All of us have encountered unstable people: the boss who yells at you for minor mistakes, or the driver who explodes into road rage. An extremely unstable driver might poke a pistol out of his car’s window and start shooting.
Weather in an unstable atmosphere can remind you of an unstable driver giving way to road rage. Instead of yelling, an unstable atmosphere produces bumpy rides, showers, gusty winds, and thunderstorms.
Like an enraged driver shooting a pistol, an extremely unstable atmosphere can go berserk. At such times pilots could face severe thunderstorms that produce winds 58 mph or faster, hail three-quarters of an inch in diameter or larger, and even tornadoes.
A stable atmosphere, on the other hand, can act like a stable person who isn’t always sweetness and light. It can produce weather such as low, thick clouds that manufacture widespread rain or snow, low ceilings, and poor visibility—but mostly calm winds.
The 1, 2, 3, 4 of stability
You need to know three scientific facts in order to grasp the concept of atmospheric stability. The atmosphere is unstable when air that’s pushed upward continues to rise after it’s no longer being pushed. In a stable atmosphere, air stops rising after the initial shove ends.
Here are the three basics:
- As air rises, it cools at a constant rate, which the temperature of the surrounding air does not affect.
- Air rises as long as it is warmer— and thus lighter—than the surrounding air.
- An unstable atmosphere is one in which rising air remains warmer than the surrounding atmosphere.
Winds (air) blowing across a mountain range illustrate what happens when the air is stable or unstable. The mountains force the wind to flow up their windward slopes. If the atmosphere is stable, the wind will blow down the mountains after reaching their crest. In an unstable atmosphere, the air will continue rising after reaching the crest.
In addition to wind blowing uphill, forces that push air upward include heating of air near the ground, an advancing cold front where arriving cold air shoves lighter warm air up, and winds that converge at the Earth’s surface.
The lapse rate
Meteorologists use the term “lapse rate” to describe changes in temperatures (and sometimes humidity) with altitude. The 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. 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.
The term “adiabatic” refers to heating or cooling of a bubble of air that’s rising or sinking because of what’s occurring inside the bubble. The temperature of the surrounding air does not affect the temperature of rising or sinking air. A bubble of rising air grows cooler because it is expanding as the pressure of the surrounding air decreases. A bubble of sinking air warms as the increasing pressure of the surrounding air compresses the bubble.
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 into drops of water. 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 of approximately 3.3 degrees per 1,000 feet.
Examples of stable, unstable atmospheres
Tables Day 1, 2, and 3 (at right)show the soundings up to 4,000 feet on three different days, and the temperatures of rising air that is cooling at the dry adiabatic lapse rate.
In Day 1 the atmosphere is stable, at least up to 4,000 feet. If the upward push ended at any of the altitudes shown, the air would stop rising. In Day 2 the rising air remains warmer at each altitude and would continue rising after the upward push ended. In other words, the atmosphere is unstable at all of the altitudes shown.
Day 3 shows a day when the atmosphere becomes unstable under certain conditions. Meteorologists call this “conditional instability.” Whether the air is stable or unstable for a sounding similar to Day 3 depends on the conditions, especially how much water vapor is in the air. In Day 3 the air is humid enough that water vapor begins condensing—thus releasing heat—at an altitude of 2,000 feet. As the air begins cooling at the moist adiabatic lapse rate of 3.3 degrees per 1,000 feet, it releases heat, which causes the air to cool at only 3.3 degrees per 1,000 feet. This means that the rising air is warmer than the air around it at both 3,000 and 4,000 feet and continues rising.
How stability affects weather
In general, if you go flying when the atmosphere is unstable, expect turbulence. Unless the air is very dry you’ll see clouds that extend vertically into the sky (cumulus clouds). Any rain or snow will come and go as showers with dry air and the possibility of clear sky between the showers. Sometimes the atmosphere will be unstable only at lower altitudes. On such days clouds won’t extend high into the sky and the air above the clouds will not be as turbulent as the air between the clouds and the ground. On the other hand, when the atmosphere is unstable to high altitudes, vigorous thunderstorms can form as long as the air isn’t too dry. On the plus side, unstable weather usually clears out haze or smog by mixing air up and down and diluting pollution.
Stable air does not guarantee clear skies. Widespread clouds and precipitation can form, such as when warm, humid air rides over cooler air ahead of a warm front at the surface. In this case the warm, humid air is rising at an angle instead of straight up as on a day with showers and thunderstorms. Nevertheless, the air is still cooling at the adiabatic lapse rate to form clouds and precipitation. This can create steady rain over an area stretching maybe 400 miles east to west and 100 or more miles north to south. Obviously, neither stable nor unstable air guarantees that you are going to have a good day for flying.
