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Stratus is a low-level layer cloud
(not to be confused with altostratus and cirrostratus which
are much higher). In appearance, it is usually a featureless
grey layer. Sometimes, when a sheet of stratus is affecting
an area, the cloud base will be right down to the ground, and
the visibility will be below fog limits. However, the usual
base will be between the ground and 1,000 feet (300 metres),
which means that hilltops may be obscured by cloud. Sometimes
stratus will produce drizzle, snow or snow grains, particularly
over hills.
Perhaps the most important indication
of its low altitude is its apparent rapid movement across
the sky in any wind more than a flat calm. For example, a
stratus cloud at 500 feet (150 metres) moving at 20 miles
per hour will appear to move much faster than altostratus
with its base at 10,000 feet (3,000 metres) moving at 60 miles
per hour.
An approximate guide to the height
of stratus may be gained by measuring the relative humidity
and subtracting it from 100. The resulting number gives you
some idea of the height of the low cloud in hundreds of feet.
For example, 94% relative humidity would indicate that the
stratus is about 600 feet (180 metres) above the ground.
Inland, the base of any stratus
will tend to lift slightly during the daytime, and in summer
will usually disperse completely unless associated with rain.
On the coast, when a moist south-westerly airstream persists,
there may be no such respite. The highest frequency of stratus
occurs in the western parts of the British Isles where moist
south-westerly airstreams are forced to rise over various
ranges of hills.
Useful links
Cirriform
Cumulonimbus
Cumulus
Stratocumulus
and Altocumulus
Summary
Nature of clouds
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