Mid-level clouds sometimes have a flattened appearance, in which case they
are called altostratus. Altostratus clouds are lower and thicker than cirrostratus.
Mid-level clouds with a puffy look are known as altocumulus.
LOW-ALTITUDE CLOUDS
The lowest clouds have bases at altitudes less than 800 m (6,000 ft). These
clouds are sometimes given the prefix strat-. Low clouds sometimes extend all
the way to the surface, in which case fog occurs.
Low, flat clouds are called stratus clouds. If rain falls from them, they are
called nimbostratus. Low, rolling, gray stratocumulus are seen on an overcast
but dry day typical of much of the United States in late autumn. The cottonlike
puffs of fair-weather cumulus are easy to recognize.
CLOUDS AT MULTIPLE ALTITUDES
Several kinds of clouds can exist at the same time, at various altitudes. It is not
uncommon, especially in storm systems, to observe three different and distinct
cloud layers. One well-known type of cloud complex, the cumulonimbus,
encompasses low, medium, and high levels, often extending from approximately
1 km (3300 ft) to the top of the troposphere (Fig. 2-9). These clouds sometimes
produce severe thunderstorms.
Cumulonimbus clouds sometimes stand alone in an otherwise clear sky. In
other storm systems, especially in front of strong cold fronts, multiple cumu-
lonimbus clouds merge into squall lines. Cumulonimbus clouds congregate in a
donut-shaped, revolving mass around the eye of a tropical storm or hurricane.
Imagine that it is a cloudy, dark day, and you board an airplane bound for a
distant place. You know the airplane will be flying at a high altitude. As you set-
tle in for the long ride, the pilot’s voice comes over the speakers. She tells you
that the cruising altitude will be 37,000 feet (approximately 11 km or 7 mi). The
plane climbs into a nimbostratus cloud bank. The clouds are so thick that you
can hardly see the wings. Then the aircraft bursts into the clear, only to reveal
another, higher cloud layer. You recognize it as altocumulus. Even at this alti-
tude, the sky is overcast. The plane climbs higher and enters the second deck of
clouds. You emerge from the second cloud deck into sunshine, but you can see
clouds in the distance at a still higher altitude. As you squint into the sun, you
discern a cumulonimbus “thunderhead” about 60 km (40 mi) away, and cirro-
stratus at about the same distance. The altocumulus clouds, through which you
CHAPTER 2 The Atmosphere
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