer, often dark, the appearance of which is rendered diffuse by more or less continuously falling rain or snow, which in most cases reaches the ground. It is thick enough throughout to blot out the sun. Low, ragged clouds frequently occur below the layer, with which they may or may not merge".
Nimbostratus is composed of water droplets (sometimes supercooled) and raindrops, of snow crystals and snowflakes or of a mixture of these liquid and solid particles. Nimbostratus produces rain, snow or ice pellets. Nimbostratus is most often formed as a result of the slow ascent of extensive layers of air.
7. Stratocumulus (Sc)
"Grey or whitish, or both grey and whitish, patch, sheet or layer of cloud which almost always has dark parts, composed of tessellations, rounded masses, rolls, etc. which are non-fibrous (except for virga) and which may or may not be merged; most of the regularly arranged small elements have an apparent width of more than five degrees".
Stratocumulus is composed of water droplets, sometimes accompanied by raindrops or snow pellets and, more rarely, by snow crystals and snowflakes. Stratocumulus is sometimes accompanied by precipitation of weak intensity in the form of rain, snow or snow pellets. Stratocumulus is frequently formed by the spreading out of cumuliform cloud.
8. Stratus (St)
"Generally grey cloud layer with a fairly uniform base, which may give drizzle, ice prisms or snow grains. When the sun is visible through the cloud, its outline is clearly discernible. Stratus does not produce halo phenomena except, possibly, at very low temperatures.
Sometimes stratus appears in the form of ragged patches".
Stratus is usually composed of small water droplets. At low temperatures it can consist of small ice particles. When dense it often contains drizzle droplets and sometimes ice prisms or snow grains. Stratus commonly forms as a results of turbulence and advection over a colder surface. It can also form due to the slow lifting of a layer of fog.
9. Cumulus (Cu)
"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".
Cumulus is mainly composed of water droplets, ice crystals may form in parts of the cloud in which the temperature is well below 00C. Cumulus of great vertical extent may produce precipitation. In the tropics, these clouds often produce heavy rain showers.
Cumulus develops in convection currents in unstable air. The commonest causes are (a) heating of the earth's surface by insolation and (b) continuous warming of the base of a cold air mass by passage over a relatively warm surface.
10. Cumulonimbus (Cb)
"Heavy and dense cloud, with a considerable vertical extent, in the form of a mountain or huge towers. At least part of its upper portion is usually smooth, of fibrous or striated, and nearly always flattened; this part often spreads out in the shape of an anvil or vast plume.
Under the base of this cloud which is often very dark, there are frequently low ragged clouds either merged with it or not, and precipitation sometimes in the form of virga ".
Cumulonimbus is composed of water droplets and, especially in the upper portion, of ice crystals. It also contains large raindrops and, often, snowflakes, snow pellets, ice pellets or hailstones. The water droplets and raindrops may be substantially supercooled. Cumulonimbus may produce thunder and lightening, intense showers of rain, snow or hail and squalls. Cumulonimbus normally evolve from large cumulus, but sometimes develop from Altocumulus castellanus.
dariusz.lipinski