Using theory to interpret weather data
Similar thinking applies to weather. A little bit of meteorological theory can go a long way toward helping you to understand what you read in a forecast, see on weather maps, or hear during a weather briefing. Understanding at least on a basic level what meteorologists mean when they say the atmosphere is stable, unstable, or even conditionally stable will not only help you to comprehend forecasts, maps, and briefings, but also to understand a little of what the clouds you see have to say about the current weather.
Think of an unstable atmosphere as being somewhat like an unstable person--little is needed to trigger violence. If a person is unstable enough, a minor incident on an expressway can lead to an explosion of road rage that ends up on the evening news. When the atmosphere is unstable enough, what seems like a minor factor--such as uncomfortable morning humidity--can become part of a weather pattern that produces a violent thunderstorm with tornadoes, softball-size hailstones, and extreme turbulence.
The weather produced by a stable atmosphere is calm with no dramatic highs and lows. Calm doesn't always mean pleasant, however. A stable atmosphere can produce clouds and precipitation that will ground pilots who are not trained and equipped for flight under instrument conditions.
When meteorologists say the air is stable, they mean that if a bubble or parcel of air is somehow given a shove upward, it will stop rising as soon as the force pushing it up is taken away. If the air is unstable, air that's given an upward push will continue to rise, at least for a while, after the upward force is removed.
To see how this works we need to look at how the pressure, temperature, and density of air change as the air rises. The air's density is directly related to its temperature and its pressure. The warmer the air, the less dense--or lighter--it is. As the air's pressure decreases it also becomes less dense. This explains why cold air aloft stays aloft instead of sinking; air aloft is at a lower pressure, which offsets the density increase caused by cooling the air.
Since the pressure of a bubble of air constantly changes to match the pressure of the surrounding air, a bubble of air will rise as long as it is warmer, and therefore lighter, than the surrounding air.
As air rises, whether it's being forced up or because it's lighter than the surrounding air, it cools at a regular rate that does not depend on the temperature of the surrounding air. The expansion of a bubble of rising air and the resultant lowering of its pressure as it rises into lower air pressure cools the air. Such cooling is known as adiabatic cooling, which means that no heat is being added from the outside or given off to the outside air. Rising air does not cool because the surrounding air is colder. If the water vapor in the air is not condensing into water droplets or turning directly into ice crystals, the rate of cooling is 5.5 degrees Fahrenheit per 1,000 feet. This is called the dry adiabatic lapse rate.
Since only air that's warmer than the surrounding air will continue rising after it's no longer being shoved up, such as by wind blowing over a mountain, the temperature of the air that's in place at various altitudes determines whether rising parcels of air will continue rising on their own--in other words, whether the air is stable or unstable. To see how this works, let's look at three soundings and what happens to air that begins rising within a few hours of the sounding, before it changes.
In the Day 1 example, the rising air stays cooler than the surrounding air. The atmosphere is stable.
| Figure 1. In the Day 1 example, the rising air stays cooler than the surrounding air. |
On Day 2, once it reaches 1,000 feet the rising air is warmer than the surrounding air; it continues to rise by itself. The air is unstable.
| Figure 2. On Day 2, once it reaches 1,000 feet, the rising air is warmer than the surrounding air and continues to rise by itself. |
Unless the air is extremely dry, the atmosphere is often more complicated than shown in these two straightforward examples of atmospheric stability. Most of the time the air is humid enough for its water vapor to begin condensing into the tiny water drops that make up a cloud when the air cools by a few degrees.
Now things become more complicated, because when water vapor condenses into water drops or turns directly to ice crystals, it releases the latent heat it took up when it evaporated to become water vapor. In a rising air bubble, this means that heat is being added as vapor condenses, which offsets some of the cooling caused by the fact the air is rising.
