Picture a hot-air balloon ...
In general, when the air is unstable, turbulence is much more likely. Clouds will extend vertically into the sky, and their sides and tops will be lumpy. Rain or snow will come and go, with dry air -- maybe even a clear sky -- between the showers. By the way, the term showers does not refer to light rain or snow, but to patchy rain or snow, which can be very heavy.
When the air is stable your flight will be smooth, and clouds will be flat. Rain or snow will tend to fall steadily over a large area.
Now, let's see how a hot air balloon can help you to understand air stability.
Even if you've never taken a lesson in flying hot-air balloons, you probably know that to leave the ground, the pilot ignites a flame that heats the air inside the balloon's envelope. The balloon rises because "warm air is lighter than cool air."
To descend, the pilot extinguishes the flame, the air cools, and the balloon descends because the air inside its envelope is no longer lighter than the surrounding air. By lighter, we really mean that warm air is less dense than cooler air; that is, a cubic yard of warm air weighs less than a cubic yard of cool air.
Air pressure, which goes up and down with weather changes and decreases with altitude, and the humidity also affect the air's density. But, the air's temperature has the greatest effect on its density. To begin understanding air stability, we need to look at only its temperature.
Even without the fabric of a balloon's envelope around it, a bubble of air will rise as long as it's warmer than the surrounding air. Rising air is one of the most important weather makers, because as air rises it cools. If the air cools enough, the water vapor - humidity -- in it will begin condensing to form clouds and eventually rain or snow. In other words, for clouds and precipitation to form, air has to rise and cool.
Both stable and unstable air can rise to form clouds and precipitation, but the different ways in which they rise make all of the difference:
- Unstable air needs a shove to begin rising, but once started, continues rising after the shove goes away.
- Stable air rises only as long as something is shoving it up.
Let's look at how unstable air keeps rising even after the initial shove disappears, and what kind of weather this creates.
For air to rise after an initial push ends, the rising air has to be warmer than the surrounding air -- that is, it has to act like a hot-air balloon with the burner turned on. When the rising air cools to the temperature of the surrounding air, it stops rising. That is, the air is no longer unstable. Rising air cools at a rate of 5.5 degrees Fahrenheit per 1,000 feet, as long as the humidity in it is not condensing. This cooling rate is the same no matter what the temperature of the surrounding air happens to be. In other words, the surrounding cold air aloft is not what cools rising air. This point is very important to understanding stability.
Imagine a bubble of rising air. As the bubble rises, the air pressure around it decreases, and the bubble expands so that its pressure matches the air around it. Since there's no energy coming from outside, some of the heat energy of the molecules of the gases that make up the air inside the bubble is used to expand the bubble, and it cools. This is dry adiabatic cooling because the air's humidity isn't involved in the cooling; adiabatic refers to a temperature change that isn't caused by outside cooling or heating.
Whenever air descends, the opposite happens. The increasing air pressure on the descending bubble squeezes it, warming the air at a rate of 5.5 degrees per 1,000 feet. The temperature change in rising or descending air is called the dry adiabatic lapse rate.
Eventually, unless rising air is very dry, it will cool enough for its humidity to begin to condense into tiny drops of water -- or, if it's cold enough, to sublimate into tiny ice crystals. Since both of these release heat, the rising air bubble is now being warmed as well as being cooled adiabatically as it continues to rise.
The heat being added by condensation or sublimation isn't enough to completely offset the adiabatic cooling, but it does slow the process. The actual cooling rate depends on the temperature and pressure of the rising air, but 3.3 degrees per 1,000 feet is a good average figure.
What all of this means is that if the temperature of the surrounding air falls at a rate faster than 5.5 degrees per 1,000 feet as the air is rising with no condensation or sublimation, or 3.3 or so degrees when water vapor in the air is condensing or sublimating, the air is unstable.
If the temperature of the surrounding air cools slower than 5.5 degrees per 1,000 feet, the air is stable, because if something shoves air up, it would remain colder than the surrounding air as it rises. As soon as the shove stopped, the bubble of rising air would be like a cold-air balloon and sink instead of rising.
