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Clouds are huge collective masses, of either tiny water droplets or minute crystals of ice or sometimes of different chemicals released in the atmosphere above the surface of the earth, which are visible to the naked eye. The appearance of clouds keeps on changing and hence we can see them in a wide variety of shapes, sizes and forms. While most of us love gazing at clouds in the sky, and admiring them from the bottom of our hearts, some of us are also keenly interested in gaining more knowledge about this natural phenomenon that has always been with humans since time immemorial.

Classification of Clouds

Clouds have been broadly classified into three types, on the basis of their location in the sky. They are high level clouds, medium level clouds and low level clouds. As the names suggest, high levels clouds are located at the highest level of the troposphere of Earth. Medium level clouds are situated below the high level ones, and the low level clouds are the ones that appear below medium level clouds.

High level Clouds

High level clouds are located at about 20,000 feet above the ground level, at the highest level of the troposphere, where the temperature is freezing cold. Owing to this, they consist of small crystals of ice. They are thin and white in appearance. There are three types of high level clouds viz.,

Cirrus

Cirrus
Credit: Ralph F. Kresge/NOAA's NWS Collection

Of the high level clouds, the cirrus clouds form at about 16,500 feet above the ground level. The water at this altitude tends to freeze, and therefore these clouds are made up of ice crystals. They are transparent, wispy and hair-like. Though they seem to be whiter than any other cloud during the day, as the sun begins to set, they tend to take on the hue of the sunset. While single or isolated cirrus clouds are an indication of stable weather, a gathering of cirrus clouds can be a forewarning of an approaching storm.

Cirrocumulus

Cirrocumulus
Credit: Ralph F. Kresge/NOAA's NWS Collection

The cirrocumulus clouds form at a height of about 16,000 feet above ground level. They are basically nothing but lots of small white cloudlets bunched together, but evenly spaced from each other. These rare clouds form a veil-like layer and occur in rippling patterns, and are almost completely made up of ice crystals. With the advent of the warm front near the cirrus clouds, they become denser and thicker and thus, convert into cirrocumulus clouds.

Cirrostratus

Cirrostratus
Credit: Quasipalm (Own work)/via Wikimedia Commons (PD)

These transparent layered clouds, that cover large portions of the sky, appear like cloud streets. These are very difficult to detect owing to their thinness. Sometimes, these clouds form 'halos' around the sun and the moon, and it is only then that their presence in the sky can be observed. They may spread out thousands of miles across the sky.

Medium level clouds

Located between 6,500 and 20,000 feet above ground level, medium level clouds, as the name indicates, are formed in the middle levels of the troposphere. These clouds are either made of water droplets or of ice crystals, or both combined. Clouds that fall under this category include the following:

Altocumulus

Altocumulus
Credit: Grant W. Goodge/NOAA's NWS Collection

These are heap-like clouds that appear in patchy formations. Much like cirrocumulus clouds, the altocumulus also form a veil-like layer across the sky. They are typically white or grayish in color or sometimes even shaded, and are found mostly in clear weather. Their presence in the sky, on a warm and a humid morning, may indicate an approaching thunderstorm.

Altostratus

Altostratus
Credit: Janne Naukkarinen/via Wikimedia Commons (PD)

These mid level clouds bearing either grayish or bluish gray shade, are composed either of water droplets, or of very tiny ice crystals. The cloud spans across the sky like a huge sheet and displays either wavy patterns (due to the wind blowing constantly through it) or appear as individual fragmented clouds with patches of white sky visible in between. Sometimes, through the thick and opaque clouds, some amount of precipitation is possible.

Nimbostratus

nimbostratus
Credit: NOAA's NWS Collection

The thick, gray, mid level clouds are predominantly rain clouds. They are so dense and opaque that they are capable of blocking out the sun. They appear in the form of a formless gray layer covering the sky, and can be easily identified by their color and opacity. They can also cause snow, alongside rain, but this depends on the climatic conditions of a particular region. However, it is notable that the nimbostratus cloud only causes rain or snow and not phenomena such as lightning and thunder.

Low level clouds

The low level clouds appear below medium level clouds that form at the height of about 6,500 feet above ground level. These clouds are almost entirely composed of water droplets. However, it is possible that they might be composed of very tiny ice crystals in regions where winters are 'super cool'. There are two kinds of low level clouds, 'stratus' which are formulated in a horizontal manner and 'cumulus' which are formed in a vertical manner.