The actual moist rate depends on the temperature and pressure of the rising air, but 3.3 degrees of cooling per 1,000 feet increase in altitude is a good average figure. Using that figure, let's see what happens on a third day, with a sounding that's different from days 1 and 2 above. In this case, we'll assume that an arriving cold air mass is pushing up the warm (85-degree) air above 2,000 feet. After that, the air is on its own.
| Figure 3. In this scenario, an arriving cold-air mass is pushing the warm air above 2,000 feet. The air is stable until that height. |
In this scenario, the air was stable until it reached 2,000 feet. As it happened in this example, at that altitude the air had cooled enough for condensation to begin. At this point, the air begins to cool at the rate of 3.3 degrees instead of 3.5 degrees per 1,000 feet. If the cold front had not pushed the air up to 2,000 feet, it would have stopped rising before reaching 1,000 feet. If condensation with its release of latent heat had not started at 2,000 feet, the air would have cooled to 68.5 degrees by 3,000 feet, which would have been colder than the air surrounding it. The air would have stopped rising.
In this case, meteorologists would have said that the air was conditionally stable. That is, it would become unstable if it were pushed up high enough for condensation to begin. Then the air would begin rising on its own because it would be warmer than surrounding air. High humidity decreases stability because as very humid air rises, its water vapor begins condensing at a lower altitude. This, in turn, means the rising air is cooling at the slower moist adiabatic lapse rare.
By the way, the term lapse rate can be confusing because it's used in different ways. The dry adiabatic lapse rate of 5.5 degrees per 1,000 feet refers to how much air cools because its pressure is decreasing as it rises. The moist adiabatic lapse rate--3.3 degrees per 1,000 feet in our example--refers to how quickly rising air that's being warmed by the release of latent heat cools as it rises.
The soundings used in our examples are the environmental lapse rates. This lapse rate changes from day to day and place to place. Obviously, the change is important to weather forecasting because it determines whether the atmosphere will be stable, conditionally stable, or unstable.
You sometimes hear that the atmosphere's lapse rate is 3.6 degrees per 1,000 feet. This refers to the drop in temperature with altitude in what's known as the standard atmosphere. You can think of the standard atmosphere as a kind of global average for all seasons. In fact, if the environmental lapse rate were always 3.6 degrees per 1,000 feet we'd never have thunderstorms because rising air, which cools at the dry adiabatic rate of 5.5 degrees per 1,000 feet, would stay colder-- and heaver--than the surrounding air.
If the air is humid enough, clouds that form will be cumuliform--that is, they will be puffy, piled-up clouds. Under the right conditions the clouds can grow into thunderstorms. The most unstable atmosphere and other conditions lead to large, fierce Plains thunderstorms that spawn tornadoes. On a more ordinary unstable day, the sky will be dotted with cumulus clouds with areas of clear sky in between, on and off (showery) precipitation, and bumpy air in adjacent updrafts and downdrafts. Visibility is likely to be good because the updrafts carry polluted air aloft and the downdrafts transport clean air from aloft to the surface.
Stable air doesn't mean clear skies. But, for air to rise far enough for its moisture to condense into clouds and precipitation, a large mass of air has to be pushed up. This often occurs when warm air moves over cold air, with a surface warm front marking the surface boundary between warm and cold air. Visibility is likely to be poorer than on an unstable day, but airplanes will encounter less low-level turbulence because there are no localized updrafts and downdrafts.
One of the reasons places in the middle latitudes, which includes all of the United States except Hawaii and the northernmost parts of Alaska, has such a variety of weather is that atmospheric stability changes as weather comes and goes. Sometimes stability changes without a new weather system arriving. This occurs commonly on clear nights when heat from the Earth radiates out into space, cooling the ground. As the ground cools, it chills the air next to it, which makes the air more stable. In fact, the air in the first few hundred feet above the ground can actually grow colder than the air above it. This creates an inversion with warm air atop cold air.
As the sun comes up on a clear day, a stable atmosphere begins to grow unstable as the sun heats the ground. As the ground warms, it heats the air next to it while the air aloft does not warm. Eventually, the air near the ground can become much warmer than the air a few hundred feet up.
In addition to overnight cooling, the air can become more stable as warm air moves in aloft but not near the ground, as cold air moves in at the surface but not aloft, or as air at the surface is cooled as it moves over cold ground. In addition to daytime heating, the air can become more unstable as winds bring in warm air at the surface but not aloft, or as cold air moves in aloft but not at the surface.
The air in a particular location doesn't have to be all stable or unstable. In the real atmosphere, it's not unusual to have unstable air at some altitudes, stable air at others.
Understanding ground effect isn't going to solve all of your problems with landings. In a similar way, understanding atmospheric stability isn't going to enable you to master meteorology. But it's a good start.
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.
