Weather

Learning to fly an airplane requires a greater mix of practical and theoretical knowledge than most other activities. For example, to learn how to perform steep turns, a student needs to understand that some of the lift that was previously vertical transitions to a horizontal component. This knowledge translates into the practical action of pulling back on the yoke harder in a steep turn than during other maneuvers. In a similar way, knowing alittle about meteorological theory helps you make sense of weather briefings and the weather you encounter in flight. Stability is one of the important meteorological concepts you need to understand.

In day-to-day life we talk about a person being stable or unstable. A stable person remains cool when things go wrong. An unstable person goes crazy, such as into road-rage mode when someone cuts him off on a highway. In aeronautical terms a properly trimmed, stable airplane tends to return to level flight if a wind gust pushes the nose up. If an airplane is unstable in pitch, the nose continues rising after an upward gust. An unstable atmosphere can be turbulent and make life unpleasant, such as with thunderstorms.

You can’t understand atmospheric stability without using the term “lapse rate,” which is sometimes confusing because it’s used in three ways: Environmental lapse rate refers to the changes in temperatures with altitude from the surface to the top of the atmosphere at a particular time and place where the air is neither rising nor sinking. Weather balloons are the major source of this data, supplemented by automatic reports from passenger and freight airliners. Meteorologists usually refer to the environmental lapse rate as a "sounding" because the figures are based on measurements.

The adiabatic lapse rate is the decrease in temperature of air that’s rising or the increase in temperature of air that’s sinking. Adiabatic refers to the heating or cooling of a bubble of rising or sinking air that’s caused by what’s occurring inside the bubble as it rises or sinks. As the air rises, its pressure decreases to match the surrounding air pressure, which cools the rising air. As air sinks the surrounding air compresses it, increasing its pressure to match the surrounding pressure.

When air rises, it cools at the rate of 5.5 degrees Fahrenheit per each 1,000 feet—as long as water vapor in the air is not condensing or freezing. This is called the dry adiabatic lapse rate. Sinking air warms at the same rate as long as water drops or ice crystals in the air are not melting or evaporating. As rising air grows cooler, water vapor begins condensing into water droplets, which releases heat. This heat offsets to some degree the dry adiabatic lapse rate, creating a moist adiabatic lapse rate that can vary depending on the pressure and humidity, but 3.3 degrees per 1,000 feet is a good average figure.

The adiabatic lapse rate remains the same no matter what the temperature of the surrounding air happens to be. In other words, the surrounding cold air aloft does not cool rising air.

When the atmosphere is unstable, rising air will continue rising after the initial upward shove ends. If the air rises and becomes cold enough, the water vapor in the air will condense to form a cloud.

Air begins rising when the sun heats the ground, making air near the ground lighter. Other forces include air that’s pushed up by gust fronts flowing out from under thunderstorms, or air pushed up as wind flows over mountains.

When the atmosphere is stable, air continues rising only as long as some force continues pushing it up. Forces that push air up far enough to form clouds and precipitation include winds flowing uphill, forcing warm air up and over heavier cold air as an advancing warm front. When the atmosphere is stable, air that’s forced up condenses to form generally flat, stratiform clouds covering large areas—in contrast to the scattered cumulus clouds an unstable atmosphere creates.

For air to continue rising after an initial push ends, the rising air has to be warmer than the surrounding air. When the rising air cools to the temperature of the surrounding air, it stops rising. A stable atmosphere becomes unstable when air near the ground is heated and the environmental lapse rate is greater than 5.5 degrees F per 1,000 feet, or cold air aloft moves in to supply the needed environmental lapse rate. Broadcast meteorologists talk about “energy aloft” or “upper-air energy” moving in. They’re describing a pool of air aloft that’s cold enough to make the atmosphere unstable.

When water vapor begins condensing into liquid, the rising air cools at the slower moist adiabatic lapse rate, which means air will rise farther and faster than it would without the added heat. This is why thunderstorms that form when air near the ground is very humid can become more powerful than those that form in drier air. It’s also why humidity is often described as the “fuel” of thunderstorms and hurricanes, which are made of individual thunderstorms.

when clouds start climbing high in the sky, it’s not unusual for them to grow into thunderstorms: a natural phenomenon that pilots should think of as a no-go region with violent turbulence, hail that can break a windshield, gushes of rain that could drown an engine, and lightning.

Towering cumulus clouds are a major characteristic of a very unstable atmos-phere. Such clouds show air is rising and at least some of this is slowly descending in the clear air around the clouds. You’ll fly into more turbulence than on a day when the atmosphere is stable. Away from clouds the visibility in an unstable atmosphere is likely to be good, since rising air carries pollutants up to be replaced by sinking, cleaner upper atmospheric air. You’ll often trade this better visibility for turbulence caused by the up-and-down air movements.

If small cumulus clouds dot the sky, you can escape the turbulence by flying above them. If you do this, keep an eye on the clouds. If they begin growing into towering cumulus clouds, you know the atmosphere has become unstable to a higher altitude because the ground has warmed up or cooler air has moved in aloft.

You should stay away from towering cumulus clouds and be alert for signs they are growing into thunderstorms. The atmosphere’s stability changes with time. You should always be prepared to cope with these normal changes.