Flight Training - May 2011 - 41

Some typical terminal velocities: Typical cloud drop: 0.02 mph Small rain drop: 4.5 mph Large rain drop: 20 mph Large hailstone: 100 mph

Lesson for pilots: Stay far away from clouds that are producing hail, even small hail. The churning winds in such clouds often extend into the clear around them.
WHAT CLOUD SHAPES TELL YOU. Flat, or at least mostly flat, stratus clouds form when a large mass of air is rising at more or less the same speed over a large part of the sky—maybe all of the sky you can see. The ride should be generally smooth under these clouds. Lumpy or piled-up cumulus clouds form when streams of air are rising into the clouds while at the same time air is sinking between the clouds. Such clouds should alert you to possible turbulence as your airplane flies through streams of rising air into streams of sinking air. The bottoms of such clouds are typically flat, while the tops are lumpy; and if conditions are right, they can rise like castles into the sky, maybe growing into thunderstorms. These are fair-weather cumulus clouds, which form on days with generally good weather. The clouds are not growing

THE HARD, cauliflowerlooking parts of cumulus congestus clouds are mostly supercooled liquid water drops that have formed in rapidly rising air. The smoother areas of the clouds are where the air is not rising as fast and ice crystals predominate. When most of the cloud takes on a smoother look, its turbulent updrafts and downdrafts are beginning to weaken.

higher to form towering cumulus clouds or thunderstorms. The ride below the clouds will be bumpy because air is sinking between the clouds and rising into them. The tops of the clouds show where the air is no longer rising. This means that climbing above the clouds will produce a smoother ride. While cruising in the generally smooth air above fair-weather cumulus clouds, you need to stay alert to the possibility that more clouds will grow, covering more of the sky—eventually blocking too much of the ground from view for a safe descent from above the clouds if you aren’t instrument-rated. Be aware that as you look toward the horizon, the clouds will look like they are closer together than the clouds under you.

To check on what they’re doing, radio the en route flight advisory service (known as Flight Watch) on 122.0 MHz, and ask for weather conditions at a couple of airports ahead of you. If the sky condition is reported as clear or as scattered clouds you should be OK, at least for now. If the sky condition is broken or overcast it means clouds cover more than five-eighths of the sky. You should descend below the clouds while there’s plenty of room between them, and continue with the knowledge you may experience a bumpier ride.
Jack Williams, a writer specializing in weather and

climate, is an instrument-rated private pilot. The latest of his six books is The AMS Weather Book: The Ultimate Guide to America’s Weather.

INSIDE A CUMULUS CLOUD
1. As air rising into the cloud cools
down, the water vapor in it begins condensing to form cloud drops.

2. After the air’s temperature falls
well below 32 degrees Fahrenheit, liquid water drops continue to form but a few ice crystals also form and begin growing.

3. Eventually, ice crystals become
predominant and grow heavy enough to begin falling.

4. As ice crystals fall into warmer
air they melt to fall to the ground as raindrops.

MAY 2011 FLIGHT TRAINING

/ 41

Flight Training - May 2011

Table of Contents for the Digital Edition of Flight Training - May 2011