Clouds & Particles

Basics

The different types of clouds in the atmosphere

Clouds are classified into a system that uses Latin words to describe their appearance and the height of cloud base. This classification was developed by the English chemist Luke Howard in 1803. The Latin words used are: cirrus which means "curl of hair"; stratus which means "layer"; cumulus which means"heap"; and nimbus which means"rain".

Cloud types are divided into four groups. The identification of the first three groups is based on the height of the cloud base above the ground:

- high level clouds with a cloud base between 5 and 13 km above the ground
- mid level clouds with a cloud base between 2 and 6 km above the ground 
- low level clouds with a cloud base from 0 to 2 km above the ground.

The fourth group consists of vertically developed clouds.  These clouds are so thick that they cannot be classified according to the height of their cloud base above the ground.

High level clouds

High level clouds are named cirrus, cirrostratus and cirrocumulus.  Air temperatures at the altitudes these clouds form at can be less than -40 _^oC so these clouds are made of millions of tiny ice crystals, rather than water droplets.

Cirrus (Ci)

Cirrus clouds are curly, featherlike clouds and are often the first clouds to appear in a clear, blue sky. The shape and movement of cirrus clouds can often indicate the strength and direction of high altitude winds.

These clouds never produce rain or snow at the Earth's surface.

Cirrocumulus (Cc)

Cirrocumulus clouds look like small white puff balls high in the sky.  The puff balls can occur individually or as long rows.  When the puffs are in rows, they give the cloud a rippling appearance that resembles the scales of a fish and distinguishes it from a cirrus or a cirrostratus cloud.

Cirrostratus (Cs)

These sheet-like, nearly transparent clouds form at least 6 km above the ground. Cirrostratus clouds are so thin that the Sun and Moon can be clearly seen through them. When sunlight or moonlight passes through the ice crystals of a cirrostratus cloud, the light is bent in such a way that a halo may form. These clouds often indicate that rain is on its way.

Mid level clouds

Mid level clouds are called altostratus and altocumulus.  The prefix "alto" indicates that they have cloud bases between 2 and  6 km above the ground.

Altostratus (As)

Altostratus clouds are made up of both water droplets and ice crystals. They cover huge areas of the sky, often over hundreds of square kilometres.  The Sun is visible through these clouds but it looks as if it is behind frosted glass.  Although altostratus clouds bring very little precipitation, they often indicate the increasing likelihood of rain so don't forget your umbrella!

Altocumulus (Ac)

Altocumulus clouds are white or grey, or a mixture of both.  They look puffy or like fuzzy bubbles in long rows.  The generally have dark shadowed undersides.  If this shading isn't visible, it's quite easy to mistake these clouds for high level cirrocumulus clouds.  In case of doubt, hold your hand at arms length: if the puff is smaller than one finger width, you are looking at a cirrocumulus cloud!

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Low level clouds

Clouds which form between the ground and 2 km in height are generally made up of water droplets and are called stratus, stratocumulus and nimbostratus clouds.

Stratus (St)

Stratus clouds form in a low layer and cover the sky like a blanket. They develop horizontally rather than vertically like cumulus clouds. They can form only a few meters above the ground. A stratus cloud at ground level is fog!

Stratocumulus (Sc)

Stratocumulus clouds are grey with dark shading and spread in a puffy layer. They do not produce rain but often form after a rainstorm.

Nimbostratus (Ns)

Nimbostratus clouds form a dark grey, wet looking, cloudy layer, and are associated with falling rain or snow. They can also be considered as mid-level clouds as they can be upto 3 km thick!  They totally mask the Sun.

Vertically developed clouds: cumulus and cumulonimbus

Cumulus (Cu)

Cumulus clouds look like white balls of cotton wool. They usually occur individually with blue sky between each cloud and they sometimes have funny shapes.  They are formed as a result of thermal convection (see the chapter on cloud formation processes) and have have flat bases and lumpy tops.

Cumulonimbus (Cb)

Cumulonimbus clouds are the King of the clouds. The top of these clouds can reach 12 km in height (much higher than the Everest!) and are commonly topped with an anvil-shaped head.  They can sometimes even reach altitudes of 18 km and penetrate into the stratosphere.  The bottom of a cumulonimbus cloud is made up mostly of water droplets whereas higher in the cloud, ice crystals dominate as the temperature is well below 0 °C. Vertical winds inside the clouds can be greater than 100 km h^-1. If you like rain, thunder, lightning and even tornadoes, cumulonimbus are your friends! If not, run quickly into your house!

About this page

author: Dr. Justine Gourdeau - LaMP, Clermont-Ferrand, France
scientific reviewer: Dr. J. Feichter, Max Planck Institute for Meteorology, Hamburg, Germany
published: 2003-06-03