Introduction, what is a cloud meteorologically?

In meteorology, a cloud is a visible mass of liquid droplets or frozen crystals made of water or various chemicals suspended in the atmosphere above the surface of a planetary body. 

These suspended particles are also known as aerosols and are studied in the cloud physics branch of meteorology.


Cumulus humilis clouds in the foreground and cumulonimbus clouds in the back, taken at Swifts Creek, in the Great Alps of East Gippsland, Victoria, Australia. image: wikipedia

Cloud formation

Terrestrial cloud formation is the result of air in Earth's atmosphere becoming saturated due to either or both of two processes; cooling of the air and adding water vapor. With sufficient saturation, precipitation will fall to the surface; an exception is virga, which evaporates before reaching the surface.

Cloud names and classification

Clouds in the troposphere, the atmospheric layer closest to Earth's surface, have Latin names due to the universal adaptation of Luke Howard's nomenclature. It was introduced in December 1802 and became the basis of a modern international system that classifies these tropospheric aerosols into:

  •  several physical forms or categories
  • then cross-classifies them into families of:
    • low,  include stratus and  stratus
    • middle and include; altostratus and altocumulus. 
    •  high - include  cirrus, cirrostratus, and cirrocumulus
  • according to cloud-base altitude range above Earth's surface. 

Clouds with significant vertical extent are often considered a separate family. 

  • One physical form shows free-convective upward growth into low or vertical heaps of cumulus.
  •  Other forms appear as non-convective layered sheets like low stratus, and as limited-convective rolls or ripples as with stratocumulus. 

Both of these layered forms have middle- and high-family variants identified respectively by the prefixes alto- and cirro-. Thin fibrous wisps of cirrus are a physical form found only at high altitudes. 


Example of cirrus type clouds. image: R.Conan-Davies

In the case of clouds with vertical extent, prefixes are used whenever necessary to express variations or complexities in their physical structures. 

These include:

  •  cumulo- for complex highly convective vertical nimbus storm clouds, and 
  • nimbo- for thick stratiform layers with sufficient vertical depth to produce moderate to heavy precipitation. 

This process of cross-classification produces ten basic genus-types or genera, most of which can be subdivided into species and varieties. 

Synoptic surface weather observations use code numbers to record and report any type of tropospheric cloud visible at scheduled observation times based on its height and physical appearance.

While a majority of clouds form in Earth's troposphere, there are occasions when they can be observed at much higher altitudes in the stratosphere and mesosphere. 

Clouds that form above the troposphere have common names for their main types, but are sub-classified alpha-numerically rather than with the elaborate system of Latin names given to cloud types in the troposphere. 

These three main atmospheric layers that can produce clouds, along with the lowest part of the cloudless thermosphere, are collectively known as the hemisphere.  

Above this lies the heterosphere (which includes the rest of the thermosphere and the exosphere) that marks the transition to outer space. 

Clouds on other planets

Clouds have been observed on other planets and moons within the Solar System, but, due to their different temperature characteristics, they are composed of other substances such as methane, ammonia, and sulfuric acid.

How clouds form

Adiabatic cooling

All weather-related clouds form in the troposphere, the lowest layer of Earth's atmosphere. This generally happens when one or more lifting agents causes air containing invisible water vapor to rise and cool to its dew point, the temperature at which the air becomes saturated. The main mechanism behind this process is adiabatic cooling.

Atmospheric pressure decreases with altitude, so the rising air expands in a process that expends energy and causes the air to cool, which reduces its capacity to hold water vapor. If the air is cooled to its dew point and becomes saturated, it normally sheds vapor it can no longer retain which condenses into cloud.

Lifted condensation level and condensation nuclei

The altitude at which this begins to happen is called the lifted condensation level, which roughly determines the height of the cloud base. Water vapor in saturated air is normally attracted to condensation nuclei such as salt particles that are small enough to be held aloft by normal circulation of the air. 

If the condensation process occurs below the freezing level in the troposphere, the nuclei help transform the vapor into very small water droplets. Clouds that form just above the freezing level are composed mostly of supercooled liquid droplets, while those that condense out at higher altitudes where the air is much colder generally take the form of ice crystals.

An absence of sufficient condensation particles at and above the condensation level causes the rising air to become supersaturated and the formation of cloud tends to be inhibited.


see also