Clouds 雲
Ref: https://cloudatlas.wmo.int/en/clouds.html
雲係一種同水有關既大氣現象
A cloud is a hydrometeor consisting of minute particles of liquid water or ice, or of both, suspended in the atmosphere and usually not touching the ground. It may also include larger particles of liquid water or ice, as well as non-aqueous liquid or solid particles such as those present in fumes, smoke or dust.
Distinguish clouds by levels
Clouds are generally encountered over a range of altitudes varying from sea level to the top of the troposphere (i.e the tropopause).
Table 6. Approximate heights of each level, and the genera occurring in each. (Information from WMO)
| Level | Genera | Polar region | Temperate region | Tropical region |
| High | Cirrus Cirrocumulus Cirrostratus |
3 – 8 km (10 000 – 25 000 ft) |
5 – 13 km (16 500 – 45 000 ft) |
6 –18 km (20 000 – 60 000 ft) |
| Middle | Altocumulus Altostratus Nimbostratus |
2 – 4km (6 500 – 13 000 ft) |
2 – 7 km (6 500 – 23 000 ft) |
2 – 8 km (6 500 – 25 000 ft) |
| Low | Stratus Stratocumulus Cumulus Cumulonimbus |
From the Earth’s surface to 2 km (0 – 6 500ft) |
From the Earth’s surface to 2 km (0 – 6 500ft) |
From the Earth’s surface to 2 km (0 – 6 500ft) |
Most clouds are confined within their level with the following few notable exceptions:
(a) Altostratus is usually found in the middle level, but it often extends higher;
(b) Nimbostratus (Ns) is almost always found in the middle level, but it usually extends into the other two levels;
(c) Cumulus and Cumulonimbus (Cb) usually have their bases in the low level, but their vertical extent is often so great that their tops may reach into the middle and high levels.
When the height of a particular cloud is known, the concept of levels may be of some help to the observer in identifying the cloud. The genus can be determined by making a choice from among the genera normally encountered in the level corresponding to its height.
Distinguish clouds by Genera
Consideration of the most typical forms of clouds leads to the recognition of (10) ten genera. The definitions of the genera given below do not cover all possible aspects, but are limited to a description of the main types and of the essential characteristics necessary to distinguish a given genus from genera having a somewhat similar appearance.
High level: Ci, Cs, Cc
Medium level: As, Ac, Ns
Low level : Sc, St, Cu, Cb
- Cirrus (Ci) – High level
- 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
- (related to) in advance of an occluded front
- Cirrocumulus (Cc) – High level
- Thin, white patch, sheet or 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 1°.
- Cirrostratus (Cs) – High level
- Transparent, whitish cloud veil of fibrous (hair-like) or smooth appearance, totally or partly covering the sky, and generally producing halo phenomena.
- Altocumulus (Ac) – Medium level
- White or grey, or both white and grey, patch, sheet or layer of cloud, generally with shading, composed of laminae (a layer or layers), 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 of between 1° and 5°.
- Altostratus (As) – Medium level
- Greyish or bluish cloud sheet or layer of striated (grooves or channels in cloud formations, arranged parallel to the flow of the air), 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 or frosted glass
(a grey featureless layer of clouds with great horizontal extent covering almost the whole sky) - Altostratus does not show halo phenomena.
- Greyish or bluish cloud sheet or layer of striated (grooves or channels in cloud formations, arranged parallel to the flow of the air), 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 or frosted glass
- Nimbostratus (Ns) – Medium level
- Grey cloud layer, often dark, the appearance of which is made 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 (i.e the cloud is thick enough throughout to obscure the Sun).
- Low, ragged clouds frequently occur below the layer, with which they may or may not merge.
- Stratocumulus (Sc) – Low level
- Grey or whitish, or both grey and whitish, patch, sheet or layer of cloud that 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 5°.
- Stratus (St) – Low level
- Generally grey cloud layer with a fairly uniform base, which may give drizzle, snow 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.
- Cumulus (Cu) – Low level
- 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 bases are relatively dark and nearly horizontal.
(White cloud – blue sky)
- 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 bases are relatively dark and nearly horizontal.
- Cumulonimbus (Cb) – Low level
- 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, or 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.
