The big problem with trying to obtain weather information by reading the sky before or during a flight is that only a few minutes of flying, even in a slow aircraft, will carry you over the horizon, maybe into huge thunderstorms that were hidden by the Earth's curvature.
No sensible pilot wants to return to the barnstorming days when a pilot's weather information was based on what could be seen from the ground and the cockpit. On the other hand, understanding the basic alphabet of the sky ? clouds - increases a pilot's ability to round out the weather picture obtained from a preflight briefing and regular in-air updates from flight watch.
Weather courses often introduce the study of clouds by teaching their Latin names, such as altocumulus or nimbostratus. This information is nice to know, and having names for different kinds of clouds is useful. But beginning to learn about clouds by learning their names is like beginning to learn a foreign language by taking a course in grammar. You want to learn how to ask questions and understand the answers: "Where's the toilet? How do I get to the airport?"
In a similar way, pilots want answers to questions such as, "Does that big, lumpy cloud directly ahead mean I should change my course?" Being able to answer questions about clouds begins with knowing what clouds are, how they form, and the major factors that dictate their appearance.
Clouds are made of tiny water droplets - perhaps 10 to 20 microns in diameter - or ice crystals that are a little bigger. Clouds form when the air is cooled to the dew point, the temperature at which water vapor begins condensing into water droplets or, if it's very cold, sublimating directly into ice crystals.
The very fact that warm air rises accounts for the fact that the air becomes cold enough for clouds to form. As air rises, it cools by approximately 5.5 degrees Fahrenheit for each 1,000 feet in altitude gained, regardless of the original temperature of the rising air and the surrounding air. This helps answer a question that bedeviled natural philosophers (we call them scientists today) well into the nineteenth century: Since clouds are made of water droplets or ice crystals, which are heavier than air, what holds clouds up? Answer: rising air.
All falling objects, including water droplets or ice crystals in a cloud, have a terminal velocity. This is the fastest speed it will fall before drag creates an upward force equal to weight. When this happens, downward acceleration stops and the object continues falling at the terminal velocity. A cloud forms and lasts when the air is rising at the same, or greater, speed than the terminal velocity of the cloud's water droplets or ice crystals. In other words, a cloud tells you that the air there is rising. Most of the time, unless the cloud is tall and lumpy, the air is rising so slowly that you aren't likely to notice it. Precipitation begins falling from a cloud when the water drops or ice crystals grow too large for the rising air to continue holding them up.
The general shapes of clouds depend on whether a large mass of air is rising more or less at the same speed at the same time, or whether relatively small parcels or bubbles of air are rising while the surrounding air is either staying in place or even sinking. When a large mass of air rises at the same time the clouds formed are called stratus clouds. These form in stable air that resists rising. If a small amount of air is given an upward shove, it sinks as soon as the shove stops. To rise far enough to create clouds, a large mass of air has to be pushed upward, such as when warm wind is forced up over cooler air. The air that forms stratus clouds rises steadily and slowly, which means that an aircraft should have a smooth ride in or under the clouds. Any precipitation is likely to be steady and cover a large area.
Lumpy clouds are called cumulus clouds. Such clouds form when the air is unstable. That is, if anything gives the air an initial upward shove, the air keeps on rising. The air that forms cumulus clouds rises in large bubbles, meaning the air will be rising in one place while it's steady or even descending only a few feet away. Such up-and-down air movements mean a bumpy ride in or below the clouds. If you're being bounced around under puffy clouds, try climbing above them. This will usually give you a smoother ride because you are above the level at which the air is rising.
The up-and-down air movement that creates cumulus clouds is called convection, although this term is usually reserved for strong convection that produces the ultimate in cumulus clouds - thunderstorms. As a cumulus cloud grows higher and higher, its appearance can tell a pilot a great deal about what's going on inside the cloud. In a cluster of cumulus clouds some may be growing rapidly while others are beginning to dissipate, some of the cumulus clouds may have a hard, cauliflower-like look while others have a softer look. The clouds that look more solid are caused because air is rising rapidly inside them. Water droplets that form at lower altitudes are carried upward so quickly that they remain liquid even though the temperatures high in the clouds are below freezing. These supercooled liquid water droplets are relatively small and scatter large amounts of light back to the camera or your eye. If the clouds don't rise as much and have a hard looks then the updrafts rising into the clouds have slowed or stopped. When this happens, the supercooled liquid water drops turn into ice crystals, which are larger than the water droplets. Cloud scientists say the cloud has glaciated.
