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Chapter 12 - Understanding Weather
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unstable, moist air and you have the pos-
sibility for cumulonimbus (nimbus
means rain) clouds, scattered thunder-
storms and rain showers in advance of
the front, as shown in Figure 62.
Fast-moving cold fronts can generate
Fig. 63 squall lines 30 to 180 miles in advance of
and parallel to the front, as shown in
Figure 63. Squall lines are lines of thun-
derstorms containing some of the most
turbulent weather known to pilots.
Some meteorologists speculate that
an isolated wave form may cause pre-
frontal lifting in the warm, moist air pre-
ceding the cold front. Think of this pre-
frontal wave (called a gravity wave) as
the form appearing in a long garden hose
that’s given a good shake. This ripple of
energy moves along the hose. A wave of
air, moving in advance of the cold front,
can generate enough prefrontal lifting
(lifting in advance of the actual front) to
instigate squall line thunderstorm
weather.
Cloud Families
Cirrus Clouds Clouds are divided into four families: High clouds, middle
clouds, low clouds and clouds with extensive vertical devel-
opment. The first three families of clouds have further classifi-
cations based on the way they are formed. They are classified
as either cumulus, stratus, nimbus (meaning rain) and fractus
(meaning fragmented).
High clouds are the cirriform family: cirrus, cirrocumulus
and cirrostratus. Their bases typically range from 16,500 to
45,000 feet. The middle cloud family contains altostratus,
altocumulus and nimbostratus.Their bases range from 6,500
feet to 23,000 feet. The low cloud family contains stratus, stra-
tocumulus and fair weather cumulus clouds. Their bases
Cirrus clouds have a thin, white feathery look and often range from near the surface to 6,500 feet. Clouds with exten-
form in patches or narrow bands. sive vertical development have bases ranging from 1,000 feet
or less to above 10,000 feet.
Cirrostratus Clouds Altostratus Clouds Nimbostratus Clouds
These clouds are transparent, These clouds are grayish or bluish These clouds are gray, dark and
whitish, smooth in appearance and sheets or layers and are often thin often block out most of the sun. They
often produce halos. enough to partially reveal the sun. are rain clouds. © leiana - Fotolia
