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Cumulus formation is often preceded by hazy spots out of
which the clouds evolve (top left photograph). The clouds
in their early stages of formation are depicted in the photograph
at top right. When completely formed, the clouds have clear
cut horizontal bases and flattened or slightly rounded tops
(centre photographs). At this stage of development they are
known as fair weather cumulus. In the photograph at bottom
left the clouds have been frayed by a fairly strong, turbulent
wind.
Over land, on clear mornings, cumulus may form as the sun
rapidly heats the ground, or may result from the transformation
of stratus CL6. Near coasts, cumulus may form
over the land by day in a sea breeze and over the sea during
the night in a land breeze.
Cumulus in the last stages of dissipation, (bottom right)
is also coded as CL=1. If at least one of the cumulus
clouds present in the sky shows moderate or strong vertical
development, the code CL=2 is used.
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