The Weather Never Sleeps

The FAA's knowledge test questions aren't the best guide to what a pilot should know about weather. Some significant weather conditions that you should know aren't even covered on the test.

There's one exception. The list of questions that you might be asked on a private pilot knowledge test includes at least a dozen on one aspect or another of air stability--a weather topic that you should try to understand thoroughly.

One reason, of course, is that one of these questions could pop up on the knowledge exam you take. An even more important reason is that understanding stability helps you to decipher weather reports and forecasts, and to "read" the kinds of clouds that you see from your airplane. Let's begin by examining some of the basic concepts of atmospheric stability and how they affect your flying decisions.

The key idea is that if the air is unstable it will continue to rise once it's given an initial upward shove. As air rises, it cools. If there is much water vapor in the air, it will begin condensing to form a cloud. Since the air is rising, the cloud will stretch upward--that is, it will have vertical development. Such clouds are called cumulus clouds; the name comes from the Latin word that gives us accumulate.

You've probably noticed that when clouds start climbing high in the sky it's not unusual for them to grow into thunderstorms: a natural phenomenon that a pilot should think of as concentrated bad stuff--violent turbulence; hail that can break an airplane's windshield; gushes of rain that could drown your engine; and lightning, which rarely causes serious damage to an airplane but is really scary.

If air that continues to rise is unstable, obviously air that stays where it is when the shoving stops--or even descends--is stable. If stable air rises enough to cool to the point where its moisture begins condensing to form a cloud, the cloud will tend to be flat, instead of climbing toward the stratosphere as a cloud does in unstable air. Such clouds are stratiform, or stratus, clouds.

You won't run into a thunderstorm when the air is stable, but that doesn't mean great flying is guaranteed. If there's enough moisture around, stable air can bring you a sky full of clouds, maybe clouds that are almost sitting on the ground. Even if the stable day doesn't have a sky full of clouds, it could bring poor visibility in haze.

Why does stable air do this? Rising unstable air is hauling pollutants and other stuff that creates haze high into the sky. At the same time, air is descending in places where it isn't rising, bringing down clean air from aloft.

Knowing even the little bit that we've discussed so far, you're ready to answer some of the knowledge test questions about stability.

What is a characteristic of stable air?

1. Cumulus clouds.
2. Unlimited visibility.
3. Stratiform clouds.

You know right away to eliminate answer "A," since cumulus clouds form in unstable air. If you missed the part above about how stable air can be hazy, you could put "B" aside for some more thought. "C" is the right answer. If clouds form in stable air they will be stratiform.

To be absolutely sure about the question below, you should learn the definitions of some meteorological terms.

Moist, stable air flowing upslope can be expected to

1. produce stratus type clouds.
2. develop convective turbulence.
3. cause showers and thunderstorms.

Even if you don't know what convective means, or aren't too clear about showers, you should have no problem seeing that answer "A" is correct. It has the magic word stratus, which goes with stable.

Convective refers to the up-and-down air movements that occur when the air is unstable. When you hear or read convective in a weather report, think thunderstorms.

A shower is not light rain. In fact, showers can produce some of the heaviest rain. A shower refers to single clouds producing rain. There can be many of them around, but there is dry air between them. If you are driving on a day with showers, you'll go in and out of rain. If you're flying, you should be diverting around the showers, giving them plenty of room.

Showery precipitation (rain or snow) is characteristic of unstable air. You can think of a thunderstorm as a shower on steroids.

Let's look a little closer at what happens when air rises. It will help you to understand what makes air stable or unstable.

When air rises, it cools at a steady rate of 5.5 degrees per 1,000 feet as long as condensation isn't taking place. As air rises the surrounding air pressure decreases. As the pressure decreases, a bubble of rising air expands so its pressure matches its surroundings. When air expands, it cools. In other words, rising air becomes cooler because it's expanding, not because it's coming in contact with cooler air.

When water vapor begins to condense, the cooling rate slows down. This is because condensing water vapor releases latent heat that it picked up when it originally evaporated. This means that as expansion is cooling the rising air, the release of latent heat is trying to warm it. The warming doesn't overcome the cooling, but the actual difference depends on various factors, such as how much condensation is occurring. A good average figure for the cooling rate when water vapor is condensing is around 3.3 degrees per thousand feet.

You're probably aware that air rises when it's warmer than the surrounding air. Air is unstable when it rises like a hot-air balloon. Because air cools as it rises, however, this balloon of rising air is controlled by the surrounding air.

To see what this means, let's assume the temperature on the ground is 85 degrees and air begins rising from the ground. It will continue to rise as long as it's warmer than the surrounding air. The table below shows what happens on two different days. The columns on the right and left are the actual air temperatures measured by a weather balloon early in the morning. The column in the middle is the temperature of rising air.

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Day 1 would be a day with a stable atmosphere because even when the air is pushed up to 4,000 feet above the ground, it remains cooler than the surrounding air. As soon as the lifting force stopped, the heavier air would sink like a hot air balloon with the burner turned off. Day 2 is unstable because at each altitude the rising air is warmer than the surrounding air. The quicker the air aloft cools, the more unstable it is.

The ultimate in stable air is an inversion, which means the air actually increases in temperature as you go higher. For instance, if the air on Day 1 had been 87 degrees at 1,000 feet with everything else the same, that would be an inversion.

Knowing what an inversion is makes the next question a snap.

What feature is associated with a temperature inversion?

1. An unstable layer of air.
2. Chinook winds on mountain slopes.
3. A stable layer of air.

The answer, of course, is "C."

You might be thinking by now that unstable air is always bad news. But, the weather isn't that simple. Consider this question:

Possible mountain wave turbulence could be anticipated when winds of 40 knots or greater blow

1. across a mountain ridge, and the air is stable.
2. down a mountain valley, and the air is unstable.
3. parallel to a mountain peak, and the air is stable.

If you think unstable always means bad, you'll pick "B." Let's take a look. If you're in a small airplane, 40-kt winds might not be too pleasant. But, this question is about mountain wave turbulence, so let's see what that is.

When the wind blows across a mountain ridge it's forced up on the windward side. What happens then depends on the stability. If the air is unstable, the air that was pushed up the mountain ridge continues to rise. Unless the air is very dry you will see what you would expect in unstable air; cumulus clouds and then maybe thunderstorms begin to form over the mountains.

But, what happens when the air is stable? Once the air reaches the top of the mountains it begins to descend; since the air is stable it wants to return to where it was before being forced up the mountains. However, the air is moving so fast that it descends too far and goes back up. This up-and-down motion creates a mountain wave, which in turn can produce severe turbulence.

In other words, when the weather forecasters are talking about unstable air, be on the alert for thunderstorms.

Just because the air is stable, don't assume that flying is going to be great. True, the ride could be smooth, but you could run into poor visibility. Knowing whether the air is stable or unstable is just the starting point for figuring out what kind of weather you'll have for a flight.

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Jack Williams is the weather editor of USAToday.com. An instrument-rated private pilot, he is the author of The USA Today Weather Book and co-author with Dr. Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.