Calculating potential storm energy
The Cessna's aviation gasoline won't work in a rocket because it needs the lower atmosphere's oxygen to transform the gasoline's chemical energy into heat energy, which the engine converts into the mechanical energy that turns the propeller. The liquid hydrogen and liquid oxygen that fuel the shuttle's main engines need to be stored in heavy cryogenic tanks that keep the oxygen colder than minus 238 degrees Fahrenheit and the hydrogen colder than minus 423 degrees F. Rocket fuel is neither practical nor safe for your Cessna 152.
The different atmospheric "engines" that produce a calm day with no fireworks or a day with thunderstorms also differ from each other, although not nearly as much as the shuttle's and Cessna's engines. In fact, the atmosphere's engines morph from one into the other with day-to-day changes in weather.
To differentiate between the two kinds of weather "engines," meteorologists say the atmosphere at a particular time and place is either stable or unstable. It's not too much of a stretch to think of a stable day as being like a Cessna 152, steady and reliable, but without fireworks. An unstable day, with a good chance of thunder, is more like the space shuttle; if you're anywhere nearby you'll hear it.
Once you learn how to read the sky you can have a pretty good idea of what kind of weather engine is energizing the atmosphere that day. When you're getting ready for a flight, however, this isn't enough. To really know what's likely to happen during your flight you need to obtain a weather briefing.
The difference between the atmosphere's two engines is quite subtle, depending on only a few degrees' temperature difference at various altitudes. To understand how this works you need to know a few basics:
- If the air pressure of a bubble of air and the air surrounding the bubble is the same, the warmer of the two will be less dense.
- If a bubble of air is less dense (or lighter) than the surrounding air it rises.
- As a bubble of air rises its pressure decreases to match the pressure of the surrounding air and the bubble cools.
- As a bubble of air sinks its pressure increases to match the pressure of the surrounding air and the bubble warms.
- The rate at which a bubble of rising air cools and sinking air warms is 5.5 degrees Fahrenheit per 1,000 feet no matter what the temperature of the surrounding air.
The atmosphere is stable when a bubble of air that begins rising for any reason cools below the temperature of the surrounding air. This means it becomes denser and sinks until its temperature is the same as the surrounding air.
The atmosphere is unstable when a bubble of rising air stays warmer than the surrounding air, and thus less dense, even though the rising air is cooling at the rate of 5.5 degrees per 1,000 feet. This rate is known as the dry adiabatic lapse rate. Adiabatic means that no heat is being added or taken away by the surrounding air. The temperatures of the surrounding air, which isn't moving up or down, is the environmental lapse rate.
A bubble of air continues rising until it cools to the temperature of the surrounding air. This can occur a couple of hundred feet above the ground, or 40,000 or more feet up.
Since rising air always cools at the same rate, the difference between a stable or unstable atmosphere depends on the temperatures of the air that's already in place at different altitudes. Meteorologists obtain these temperatures mainly from weather balloons that are normally launched twice a day from locations around the world. If the temperature profile is like the one shown in Figure 1 the air is stable because the rising air will grow colder than the surrounding air. If it's more like that of Figure 2, the air is unstable because the rising air stays warmer than the surrounding air. These figures show the atmosphere only up to 4,000 feet, but the idea applies no matter how high you go. The basic concept is the same for all altitudes; as long as the rising air stays warmer than the surrounding air it continues rising.
Why do we say this means the air is unstable? The term unstable is applied to physical systems--including aircraft and the atmosphere--and to people in slightly different but related ways. One way to remember what we mean when we say the atmosphere is unstable is to think of what an "unstable" person is like. He or she overreacts to a minor disturbance. When the atmosphere is unstable, once anything causes the air to begin rising, it will continue rising--so high it will cause violent weather if the air is unstable enough.
When the air is stable, air that's given an upward shove stops rising when the shove ends. The atmosphere stays calm. An arriving cold front and various other kinds of atmospheric disturbances can give the air an initial upward shove. Or, air can begin rising when incoming sunlight, which doesn't warm the air on the way down, warms the ground, which in turn does warm the air next to it.
If the atmosphere is unstable, and unless rising air is very dry, it will cool enough for its humidity to begin condensing into tiny drops of water. When water vapor condenses it releases the latent heat that it gained when the water vapor evaporated. The release of latent heat warms the rising air bubble at the same time as it's being cooled adiabatically while it continues rising.
