Figure 1-8.Cumulus mediocris cloud.
extending upwards from the base (fig. 1-9, view A), and
also by their overall size. It is not uncommon to see one
or two cumulus congestus cells in the sky surrounded by
cumulus mediocris cells.
Rapidly building cumulus congestus cloud cells
may produce clouds of great vertical extent with
relatively small base areas so that they appear to be in
the form of large towers. The cloud is still classified a
cumulus congestus cloud, but this appearance is
commonly called towering cumulus (TCU). Towering
cumulus clouds normally do not develop as fast
horizontally as they do vertically. A rule of thumb for
identifying a cumulus congestus cloud as a towering
cumulus is that the height appears to be twice the width
of the base (fig. 1-9, view B).
Cumulus congestus cells, and especially towering
cumulus (congestus) clouds may produce light to
moderate showers. Over warm ocean waters, towering
cumulus may produce waterspouts. When a large
cumulus congestus cloud begins to produce either a
wispy cirrus blow-off or a well-defined anvil-shaped
top (the upper portion of the cloud column begins
bulging horizontally) or if lightning is seen or thunder is
heard, the cloud is automatically classified a different
type of cloud: the cumulonimbus.
Cumulonimbus clouds are generated from large
cumulus congestus clouds. These clouds cells are
distinguished from cumulus congestus by their massive
appearance and extensive vertical development. The
presence of thunder, lightning, or an anvil top
automatically classifies the cloud a cumulonimbus.
Although cumulonimbus may develop cirrus blow-
off in the polar regions or during the winter in the mid-
latitudes at 20,000 feet, most commonly the cirrus
blow-off or top of the anvil will be somewhere between
25,000 to 45,000 feet in the mid latitudes. Tops of the
larger cumulonimbus cells have been measured in the
tropics in excess of 60,000 feet.