Weather

Pilots who have not yet earned an instrument rating must stay away from clouds because flying into them is both illegal and dangerous. This doesn’t mean, however, that clouds are merely something new pilots must avoid.

Clouds can sometimes help you better understand what the weather is doing and where to find the smoothest air in which to fly. And when you start training for an instrument rating you’ll have to learn more about what happens in the clouds you’ll be flying through.

Clouds need rising air. Clouds are made of very tiny ice crystals or drops of water that are always being pulled down by the force of gravity. Like any falling object, the crystals or drops create drag, which slows the speed at which they fall, until they reach a terminal velocity—when the forces of gravity and drag are equal. At the terminal velocity an object falls at a steady rate.

The ice crystals or water drops that make up a cloud stay in place because air is rising into a cloud as least as fast as the terminal velocity of the cloud’s ice crystals and water drops.

An object’s terminal velocity depends mostly on its mass, size, and shape. The approximate typical terminal velocities in the chart on page 39 show how fast the air has to be rising for clouds and raindrops of certain sizes to stay aloft. When they grow large enough for their terminal velocity to exceed the velocity of rising air, ice crystals begin falling as snow or hail; water drops begin falling as rain.

Terminal velocities of ice crystals and hail are harder to characterize because their complex shapes affect terminal velocities in various ways.

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 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.