March 25, 2013
BY JACK WILLIAMS (From Flight Training, November 1996)
Air stability is one of those theoretical-sounding subjects that pilots should learn for good, practical reasons. This knowledge helps you make sense of preflight weather forecasts and actual weather you encounter in flight.
Also, the FAA likes to ask questions about stability on its knowledge tests.
Let's begin with some simple, key ideas about air stability and then see what they mean to pilots.
Unstable air is like an unstable person. When we see someone who begins screaming for a trivial reason, we're likely to say he's "unstable." Most of us try to avoid unstable people because we fear something minor will trigger violence.
It's the same thing with unstable air. If the weather forecast says the air is unstable, there's a good chance we're going to see towering, bumpy, cumulus clouds, and maybe thunderstorms.
In contrast, a stable person stays cool when things go wrong. But this doesn't necessarily mean you'd want to spend a lot of time around him. A stable person may be gloomy, no fun at all.
Stable air isn't likely to cause extreme turbulence, as a thunderstorm could. But it can bring widespread low clouds, steady precipitation, and poor visibility-gloomy weather.
To visualize what makes the air stable one day and unstable another, let's fly a hot air balloon. As our balloon is being readied for flight, we see its envelope rise as the air in it is heated by the propane burner attached to the basket. The crew holds the balloon down until we are in the basket. When the pilot tells the crew to let go, we lift off the ground. When our ascent slows, the pilot turns on the burner, and we continue rising. Why?
Even if you know nothing about balloons, you're likely to say that the balloon rises because the hot air in the envelope is lighter than the surrounding air. This nylon-shrouded light air is floating through the cooler, heavier air around it, just as a ping-pong ball rises when released at the bottom of a pool.
The pilot turns on the burner when the air in the envelope cools to the point where it is no longer buoyant. You can say the balloon rises because the air in the envelope is unstable. The pilot uses the burner to keep the air in the envelope unstable.
This is air stability in a nutshell. When a bubble of air-whether it's in a balloon's envelope or surrounded by nothing but air that's not rising-is warmer than the surrounding air, it rises. When air rises, we say it's unstable.
A balloon pilot uses the burner to keep the air inside an envelope unstable. But how does nature do it? Because nature doesn't use a propane burner, the answer is a little more complicated. But it's not too complicated if we clarify the meaning of a somewhat murky term-lapse rate.
Just what is lapse rate? It's really two different terms, which are often confused with each other. Without understanding the difference between the two, you'll never understand stability.
The first is environmental lapse rate. It's nothing more nor less than the rate at which temperature decreases-or sometimes increases-as you climb higher in air that is neither rising nor sinking.
You often hear that the lapse rate is a decrease of 3.6?F for each 1,000 feet of altitude gained. This is the decrease in a standard atmosphere, which you can think of as the average for the entire world for all seasons. At sea level, a standard atmosphere has a barometric pressure of 29.92 inches of mercury and a temperature of 59°F.
If the actual environmental lapse rate were always 3.6?F per 1,000 feet, you'd never see cumulus clouds or thunderstorms. In the real world, the environmental lapse rate is hardly ever 3.6?F per 1,000 feet, and it usually doesn't stay the same as you climb. Meteorologists use balloons with thermometers that radio information to earth, or automated radio temperature reports from some airliners, to keep up with the actual environmental lapse rate. These measurements, along with surface temperature and humidity measurements help determine whether the air is stable or unstable at a particular time and place.
The second term is the adiabatic lapse rate. This describes rising air that cools or warms without being cooled or heated from the outside. Adiabatic lapse rate is independent of the temperature of the surrounding air. What's happening inside parcels of rising or sinking air causes the air to cool or warm.
Imagine a parcel, or bubble, of air enclosed by an invisible, stretchable envelope. As the parcel rises, the pressure around it decreases. The air inside the parcel expands to match the outside pressure. Because there's no energy coming from outside, some of the air's heat energy is expended. This causes the parcel to cool.
There are two adiabatic lapse rates. The difference between them is the moisture content of the air. Rising air cools at 5.5?F per 1,000 feet no matter what the temperature of the surrounding air-as long as water vapor in the air doesn't condense into liquid or freeze into ice. This is dry adiabatic lapse rate.
If rising air cools enough for water vapor to condense, things become more complicated. When water vapor condenses or turns directly into ice (sublimation), or when water freezes, it releases latent heat. In a rising parcel of air, this means heat is generated as vapor condenses. This tends to offset some of the cooling caused by the air rising and expanding.
