Flight Training - October 2013 - 43

Stratocumulus clouds are sheets of
lumpy clouds.
To tell stratocumulus, altocumulus, and
cirrocumulus clouds apart, extend your
arm toward the lumps in the clouds. If
your little finger covers a lump, the clouds
are cirrocumulus. If your thumb roughly
covers one of the lumps, the clouds are
altocumulus. If your fist is needed to
cover a lump, you’re looking at stratocumulus clouds.
RISING AIR HOLDS CLOUDS UP. Clouds
are made of tiny drops of water or small
ice crystals that are always falling. For
a cloud to stay together and in the sky,
air must be rising into it at least as fast
as the water drops or ice crystals are
falling at what’s known as their terminal
velocity, which depends on their size
and weight.
Air cools as it rises; when it cools to
the dew point, the water vapor in the
air begins condensing into tiny drops of
water or sublimating into tiny ice crystals.
These create the clouds we see. In a
strong thunderstorm, that air can be rising
at 100 mph. In an ordinary stratus cloud

the air will be rising as slowly as a few
inches an hour. Air does not have to be
rising straight up to form clouds; it can be
ascending at an angle, such as when warm
air ahead of a surface warm front is sliding
up and over colder air next to the ground.
The typical terminal velocities in this
chart show how fast the air has to be rising for clouds, rain drops of certain sizes,
and even hail to form.

SOME TYPICAL
TERMINAL VELOCITIES
Typical cloud drop:
Small raindrop:

0.02 mph
4.5 mph

Large raindrop:

20 mph

Large hailstone:

100 mph

Growing cumulus clouds have generally
flat bottoms showing where the rising air
has cooled enough for water vapor in the
air to begin condensing into cloud drops
or—if the air becomes cold enough—
depositing directly into ice crystals.
Jack Williams is an instrument-rated private pilot.
His latest book is The AMS Weather Book: The Ultimate
Guide to America’s Weather.

CHARLES FLOYD

and hardly at all in others. You’ll usually
see breaks between the clouds.
Clouds higher than 20,000 feet are
called cirrus clouds or have names
beginning with “cirro.” Since they’re
so high, they are made mostly of ice
crystals, although scientists have found
supercooled water drops (water that
is still liquid although colder than 32
degrees Fahrenheit). Cirrus clouds
have wisps of snow falling from them
(often called “mares’ tales”) that quickly
evaporate. More or less solid sheets of
these high clouds are called cirrostratus
clouds, while those with lumps are
cirrocumulus clouds.
Clouds between roughly 6,500 feet
above the ground and 20,000 feet have
names beginning with “alto,” such as
altostratus and altocumulus.
Clouds below 6,000 feet don’t have
a prefix indicating their height. If they
are lumpy, they are cumulus clouds of
various kinds. If they are flat, they are
stratus clouds or have “strato” or
“stratus” in their name, such as nimbostratus. The “nimbo” tells you that rain
or snow is falling from the clouds.

AS ADVANCING warm air rises over colder air at the surface, it creates
several kinds of clouds that can stretch more than 600 miles. The surface
boundary with warm air moving toward the northeast is west of St. Louis. This
is the surface warm front. The warm/cold air boundary stretches all the way to
Pittsburgh, where a few cirrus clouds more than 20,000 feet above offer the
first hint that lower clouds and precipitation are on the way.
If you were flying from Pittsburgh to St. Louis you would be flying into
thicker clouds and lower ceilings. The bottoms of the ever-lowering clouds
you'd see between Pittsburgh and St. Louis would be parallel to the ground,

not following the warm/cold air boundary. After taking off from Pittsburgh,
cirrus clouds would give away to cirrocumulus or cirrostratus clouds. These
would be thin enough to see the sun through them.
As you continue west, these clouds will grow thicker and lower, becoming
altocumulus or altostratus clouds. If you fly into these clouds, you'll have to
worry about potential icing. The surface front is moving toward you. You will
fly into the lower nimbostratus clouds with rain and snow. If the surface front
has passed St. Louis, you might end up landing in the clearing sky behind the
front.

OCTOBER 2013 FLIGHT TRAINING

/ 43
Flight Training - October 2013

Table of Contents for the Digital Edition of Flight Training - October 2013
