Classification of Clouds
Clouds are usually classified according to their height and appearance. For convenience, we can list them in descending order viz., high clouds, middle clouds and low clouds. We must exercise some caution in relying on height data. There is some seasonal as well as latitudinal variation and there is some overlapping form time to time. However, the appearance of clouds is quite distinctive for each height category.
WMO cloud classification (1957) – The WMO classified the clouds according to their height and appearance into 10 categories. From the height, clouds are grouped into 4 categories (viz., family A, B, C and D) as stated below and there are sub categories in each of these main categories.
A. High clouds (mean heights 5–13 km) (Mean lower level 20000 ft)
• Cirrus (ci)
• Cirrocumulus (cc)
• Cirrostratus(cs)
B. Middle clouds (Mean heights 2–7 km) (6500–20000 ft)
• Altostratus (As)
• Altocumulus (Ac)
C. Low (clouds mean heights 0–2 km) (Close to earth’s surface–6500 ft.)
• Nimbostratus (Ns)
• Stratocumulus (Sc)
• Stratus (St)
• Cumulus (Cu)
• Cumulonimbus(Cn)
Family A – The clouds in this category are high clouds. The mean lower level is 7 km and the mean upper level is 12 km in tropics and sub-tropics. In this family, there are three sub-categories:
1. Cirrus (Ci) – In these clouds ice crystals are present. It looks like wispy and feathery, and it is delicate, desist, white fibrous, and silky appearance. Sunrays pass through these clouds and sunshine without shadow. It does not produce precipitation.
2. Cirrocumulus (Cc) – Like cirrus clouds, ice crystals are present in these clouds also. It looks like rippled sand or waves of the seashore. It contains white globular masses and transparent with no shading effect. The sky is mackerel sky.
3. Cirrostratus (Cs) – Like the above two clouds, ice crystals are present in these clouds also. It looks like whitish veil and covers the entire sky with milky white appearance. It produces “Halo”.
Family B – The clouds in this category are middle clouds. The mean lower level is 2.5 km and the mean upper level is 7 km in tropics and sub-tropics. In this family, there are 2 sub-categories.
1. Altocumulus (Ac) – In these clouds ice water is present. It has grayish or bluish globular masses. It looks like sheep back and also known as flock clouds or wool packed clouds.
2. Alto-stratus (As) – In these clouds, water and ice are present separately. It looks like fibrous veil or sheet and grey or bluish in colour. It produces coronas and cast shadow. Rain occurs in middle and high latitudes.
Family C – The clouds in this category are lower clouds. The height of these clouds extends from ground to upper level of 2.5 km in tropics and sub-tropics. In this family, there are 3 sub-categorizes.
1. Strato cumulus (Sc) – These clouds are composed of water. It looks soft and grey, large globular masses and darker than altocumulus. Long parallel rolls pushed together or broken masses. The air is smooth above these clouds but strong updrafts occur below.
2. Stratus (St) – These clouds are also composed of water. It looks like for as these clouds resemble grayish white sheet covering the entire portion of the sky (cloud near the ground). It is mainly seen in winter season and occasional drizzle occurs.
3. Nimbostratus (Ns) – These clouds are composed of water or ice crystals. It looks thick dark, grey and uniform layer, which reduces the daylight effectively. It gives steady precipitation. Sometimes, It looks like irregular, broken and shapeless sheet like.
Family D – These clouds form due to vertical development i.e., due to convection. The mean low level is 0.5 km and means upper level goes up to 16 km. In this family, there are two sub-categories.
1. Cumulus (Cu) – These clouds are composed of water with white majestic appearance with flat base. Irregular dome shaped and looks like cauliflower with wool pack and dark appearance below due to shadow. These clouds usually develop into cumulo-nimbus clouds with flat base.
2. Cumulonimbus (Cb) – The upper levels of these clouds possess ice and water is present at the lower levels. These clouds have thunderhead with towering anvil top and develop vertically. These clouds produce violent winds, thunderstorms, hails and lightening, during summer.
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