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Types of Clouds

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Clouds are aerosols found in different layers of the atmosphere and made up of suspended tiny droplets of water or ice crystals. They are formed by the upward movement of air, which may be due to convection currents, forceful rising by mountains and hills, or upward movement associated with fronts and cyclones. During the upward movement, the air cools adiabatically, increasing the relative humidity (RH). When RH is 100%, water vapor condenses into liquid, and clouds are formed. Clouds are indicators of weather and the source of rainfall. They have an important role in reflecting a part of solar rays back into space, thus contributing to the energy balance of the earth. The average albedo of clouds is 0.5.  

Clouds can be classified based on their height, expanse, and density. Based on the height from the ground, tropospheric clouds can be grouped into three groups: high, mid, and low-level clouds. Each level of cloud is variable and depends on the latitude. The pressure on the clouds decreases with height, and hence density decreases as well. Hence, the density-based classification runs parallel to the height-based classification. The classification based on expanse is not clearly defined and has been dealt with in the height-basis classification.

The widely accepted criterion for cloud classification divides tropospheric clouds into 10 genera.

The high-level clouds are those which are found between the heights of 3-8 km in polar regions and 6-18 km in tropical regions. Since there are upper tropospheric layers, the temperature is as low as -80°C to -50°C. Due to this, these clouds consist of ice crystals and are white. All high clouds are form cirrus clouds.

The High-Level Clouds occur in 3 broad shapes:

1.  Cirrus:

These clouds have the shape of hair-like streaks and are the whitest among all clouds. Halos are formed sometimes due to refraction and scattering of light by the ice crystals. The form of precipitation is virga (water evaporates before reaching the ground). These clouds generally signal the approaching warm front and hence change weather conditions. In terms of expanse, individual clouds don’t cover many areas.

2. Cirrostratus:

As warm front approaches, due to the continuous gentle rising of warm air mass over cold air mass, cirrus clouds transform into thicker cirrostratus, covering large areas of the sky. These clouds may indicate that precipitation is likely within a day. Their appearance is like many cirrus clouds have been aggregated to give a stratified appearance. When light passes through the hexagonal crystals of Ice, light is refracted in such a way that a halo is seen often around Sun or Moon. Sometimes, these clouds are so thin that their presence is only deduced by halos.

3. Cirrocumulus:

These are relatively rare and are formed either due to turbulent convection currents meeting the cirrus clouds (inducing the puffiness) or drifting apart a piece of the anvil of cumulonimbus clouds.  They are bulkier than the other two in high-level clouds. The cirrocumulus clouds also have some liquid water component, which exists in a supercooled state (liquid state of water below freezing point) and may very quickly freeze to form cirrostratus clouds. These may form parallel rows of clouds due to localized ascent and descent of air. Their precipitation is virga-type.

The Mid-Level Clouds:

The mid-level clouds have been given the prefix alto. These clouds exist in the altitudinal range of 2-4 km at the poles and 2-8 km above the tropics. The cloud water content consists of both solid ice and liquid water.

1. Altostratus

They are stratified with a greyish or bluish color. It has some areas which are very thin and hence vaguely reveal the sun. The halo phenomenon is not shown by these clouds. A descending cirrostratus cloud from above usually forms them.  If the presence of the sun or the moon can be felt, then the cloud is altostratus rather than nimbostratus. These are often formed when a warm front or occluded front is approaching. In terms of expanse, they are thin and cover a wide area.

2. Nimbostratus:

The alto stratus clouds thicken as the cyclone passes and transforms into nimbostratus, which brings about continuous precipitation for hours till the associated front passes.  They are thicker than altostratus clouds. Due to their thickness, they block out the sun completely and hence are dark and grey in color. If there is thunder and lightning accompanied by precipitation, then the cloud is cumulonimbus and not nimbostratus. Like altostratus, these also cover the entire sky and are featureless. These may extend into lower levels as well.

3. Altocumulus:

These are heap-like, appear in patches, and are white or grey in color. Individual cloud doesn’t cover many areas. These signify stable weather and produce precipitation of virga type. If the cloud elements exhibit any shading, the cloud is altocumulus, rather than cirrocumulus or stratocumulus. Unlike the other two clouds in midlevel, these occur in a variety of shapes. Due to the presence of ice and water both, both, appear more graceful than the lower-level huge and fluffy cumulus clouds. A part of the altostratus cloud may drift apart to form an altocumulus cloud. These may also be formed due to condensation of moisture present in convection currents by light turbulence or they can also have an orographic origin.

The Low-Level Clouds:

The low-level clouds exist up to 2 km. These clouds mainly contain water in a liquid state. These are mostly responsible for precipitation reaching the ground.

1. Stratus:

These are flat, grey in color, and featureless, covering almost the entire sky. These are associated with gloomy weather and sometimes bring about rainfall. These are the lowest lying among all the clouds, sometimes reaching the surface in the form of fog or mist. These are formed when a warm moist air mass rises gently over a cold air mass. Just like fog, these are formed under calm and stable conditions. Like fog, these may bring about light drizzle.

2. Stratocumulus:

These are white in color and resemble cumulus clouds, but are darker, thicker, and flatter. Their occurrence is either before or after a front or at times when there are slight convection currents in the atmosphere.

3. Cumulus:

Cauliflower-shaped cumulus clouds are formed as a result of convection and are the most common among all the cloud genera. These occur in fair, sunny, and bright weather conditions. The moist sea breeze along the coasts may get warmed up on land and sometimes rises up to forming cumulus clouds in coastal areas. They are detached clouds with shiny tops due to the reflection of sunlight and relatively darker bases.  

4. Cumulonimbus:

If the convection current continues for a long time, cumulus clouds transform into thicker cumulonimbus clouds. These are also formed when a cold air mass forces a warm moist air mass to move up steeply. Nicknamed “The King of Clouds”, cumulonimbus clouds extend throughout the tropospheric vertical extent. They are characterized by heavy precipitation, accompanied by thunder, lightning, and hail. This store the same amount of energy as the 10-atom bombs of Hiroshima. The upper portion of these clouds shifts in one direction under the influence of upper tropospheric westerlies making it anvil-shaped.

Apart from the above tropospheric clouds, there are polar stratospheric clouds (PSC) formed during winters within polar vortex at a height of 15-30 km, and polar mesospheric clouds (PMC) or noctilucent clouds formed at a height of 80-85 km in the mesosphere. PSC is less common over the Arctic than over Antarctica because of the less stable polar vortex. PSCs are the only clouds that materialize both, below and above the ice point. Those formed below the ice point are composed mainly of ice, while those formed above ice formed contain water and nitric acid both.


Last Updated : 02 Dec, 2022
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