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  Type of clouds with their base height

By convention, the part of the atmosphere in which clouds are usually present has been vertically divided into three "étages": high, middle and low. Each étage is defined by the range of levels at which clouds of certain genera occur most frequently. In the table below the high clouds are in yellow, the middle in orange and the low in blue. The étages overlap and their limits vary with altitude.

A tabular form is given below for the mid latitudes.

Remarks:
1) Altostratus is usually found in the middle étage, but often extends higher.
2) Nimbostratus is almost invariably found in the middle étage, but it usually extends into the other étages.
3) Cumulus and Cumulonimbus usually have their bases in the low étage, but their vertical extent is often so great that their tops may reach into the middle and high étages.

When the height of a particular cloud is known, the concept of étages may be of some help in identifying this cloud. Its genus can be determined by making a choice from among the genera normally encountered in the étage corresponding to its height.
High clouds, CH    base usually 20,000 ft (6km) or above
Type
Precipitation (ppn) etc.
Range of cloud base C code
Cirrus (Ci) No ppn. Halo may occur. Dense patches may veil or hide the sun
Usually 20,000-40,000 ft (6-12km)
0
Cirrocumulus (Cc)
No ppn. Position of sun/moon can usually be seen
Similar 1
Cirrostratus (Cs)
No ppn. Halo often occurs. Outline of sun normally visible
Cs can progressive lower of base and become As 2

 

 

Medium clouds, CM  base usually between 6,500 ft (2km)000 ft (6km)
Type Precipitation (ppn) etc. Range of cloud base C code
Altocumulus (Ac) Ac Castellanus occasionally produces ppn. Can be thick enough to hide sun/moon Usually 6,500-20,000 ft (2-6km).  3
Altostratus (As) Often continuous ppn reaching the ground with sun/moon hidden. Thinner As shows sun/moon as ground glass appearance Altostratus may thicken with progressive lowering of the base to become Ns 4
Nimbostratus (Ns) Normally continuous ppn (sometimes moderate/heavy) with sun/moon hidden Usually between the surface and 10,000 ft (3km) 5

 

 

Low clouds, CL  base usually below 6,500 ft (2km)
Type Precipitation (ppn) etc. Range of cloud base C code
Stratocumulus (Sc) Normally no ppn, but slight ppn possible over coasts/hills. Can be thick enough to hide sun/moon Usually between 1,000 ft* (300m) and 4,500 ft (1,350m) but may often be observed to 6,500 ft (2km) 6
Stratus (St) Near coasts/hills, ppn can be considerable, but it may be falling from higher cloud such as Ns. Can be thick enough to hide sun/moon. However when thin, sun/moon can be clearly visible Usually between the surface and 2,000 ft (600m) but may sometimes be observed to 4,000 ft (1,200m) 7
Cumulus (Cu) Light showers are possible Usually between 1,000 ft* (300m) and 5,000 ft (1,500m), but may sometimes be observed to 6,500 ft (2km). After initial formation, a rise in temperature often leads to a rise in cloud base 8
Cumulonimbus (Cb) Always reported when showers/thunderstorms/hail occurs. Squally winds are also common Usually between 2,000 ft* (600m) and 5,000 ft (1,500m), but may sometimes lower to near surface, or be as high as 6,500 ft (2km) 9

 

*At stations substantially over 500 ft (150m) above sea level the base will often be less.