The release of latent heat is nature's propane burner. It makes air more unstable than it otherwise would be. Now the air is cooling at the moist adiabatic lapse rate. The actual moist rate depends on the temperature and pressure of the rising air, but 3.3? per 1,000 feet is a good average figure.
To see how environmental, dry adiabatic, and moist adiabatic lapse rates combine to determine whether air is unstable, let's fly two more balloons. We've brought along some small balloons, the kind you use at kids' parties. We've blown them up, and the temperature inside the small balloons equals the surrounding air.
As we climb, the small balloons will expand, and the air inside them will cool adiabatically.
The environmental lapse rate for our first flight is the standard-atmosphere rate. The temperature de-creases by 3.6°F per 1,000 feet.
Our second flight takes place several days later. The surface temperature is 59?F, as it was on the first flight, but the air cools rapidly as we ascend.
Let's draw some conclusions from our two balloon trips. The air is stable when the environmental lapse rate is relatively low. If the air cools rapidly as we ascend, the air is unstable.
If the rising air is humid, it's even more unstable because water vapor in the air will condense. It decreases the adiabatic lapse rate of the rising air. In this case, rising air stays warmer in relation to the surrounding air than when the air is dry.
Stable air creates generally flat clouds when it's forced upwards. This happens when warm air flows over cold air ahead of a surface warm front.
Unstable air creates cumulus clouds and thunderstorms because it rises on its own. Eventually, it's cool enough for humidity to condense into clouds and release latent heat, which makes the rising air even more unstable. Latent heat released by condensation or formation of ice as unstable air rises is the major source of energy for thunderstorms and hurricanes.
In the real atmosphere, air can be stable at some altitudes and unstable at others. For example, on a day when puffy clouds form but aren't able to grow, the atmosphere is unstable at lower altitudes, but stable above the tops of the clouds.
The most stable times occur when air temperature actually increases with altitude. This situation is called an inversion. It can cause poor visibility because the inversion acts like a lid to keep foggy or polluted air from rising and being replaced by cleaner air from aloft.
The most unstable days-the ones with towering thunderstorms-are hot and humid near the ground, but very cold aloft. This usually happens when cold, upper-air disturbances move over warm air.
On stable days, the air has little up and down motion. Unstable days bring rising and sinking air. As a result, stable air tends to produce flat clouds that might cover most or all of the sky. Any rain or snow will be steady, but probably not heavy. Visibility is likely to be poor. The one good fact for pilots is that the air will tend to be smooth.
Unstable air produces puffy, cumulus clouds that can grow into towering cumulus or thunderstorms when the air is unstable and humid enough. Such clouds aren't likely to cover all of the sky all day. Rain or snow can be heavy, but it tends to come in bursts; that is, it's showery. Because the up and down air motions mix up the air, visibility is likely to be good when you're away from the clouds. But this motion makes for bumpy rides.
Short-term weather forecasts are based to a large extent on how stable the air is and how stable, or unstable, it is forecast to become. You can find clues by paying attention to your aircraft's outside air thermometer and comparing the actual temperatures to those in the winds aloft forecast.
If the temperatures aloft are colder than forecast, this could mean that the air is more unstable than the meteorologists expected. Thunderstorms could be more likely, or the smooth ride you were expecting could turn rough.
If the temperatures aloft are warmer than forecast, the air could be more stable than expected. In this case, visibility could turn out to be worse.
As we go about our daily business, we try to avoid unstable people, the kind who might try to ram our car if they think we cut them off on the expressway. Pilots don't always have to avoid unstable air. But, they ought to know what to expect when the air is unstable.
The air we fly through is seldom neutrally stable. Convection can give us a bumpy ride, and at times, convection can be violent enough to be hazardous. Stable air usually gives a smooth ride, but it often means poor visibility and a low ceiling.
During preflight planning, we can observe the following signs to plan accordingly for associated hazards.
When outside air temperature (OAT) decreases uniformly and rapidly as you climb (approaching 5 to 6?F per 1,000 feet), the air is likely to be unstable.
If the OAT remains constant or decreases only slightly with altitude, the air tends to be stable.
If the OAT increases with altitude through a layer-an inversion-the layer is stable, suppressing convection. The air may be unstable below the inversion, however.
Warm, moist air near the surface generally indicates instability. Surface heating, cooling aloft, converging or upslope winds, or an invading mass of colder air may lead to instability and cumuliform clouds.
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