Clouds – How to Distinguish the Different Types of Clouds?

What is International Cloud Atlas?

• The International Cloud Atlas describes the classification system for clouds and meteorological phenomena used by all World Meteorological Organization Members.
• It includes a manual of standards and photographs of clouds and weather phenomenon.
• It was first published in the 19th century and was last updated 30 years ago.
• The new 2017 version of  International Cloud Atlas was a digitalized one and has many additions.

The new cloud classifications that were introduced the International Cloud Atlas (2017)

1) The Species

Volutus

• They are long, typically low, horizontal, detached, tube-shaped cloud mass.
• They often appear to roll slowly about a horizontal axis.
• The species volutus is a soliton and hence not attached to other clouds.
• This species applies mostly to Stratocumulus and rarely Altocumulus.

2) The Supplementary Features

(a) Asperitas

• There are well-defined, wave-like structures in the underside of the cloud.
• Asperitas is characterised by localised waves in the cloud base, either smooth or dappled with smaller features, sometimes descending into sharp points, as if viewing a roughened sea surface from below.
• The varying levels of illumination and thickness of the cloud can lead to dramatic visual effects.
• They occur mostly with Stratocumulus and Altocumulus.

(b) Fluctus

• They are relatively short-lived wave formation, usually seen on the top surface of the cloud, in the form of curls or breaking waves (Kelvin-Helmholtz waves).
• They occur mostly with Cirrus, Altocumulus, Stratocumulus, Stratus and occasionally Cumulus.

(c) Cavum

• These are a well-defined generally circular hole in a thin layer of supercooled water droplet cloud.
• The Cavum is typically a circular feature when viewed from directly beneath, but may appear oval-shaped when viewed from a distance. When resulting directly from the interaction of an aircraft with the cloud, it is generally linear.
• They occur in Altocumulus and Cirrocumulus and rarely Stratocumulus.

(d) Murus

• It is a localised, persistent, and often abrupt lowering of cloud from the base of a Cumulonimbus from which tuba (spouts) sometimes form.
• Usually associated with a supercell or severe multi-cell storm.
• Murus showing significant rotation and vertical motion may result in the formation of tuba (spouts), Commonly known as a ‘wall cloud’.

(e) Cauda

• A horizontal, tail-shaped cloud (not a funnel) at low levels extending from the main precipitation region of a supercell Cumulonimbus to the murus (wall cloud).
• It is typically attached to the wall cloud, and the bases of both are typically at the same height.
• Cloud motion is away from the precipitation area and towards the murus, with rapid upward motion often observed near the junction of the tail and wall clouds and are commonly known as a ‘tail cloud’.

3) Accessory Cloud

Flumen

• They are bands of low clouds associated with a supercell severe convective storm (Cumulonimbus), arranged parallel to the low-level winds and moving into or towards the supercell.
• These accessory clouds form on an inflow band into a supercell storm along the pseudo-warm front.
• One particular type of inflow band cloud is the ‘Beaver’s tail’. This is distinguished by a relatively broad, flat appearance suggestive of a beaver’s tail.

4) Special Clouds

(a) Flammagenitus

• These are clearly observed to have originated as a consequence of localised natural heat sources (forest fires, wildfires or volcanic activity) and consist of water drops.

(b) Homogenitus

• These are originated specifically as a consequence of human activity.
• They include aircraft condensation trails (contrails), or clouds resulting from industrial processes, such as cumuliform clouds generated by rising thermals above power station cooling towers.

(c) Homomutatus

• These are formed as a result of persistent contrails (Cirrus homogenitus) that may be observed, over a period of time and under the influence of strong upper winds, to grow and spread out over a larger portion of the sky, and undergo internal transformation such that the cloud eventually takes on the appearance of more natural cirriform cloud.

(d) Cataractagenitus

• They may develop locally in the vicinity of large waterfalls as a consequence of water broken up into spray by the falls.
• The Cataractagenitus are formed when the downdraft caused by the falling water is compensated for by the locally ascending motion of air.

(e) Silvagenitus

• These are the clouds that may develop locally over the forests as a result of an increased humidity due to evaporation and evapotranspiration from the tree canopy.

What is Asperitas Cloud?

• Asperitas is formerly known as Undulatus asperitus.
• It is a cloud formation proposed by Gavin Pretor-Pinney of the Cloud Appreciation Society (2009).
• It is recently been accepted and added to the International Cloud Atlas on March 23, 2017, on the occasion of World Meteorological Day.
• The ‘Asperitas’ is a Latin word and its meaning is ‘Rough‘.
• The Asperitas clouds tend to be low-lying and are caused by weather fronts that create undulating waves in the atmosphere.

Why clouds appear white in colour?

• The clouds usually appear white because the tiny water droplets and ice crystals inside them are tightly packed, and they reflect most of the sunlight that falls on these masses (scattering).
• The tiny cloud particles equally scatter all colours of light, which make the viewer to perceive all wavelengths of sunlight mixed together as white light.

Why do clouds darken at the time of rain?

• The clouds appear dark or grey in colour at the time of rain is due to their particulate density.
• The water vapour will bind together into raindrops, leaving larger spaces between these drops of water and hence less amount of light is reflected, lending a darker appearance of the rain clouds.

Article by: Priyanka Sunil