In the absence of instrumental aids the cloud-base height must be estimated. In order to
improve their ability to do this, observers should be encouraged to take every opportunity of
checking their estimates against known heights, e.g. when a cloud base is seen to intercept a
mountainous coast (although in such circumstances the cloud base may be lower at the
mountain than at sea). At stations where the observer has reports available from aircraft
descending or ascending in the vicinity he can relate these to what he sees and so provide
reports sufficiently reliable for meteorological purposes. At other stations estimates can
sometimes be widely in error.
As a rough guide, the heights of the bases of the various types may be expected to be
between the following limits:
Low Clouds (Cl)
Stratus: usually below 600 m (2000 ft) and sometimes nearly down to the surface. Cumulus: 450 - 1500 m (1500 - 5000 ft).
Middle Clouds (Cm)
Nimbostratus: 150 - 2000 m (500 - 6500 ft), usually below 600 m (2000 ft) in moderate rain or
snow.
High Clouds (Ch)
usually above 5500 m (18000 ft).
Note
- These limits tend to be
considerably higher in low latitudes; this applies particularly to high
clouds.
Note (auxiliary ships only)
- Auxiliary ships do not
report the height of base of lowest cloud in the sky.