A pilot who flies into a cloud that has a hard look, like those we just mentioned, is likely to have a rough ride since the cloud has strong updrafts. The pilot also has to worry about the supercooled water droplets freezing when they hit the airplane, forming ice.
A pilot who flies into a cloud that has a softer lighter look should have an easier time. The updrafts are weaker and the icing hazard should be less. But there are no guarantees. The soft-looking cloud you fly into could be the side where the updrafts have weakened. As you bore through the cloud you could run into a region of stronger updrafts that was hidden from view.
Life would be easier for meteorologists and pilots alike if the sky at a particular time and place always had only stratus clouds or only cumulus clouds. Nature isn't this simple, however. Anyone who has spent much time watching the sky knows that cumulus clouds can share the sky with stratus clouds. Pilots flying in instrument conditions sometimes have to worry about embedded thunderstorms - thunderstorms hidden by surrounding stratus clouds.
Mixtures of stratus and cumulus clouds occur because different layers of the atmosphere over the same location can have different degrees of stability. Also, no sharp line divides stable and unstable air. Clouds that form when air is on the vague boundary between stable and unstable can have characteristics of both stratus and cumulus clouds.
Clouds are shaped by wind as well as by rising and sinking air. Lenticular clouds - the name tells you that they are lens-shaped - mark otherwise invisible up-and-down waves in the winds. Such waves are usually caused by stable air flowing over mountains. The air is forced upward over the mountains, but since it's stable, it begins sinking once it's clear of the mountains. Momentum carries the air down past its original level, and the air then rises again. This up-and-down motion can create a series of waves stretching hundreds of miles downwind from the mountains.
Lenticular clouds form at the crests of the waves downwind from the mountains. At the edge of the cloud nearest the mountains, the air is rising and cooling until water vapor condenses into cloud droplets. At the edge of the cloud away from the mountains, the air is sinking and warming, which evaporates the cloud droplets. Such clouds can warn of turbulence, sometimes extreme turbulence because the wind may act somewhat like the water of a breaking ocean wave. Don't count on lenticular clouds to warn you of mountain-wave turbulence, however, because the air might be too dry for clouds to form.
Bottoms of clouds that look ragged or have virga - rain or snow that evaporates before reaching the ground - falling from them warn of the potential for turbulence. If a cloud has a ragged base, you are probably seeing virga that isn't falling very far before evaporating.
Any time water drops or ice crystals evaporate to become invisible water vapor, the air around them cools, making it heavier so that it plunges toward the ground. If the air is moving fast enough, it can spread out when it hits the ground, causing a sudden change in wind speed and direction known as a dry microburst. Usually the only sign of a dry microburst is a ring of dust kicked up by falling air hitting the ground. A strong blast of cooled air that comes down with rain and reaches the ground is known as a wet microburst. The sudden wind shifts, or wind shear, caused by microbursts is especially dangerous to aircraft near the ground because a sudden shift from a head to a tail wind decreases lift and the aircraft can strike the ground.
Winds blowing at different speeds in different layers of air can create waves like those in the ocean - especially when the different layers of air have different temperatures. And, like ocean waves, these waves can break. Such breaking waves of wind are usually invisible, but if the air is humid enough clouds can form and outline the shape of the breaking waves these are called billow clouds. Billow clouds almost guarantee severe or extreme turbulence.
Next time you fly, look at the sky and imagine you're a pilot of the 1920s - maybe even Charles Lindbergh - whose safety depends on making sense of the sky. Being able to read the sky gives you an edge in airmanship that brings you a little closer to being in the same class as the Lindberghs of the world.