The heat being added by condensation isn't enough to completely offset adiabatic cooling, but it slows the cooling with the rate depending on the temperature and pressure of the rising air. A good average value is about 3.3 degrees per 1,000 feet. In other words, when the air near the ground is humid, the atmosphere is even more unstable than when the air is dry.
The latent heat released when water vapor condenses is why meteorologists talk of humidity being the "fuel" for thunderstorms. The faster and higher the air rises, the more powerful the thunderstorms it creates will be. The speed of the rising air, in turn, depends on the temperature difference between the rising air and the air it's rising through. The greater the difference, the faster the air rises.
Over the years meteorologists developed various ways of calculating the amount of instability. One of the common measures of instability is the lifted index. It's calculated by comparing the measured temperature at 18,000 feet and the calculated temperature the air would be at that altitude if it rose from the ground, cooling at the adiabatic lapse rate. If the air that rises would be as much as 2 degrees warmer than the surrounding air, the atmosphere would be somewhat unstable and thunderstorms are probable. If the rising air would be 4 degrees or warmer than the surrounding air, severe thunderstorms are possible. (A severe thunderstorm has winds of 50 knots or faster at the ground, hail three-quarters of an inch in diameter or greater, or creates a tornado.)
A problem with the lifted index is that it considers only one level of the atmosphere. Other indexes use more than one level and also consider the air's humidity at various altitudes. Still, they look at only a small part of the atmosphere.
As far back as the 1920s, meteorologists had developed a measure of potential thunderstorm energy that uses actual and calculated temperatures and humidity at all altitudes up to more than 40,000 feet, known as convective available potential energy (CAPE). Convective refers to up-and-down air movements such as those found in thunderstorms, and meteorologists use the term to refer to thunderstorms.
As the name says, CAPE is a measure of the potential energy available in the atmosphere to power thunderstorms. It doesn't say whether that day's thunderstorms will tap all of the potential energy, but it's a good measure of the potential danger.
Calculations of this index, however, were too complex to be done for day-to-day forecasting until the late 1970s when the National Weather Service began installing computers in its offices around the country that were able to calculate CAPE. This is done using the latest weather balloon observations from each office's forecast area. Today you are likely to see CAPE mentioned if you read the technical discussions from the National Weather Service's Storms Prediction Center or local Weather Service offices when severe thunderstorms or tornadoes are possible.
CAPE estimates the "fuel" available to accelerate air upward. Meteorologists use a rule of thumb (an approximate mathematical formula) to estimate the fastest possible thunderstorm updrafts: Multiply CAPE by two and take the square root of the answer for updraft speed in knots. This is only a rough guess of possible updraft speeds, but it gives you an idea of how violent thunderstorms could be.
In other words, if the CAPE is 3,000, which is in the middle of the very unstable range (see Figure 3), you could expect updrafts of approximately 77 kt. Another rule of thumb is that a thunderstorm's downdrafts will be about half as fast as the updrafts; in this case, say, 38 kt. Now, imagine flying into a 38-kt downdraft and then seconds later into a 77-kt updraft. That is a wind shear of 115 kt, easily enough to throw even a large aircraft out of control, if not worse.
Even if the CAPE were "only" 500, in the "marginally unstable" range, you wouldn't want to fly into or very close to any growing cumulus clouds on such a day since the updraft speed could be in the range of 32 kt, which would give you a pretty bumpy ride.
While powerful thunderstorms wouldn't be likely on such a day, you should expect cumulus--piled-up--clouds and a bumpy ride. The visibility is likely to be good because the rising air carries pollution up to be blown away and brings down fresher air from aloft. If any rain or snow is falling, it's likely to be in showers--the precipitation will be on and off with clear sky between the showers.
If you go flying on such a day, you are likely to have a smoother ride if you climb above the tops of any puffy clouds in the sky. On such a day, the clouds top out where the rising air is no longer warmer than the surrounding air; thus no longer rising.
On a day when the atmosphere is stable, your flight is likely to be smooth since you won't run into rising and sinking air. Clouds will be generally flat. If it's raining or snowing, the precipitation is likely to cover a wide area.
Obviously, stability is only one of the factors that determine what a day's weather will be like. It is important, however--and one of the many aspects of meteorology you should try to master whether you'll be piloting a Cessna 150 or the space shuttle.
Jack Williams is coordinator of public outreach for the American Meteorological Society. An instrument-rated private pilot, he is the author of The USA Today Weather Book and The Complete Idiot's Guide to the Arctic and Antarctic, and co-author with Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.
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