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What height are the clouds?

In the troposphere (the 'weather' zone ... see here, the layers are divided up into three broad levels: (approx heights only)

Polar latitudes Temperate regions Tropics
High 10 000 - 25 000 ft
3 - 8 km
16 500 - 45 000 ft
5 - 14 km
20 000 - 60 000 ft
6 - 18 km
Medium 6 500 - 13 000 ft
2 - 4 km
6 500 - 23 000 ft
2 - 7 km
6 500 - 25 000 ft
2 - 8 km
Low Surface -- 6 500 ft
up to 2 km
Surface -- 6 500 ft
up to 2 km
Surface -- 6 500 ft
up to 2 km


The heights assigned to the 'divisions' between levels should not be followed slavishly, and assignment of clouds to the various 'groups' should be made with the appearance and composition in mind.

High clouds are primarily composed of ice crystals; Medium clouds are a mixture of water droplets (usually super-cooled) and ice crystals, in varying proportion, and low clouds primarily water droplets, but in individual cases these descriptions are probably simplistic.
(NB: Super-cooled: means that although the temperature of the droplet is below 0 deg.C, it remains liquid - this is a common state in the middle part of the troposphere.)

In the 'Low' cloud classification come: Stratus (St); Stratocumulus (Sc); Cumulus (Cu) and Cumulonimbus (Cb). However, note that both Cumulus and Cumulonimbus clouds often extend well into 'medium' levels, and towering Cu, and Cb extend to 'high' levels.

In the 'Medium' cloud class come: Altostratus (As); Altocumulus (Ac) and Nimbostratus (Ns). Nimbostratus often has a base within the 'low' cloud category.

In the 'High' cloud group are: Cirrus (Ci); Cirrocumulus (Cc) and Cirrostratus (Cs).