with all hands after being struck by a waterspout. This
strong waterspout was associated with a tornado
outbreak over the Great Lakes region. Off the west coast
of Mexico in late 1989, a cruise liner was hit by a weaker
waterspout that broke many windows and caused some
minor structural damage. This weaker waterspout
formed in conditions that were NOT favorable for
tornado development. Nontomadic waterspouts are
thought to be well-developed, cold-air funnels.
Hail may begin forming in the building stage as
water drops are carried above the freezing level. It may
fall from the cloud base as the updrafts weaken, or
become more concentrated during the dissipating stage.
Hail frequently is thrown out the top and sides of
building and mature CB cells, and may travel up to 25
miles from the cell. This hail rarely reaches the surface
before melting. During the dissipation stage, however,
the stronger, colder downdrafts tend to bring the hail to
the surface UNDER the cloud base before it has a
chance to melt. Most frequently, hail reaches the
ground in the general vicinity of the wall cloud. The size
of the hail is dependent on many factors within the
cloud, which you will study later in preparation for
Another accessory cloud associated with large
cumulonimbus buildups are the layers of altocumulus
or altostratus clouds, often seen developing from the
mid-portion of the cumulonimbus column. These
clouds are formed by the spreading out of moisture into
a thin, somewhat more stable layer of atmosphere as the
unstable column of air in the building cumulus punches
through the layer. These cloud layers are considered
part of the cumulonimbus cell and are called velum.
Occasionally, a rapidly building cumulus will
approach a layer of mid-level or upper-level stratiform
clouds and appear to push the cloud layer upwards,
forming a cap-shaped feature above the cauliflower top
of the cumulus or cumulonimbus cell. Until the
cumulus cell makes contact with the bulging cloud
layer, the cap-shaped feature is called pileus.
Clouds in the genus stratocumulus appear to be a
combination of the smooth, even, stratiform cloud and
the puffy cells of the cumuliform clouds.
Stratocumulus clouds are distinguished from cumulus
clouds by their flatter appearance.
typically appear to be tightly-packed, flattened cumulus
cells but with less distinct edges (fig. 1-19). As
stratocumulus cloud elements merge into a continuous
layer, they appear gray with dark areas. These dark
areas are the thicker portions of the SC clouds.
Stratocumulus is sometimes mistaken for
altocumulus, which is the same type of cloud form in the
Figure 1-19.Stratocumulus cloud.