Identifying the types of Cloud present

Cloud Composition

The measured temperature of a cloud throughout its vertical extent will indicate its likely composition as either water droplets or ice crystals, or a combination of the two.

Water droplets can be further classified as ordinary (or warm) water droplets, and supercooled water droplets; the latter being droplets colder than 0°C yet still in the liquid state.

The temperature at which the constituents of a cloud will exist as ice crystals depends on a number of complex factors. A simple empirical rule suggests the temperature of -20°C (and colder) can be used as a guide to indicate the predominance of ice crystals within a cloud.

Examination of an aerological diagram will show the temperature, or temperature range within an observed cloud; this temperature will give an indication of its predominant composition, which in turn can help with cloud identification.

The table below is derived from the International Cloud Atlas.

Cloud

Composition

Cirrus

Almost exclusively ice crystals.

Cirrocumulus

Almost exclusively ice crystals; strongly supercooled water droplets may occur but are usually rapidly replaced by ice crystals.

Cirrostratus

Mainly ice crystals.

Altocumulus

Almost invariably water droplets; when the temperature is very low, ice crystals may form.

Altostratus

Water droplets and ice crystals. In the most complete case, three superposed parts may be distinguished:

  • Upper part – wholly or mainly ice crystals

  • Middle part – mixture of supercooled water droplets and ice crystals

  • Lower part – wholly or mainly ordinary or supercooled water droplets.

Nimbostratus

As per Altostratus.

Stratocumulus

Water droplets; ice crystals may be present in extremely cold weather.

Stratus

Usually small water droplets; ice particles at low temperatures.

Cumulus

Towering Cumulus

Mainly water droplets; ice crystals may form in those parts with a temperature well below 0°C.

Cumulonimbus

Water droplets and, especially in its upper portion, ice crystals; the water droplets may be substantially supercooled.