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c. If the warm front air moving over the colder air mass is unstable, cumulus clouds (Cu),

cumulonimbus clouds (Cb), and altocumulus clouds (Ac) will form and frequently produce

thunderstorms and showers.

d. As a last step of warm front air movement, the warm air is. Forced up near the

stratosphere. In the freezing temperatures at that height, the condensed moisture appears

as cirrus clouds (Ci). The up-slope movement of the warm front air over the wedge of cold air

is very gradual. It rises approximately 1,000 feet every 20 miles. Cirrus clouds may form at an
altitude of approximately 25,000 feet and 500 miles in advance of the point on the surface

that marks the position of the front.