Cloud Classification

Clouds appear in an infinite variety of forms; observations will show them to be in a continuous process of evolution, and at first it may appear almost impossible to identify them. However, there are several principal ways in which clouds form, and each of these processes produces a cloud with certain distinctive features or characteristics.

Cloud classification is based on the recognition of these features, from which it is possible to identify 10 main cloud groups known as cloud genera. Often clouds of the same genera can vary greatly in appearance. To account for this, most of the genera are further subdivided into species.

The species of a cloud is determined by its shape or internal structure. A cloud observed in the sky of a particular genera, may bear the name of only one species at any given time.

Additional characteristics possessed by a cloud may determine that it is of a particular variety. The variety of a cloud considers characteristics such as its transparency and the arrangement of its observable elements.

Any supplementary features and accessory clouds, as well as an indication of the mother-cloud can be used to further classify a particular cloud. A single cloud may simultaneously bear multiple varieties and supplementary features and accessory clouds.

Here is an example of the concept:

classification example

In addition to the 10 cloud genera, a cloud known as Towering Cumulus is reported for aeronautical purposes due to its significance as a hazard and an indicator of extensive convection occurring in the atmosphere. This section will refer to the 10 cloud genera plus Towering Cumulus as the 11 basic cloud types.

The 11 basic cloud types are:

  • Cirrus

  • Cirrostratus

  • Cirrocumulus

  • Altocumulus

  • Altostratus

  • Nimbostratus

  • Stratocumulus

  • Cumulus

  • Towering Cumulus

  • Cumulonimbus

  • Stratus

“What about the different cloud species and varieties? Just how much do I need to know to perform an accurate observation?”

The required level of knowledge of the observer will largely depend upon the purpose of the observations they perform. A particular cloud observed in the sky may be described in several different ways depending on the reporting format being used. For instance:

  • Twenty-seven variations and combinations of clouds are considered when performing a synoptic cloud observation

  • An aerodrome weather report requires an observer to consider just the 11 basic cloud types.

  • Take-off and landing reports via the ATIS broadcast include the cloud types of Cumulonimbus and Towering Cumulus only; for all other clouds the type is not identified – just the amount and height is reported.

  • Often a cloud may not be deemed ‘significant’ in an observation and no mention of its existence will be reported whatsoever.

Regardless of how a cloud observation is reported, the specific relationship between the cloud types and the weather they produce, such as rain, drizzle or hail, requires that all observers be able to identify at least the 11 basic cloud types if an accurate assessment of the weather is to be made.

Observing conditions to which the cloud descriptions apply

The following pages give a general description of each of the cloud types. These descriptions, unless otherwise specified, assume an observation is carried out under the following conditions:

  • The observer is at the earth’s surface, either on land in areas without mountainous relief or at sea;

  • The air is clear – no obscuring phenomena such as mist, haze, dust, smoke, etc, are present;

  • The sun is sufficiently high to provide the usual luminance and colouration;

  • The clouds are high enough above the horizon such that effects of perspective are negligible.

There will always be the need to adapt the descriptions to other observing conditions. Some guidance is given throughout this section to assist observers when these conditions cannot be met.

In addition to the cloud types, this section describes the precipitation type associated with each cloud. Further details of precipitation types are covered in Weather Observations.