The rate that the temperature of the surrounding air decreases, or even increases, is also called a lapse rate, but this is the environmental lapse rate. In the standard atmosphere, which you can think of as an average for all seasons, the environmental rate is 3.6 degrees per 1,000 feet.
What you find on any day, however, is likely to differ considerably from the standard atmosphere. On some days you'll find an inversion as you climb; that is, the air at some level aloft is actually warmer than the air below. On other days, if you noted the readings of your airplane's outside-air thermometer each 1,000 feet as you climbed, you'd see that the air cools faster than 3.6 degrees per 1,000 feet. If it cools faster than 5.5 degrees per 1,000 feet, the air is unstable.
To take a closer look at this, let's assume the temperature on the ground is 85 degrees Fahrenheit and air begins rising from the ground on two different days. Remember, air will continue rising as long as it's warmer than the surrounding air. In the accompanying chart (see p. 51), 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, which always cools 5.5 degrees per 1,000 feet no matter what the temperature of the surrounding air happens to be.
Obviously, the air is stable on Day 1 since the rising air's temperature (in the middle column) is cooler than the surrounding air's temperature in the Day 1 column. But, on Day 2, the air is unstable because the temperature of the rising air stays warmer than the surrounding air temperatures.
The chart, of course, shows only the stability of the air from the surface to 4,000 feet. On the day when the first 4,000 feet of the air is stable, the air higher up could be unstable. If air is shoved up past 4,000 feet, it could continue to rise on its own. And, on the day when the first 4,000 feet of the air is unstable, it could be stable higher up. In this case, puffy, cumulus clouds could form at lower altitudes, but top out somewhere above 4,000 feet.
When the air is unstable to high altitudes, vigorous thunderstorms can form if the air is humid enough. Whether vigorous thunderstorms or smaller clouds -- maybe with showery rain but no lighting or thunder -- form on an unstable day, the rising air currents and air descending nearby will make for turbulence. The initial upward shove that starts unstable air moving up can be heating of the ground that makes bubbles of air warmer than surrounding air; converging winds; or a cold front, which acts like a plow that pushes warm air up.
Stable air, of course, does not guarantee clear skies. Wind blowing uphill can push stable air up to form fog or clouds. Widespread clouds and precipitation also form in stable air when a warm front pushes the air up as warm air replaces cool air.
A warm front -- the boundary between warm and cold air at the Earth's surface -- might be 300 or 400 miles long. If it's running east to west across part of the United States, the upper-air warm/cold boundary might stretch 200 miles to the north, marked by solid clouds where the humidity in the warm air is condensing as the warm air rises and cools.
Unlike on a stable day, the air isn't rising more or less straight up, but on a slant. Still, it cools at the 5.5-degree rate per 1,000 feet of altitude gain. Steady rain could be falling over an area stretching maybe 400 miles east to west and 100 or more miles north to south ahead of a warm front. Obviously, neither stable nor unstable air guarantees that you are going to have a good day for flying.
The air's stability, combined with other weather factors such as the amount of humidity in the air and the movement of large storm systems, will determine the weather. Since we're now moving into spring, however, the kinds of bad weather caused by unstable air are becoming more likely.
In general, the atmosphere is more unstable in the spring than during other seasons because as the days grow longer and the sun moves higher into the sky, the ground warms up and heats the air close to it. But, since most solar energy passes through the air without warming the air, the air aloft is slower to lose winter's chill than air near the ground.
Because anything that makes the air aloft cooler and the air near the surface warmer creates instability, spring days are more likely to be unstable. Heating of the ground during the day during any season also tends to make the air more unstable, which is why thunderstorms generally grow strongest in the afternoon.
This is a good reason for flight instructors taking new students aloft, or pilots who are celebrating their new pilot certificate by taking a nonflying friend up, to learn another lesson from hot-air balloon pilots: Go flying in the early morning or late afternoon, unless the day has been so unstable that thunderstorms threaten. Without the sun's heat warming the ground, the air tends to be more stable; you will encounter less turbulence to make a new flier uneasy.
Who knows, maybe your dawn flight will be such a great experience that you'll indulge in another ballooning tradition: the champagne breakfast after landing.
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 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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