Stratus

stratus
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The stratus clouds, as mentioned above, cover the surface of the sky horizontally. They form a flat but thick layer of cloud cover, gray in color. Interestingly, sometimes, they appear in the form of mist or fog at the ground level. They may cause some amount of drizzle or snow. Stratus clouds are usually not very opaque and so, in case there are no clouds above them, the sun/moon may be seen.

Stratocumulus

stratocumulus
Credit: Janne Naukkarinen/ via Wikimedia Commons (PD)

These are clouds that appear quiet often in the sky and form a horizontal layer of cloud agglomeration. These clouds, which seem to have rounded bases, are joined together to each other and so, the layer that is formed have gaps in between. That is to say that the thickness of the layer is not uniform at all places. These clouds are present in the sky in all kinds of weathers.

Cumulus

cumulus
Credit: Paula Campbell/NWS National Data Buoy Center

These vertically developed clouds have a fragmented occurrence, and look like heaps of cauliflowers put together. They bear clearly noticeable edges and have a puffy appearance. Although they usually indicate fair weather, they may cause small amounts of showers if they tend to get bigger in size. Though generally composed of water droplets, they may also sometimes contain a mixture of water droplets and ice crystals.

Cumulonimbus

cumulonimbus
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These vertically towering clouds are always accompanied by the cumulus clouds. These are typically thunderclouds which are extremely heavy and dense. Owing to their weight, they are located only a few hundred feet above the surface of the earth. The cumulonimbus clouds are strong indicators of extreme weather conditions that include phenomena such as hailstorms, blizzards, heavy torrential rains and tornadoes.

These main types of clouds have numerous other sub-types, which in turn possess some of their own unique properties. Also, some of the occurrences may be extremely rare, once in a lifetime opportunities for cloud enthusiasts. Given below are some of the rarely occurring cloud sub-types:

Kelvin Helmholtz Clouds

Kelvin Helmholtz
Credit: Kr-val (Own work)/via Wikimedia Commons (PD)

Also known as 'billow clouds', these are extremely rare occurrences resulting from the clash of two different layers of air blowing at two different speeds. The clouds that are formed thus, show distinct but regular wavy pattern. They indicate atmospheric instability.

Mammatus Clouds

Mammatus
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Also known as 'Mammatocumulus', these clouds blanket large portions of the sky and appear as having sagging pouch-like structures. They are often indicative of stormy weather and often, of approaching tornadoes. However, all the mammatus clouds may not necessarily indicate bad weather.

Noctilucent Clouds

Noctilucent Clouds
Credit: Image Science and Analysis Laboratory, NASA-Johnson Space Center

These clouds are visible only during the night. Their location in the mesosphere makes them the highest clouds that are formed in the atmosphere of the Earth. They have a very bright and a shiny appearance, and recent researches show that they may be linked to the phenomenon of climate change.

Wall Cloud

Wall Cloud
Credit: NOAA Photo Library/OAR/ERL/NSSL

Wall cloud, a.k.a 'pedestal cloud', is a huge cloud that is often formed under the base of a cumulonimbus cloud, and is commonly associated with the occurrences of tornadoes. Some wall clouds have features that resemble an eye or a tail.

Shelf Cloud

Shelf Cloud
Credit: Sensenmann (own work)/via Wikimedia Commons (PD)

These are horizontal wedge-shaped clouds, commonly associated with severe thunderstorms. This cloud is not always very opaque. Sometimes, the sun can shine through. Whenever this happens, the cloud takes the brightness of the sun and appears as if it is on fire.

Roll Cloud

Roll Cloud
Credit: Eazydp (personal photo)/via Wikimedia Commons (PD)

Roll cloud is a low level cloud which assumes the shape of a tube. This 'tube' spans a mile across the sky, and is an extremely rare occurrence. Interestingly enough, this cloud appears like a huge tube rolling in the sky, around its defined horizontal axis.

Lenticular Cloud

Lenticular Cloud
Credit: Jackiemu (Own work)/via Wikimedia Commons (PD)

These are stationery saucer-shaped clouds which form at very high altitudes. They generally are found in mountainous regions due to perpendicular movement of air. Sometimes, their edges seem to be brightly colored. They resemble flying saucers and have also been mistaken as UFOs in the past.

There are numerous kinds of clouds that have been discovered and studied till date. Many more are still waiting to be discovered. Climatic changes and other natural and man-made phenomenon give way to the creation of a number of new kinds of clouds, thus making this an ongoing process.