|
Cloud classification
Providing that there is sufficient moisture, clouds can occur at any level
of the atmosphere. However, most cloud formations exist in the troposphere,
the lowest region of the atmosphere extending from the earth’s surface to a
height of some 6-10 km. The structure of clouds, a combination of ice
crystals and water, depends on the wide range of temperatures and movement
in the troposphere. Clouds are classified in three main groups: lower,
middle and high level.
Low level clouds: are those in the lower layers of the
atmosphere. Less well defined, with a constant change in structure. Low
level clouds include the following:
Cumulus: cauliflower shaped, dark at the bottom and bright
at the top.
Stratocumulus: low clouds, usually faster moving than
cumulus and also less well defined, more horizontal then vertical and also
often darker at the bottom than the top.
Stratus: thin low cloud, sometimes forming a layer or sheet
and usually moving with the wind as they are closest to the ground.
Middle level clouds: are those that form in the central
layers of the atmosphere, brighter, less fragmented, with a higher
composition of ice crystals. Their appearance can vary from flat layers to
cumulus shaped and include the following:
Altocumulus: middle level clouds, similar to cumulus, dark
at the bottom and light at the top, can be widespread or patchy, smooth or
inconsistent and vary in depth.
Altostratus: also middle level clouds forming a large,
smooth, dark grey sheet, often beginning as fragmented elements, quickly
developing into large areas.
Nimbostratus: widespread sheets of cloud, light grey or
almost white in colour, lacking in contrast and producing showers or
persistent rain.
High level clouds: are those in the highest level of the
troposphere, bright white because of the ice crystals at that altitude. They
vary in form, thickness and cover and include the following:
Cirrus and cirrostratus: clouds that
develop in threads or patches, are bright white, with little contrast
between the bottom and top. They form sheets interspersed with large areas
of sky and can vary in shape, both vertical and horizontal because of winds
at those altitudes.
Cirrocumulus: also bright white, but with a spotted
appearance indicating vertical turbulence within the cloud.
Cumulonimbus: a combination of some or all types of clouds,
usually associated with thunderstorms. The height from the bottom to the top
of the clouds is often many kilometres and can even reach into the
tropopause, the interface between the troposphere and stratosphere.
|
