Clouds are formed when rising air expands and cools to form clusters of water vapor molecules, which condense gradually to become very small water droplets or ice crystals, visible as clouds. Clouds can be convective, that is, when they are created from warm air pockets rising directly from the surface, or larger stratiform clouds which are caused by slowly rising air, often accompanied with cyclonic activity. Clouds are classified according to their physical characteristics or the height at which they form in the troposphere. Let’s take a look at all the categories and sub-categories in detail.

Types of Clouds by Structure and Formation Process

On the basis of physical structure and the process by which they were formed, clouds can be classified into the following types:

• Cirriform - These clouds look wispy and form at high altitudes in the atmosphere, generally above 18,000 feet. They do not produce much precipitation and are visible when the weather is stable. They are usually accompanied by low clouds and are non-convective in general.
• Cumuliform - These clouds have puffed up domes with a flat base. They form at a lower height, within 7,000 feet and appear by themselves or in clusters.
• Cumulonimbiform - They are large free-convective clouds with very strong updrafts reaching heights of even up to 60,000 feet in some regions. They can cause thunderstorms and lightning and other severe weather conditions like hail storms, snow showers, tornadoes and heavy rainfall.
• Stratocumuliform - They can form at any height in the troposphere as long as they get sufficient moisture, and are created due to limited convection in unstable air. They have physical characteristics of both cumuliform and stratiform clouds and those forming at higher levels also display cirriform features.

Types of Clouds Based on Height

On the basis of the height at which they occur, clouds can be classified as the following:

• High Clouds - These clouds form at the height of 10,000 to 25,000 feet in the polar regions, 16,500 to 40,000 feet in temperate areas and 20,000 to 60,000 in tropical areas. All cirriform clouds are high clouds and are recognised by the prefix “cirro” like cirrocumulus and cirrostratus .
• Middle Clouds - These clouds are created at heights of 6,500 feet to 13,000 feet at polar regions, 23,000 feet at temperate regions and 25,000 feet at the tropics. They are categorized with the prefix “alto” like altocumulus and altostratus clouds.
• Low Clouds - They form near the surface of the earth within 6,500 feet height. Cumulus and stratocumulus are examples of this type.

There are some other categories which are not limited by process of formation or height:

• Opacity-based - Low and middle clouds have opacity features called translucidus or translucent, perlucidus or opaque with translucent breaks and opacus or opaque. These are not applied to high clouds as they are always translucent, and moderate and towering vertical clouds which are always opaque.
• Pattern-based - Due to certain favorable atmospheric conditions, some clouds always display specific patterns. For example, intortus and vertebratus forming with cirrus fibratus clouds are shaped like twisted filaments or fishbone patterns, and radiatus clouds look like they are converging from the horizon.
• Combinations - Some types of clouds combine, such as a combination of opacity-based clouds and pattern-based clouds. For example, an opaque layer of altocumulus stratiformis clouds radiating in converging rows.

Another classification category is special cloud formations and features which occur in association with other varieties. These features have specific Latin names:

• Precipitation-based - Precipitation occurs when the water droplets that make up the clouds become too heavy and fall as rain. When clouds produce precipitation which evaporates before it reaches the surface of the earth, it is termed as ‘Virga’. For example, cirrocumulus, altocumulus, altostratus clouds etc. When the precipitation doesn’t evaporate completely, it is called praecipitatio, causing light rain. For example, altostratus opacus clouds.
• Cloud-based - Heavy precipitation clouds like nimbostratus and cumulonimbus display the ‘Pannus’ feature, like ragged formations belonging to the cumulus fractus and stratus fractus category. These formations and other similar features are called accessory clouds. Other common patterns are ‘Pileus” which forms like a cap over a big cumulus cloud or ‘Velum’ that develops like a thin sheet in front of the parent cloud.

Clouds are thus a fascinating weather phenomenon, occurring at various heights and displaying many appearances and precipitation capacities. At times, the features of one category overlap the other. There are many further categories which are made in order to undertake detailed study of clouds. Yet, no two clouds are exactly the same, from feathery to puffy, low and dark to light and wispy, different features which makes their study even more interesting.