ZIMSEC O Level Geography Notes: Clouds
Refers to the fraction of the sky obscured by clouds when observed from a particular location. Okta is the usual unit of measurement of the cloud cover. Sky conditions are estimated in terms of how many eighths of the sky are covered in cloud, ranging from 0 oktas (completely clear sky) through to 8 oktas (completely overcast).
The diagram above shows how cloud cover can be estimated. An eye observer makes the necessary estimates at a given point of measurement. 3D cameras and computing software can also be used to make more accurate estimates of cloud cover nowadays.
Classification of clouds.
Clouds are classified according to height and form of appearance. There are high level, middle level and low level clouds. Various adjectives are used to denote height and form (shape).
High Level clouds
- Cirro is latin for high/crest.
- The adjectives are used to describe high level clouds.
- It is used for clouds found between 6 000m to 12 000m.
- Examples include cirrus, cirrostatus and cirro-cumulous clouds.
Middle Level Clouds.
- Alto means middle.
- These range between 2 100m and 6 000m.
- Examples include alto-cumulus, altostratus and strato-cumulous.
Low level clouds
- Nimbus means water bearing.
- These are below 2 100m.
- Examples include cumulonimbus, cumulous, nimbostratus and stratus clouds.
NB. Some clouds transverse all heights. These are refered to as clouds of great-vertical extent for example cumulonimbus clouds.
Adjectives used to describe form.
- cumulus means heaped.
- stratus means layered.
- cirrus means curly.
Common/important cloud types.
- Wispy white clouds with a feathery and patchy cover.
- Have a silky sheen appearance.
- Are composed of ice crystals.
- They hardly block sunlight.
- They give the sun a red or yellow color during sunset and sunrise.
- They are usually anvil shaped at the top.
- Extend of great vertical distance.
- They are black or white, heaped.
- Are associated with torrential rainfall and thunderstorms.
- They are dark grey.
- Dense, shapeless.
- Are associated with rain.
- White or grey and patched.
- Are made up of laminae (plates)
- When the sun passes over them they form a corona.