Atmospheric Stability

Because an unstable atmosphere can lead to violent storms and turbulence, flight crew members must be aware of the nature of the atmosphere in which they are flying. What are the characteristics of a stable versus an unstable atmosphere? How can atmospheric stability be predicted?

Properties of the atmosphere are partially determined by its stability, or resistance to change. Change in air temperature relative to altitude is a major factor in predicting atmospheric stability. From the earth's surface to around 36,000 feet, the temperature decreases with altitude. This occurs because the atmosphere is heated by conduction from the warm earth. Incoming solar radiation is absorbed by and heats up the earth, which in turn warms the adjacent atmosphere. The farther one travels from the warm earth, the colder the air becomes. The temperature decrease that occurs as altitude increases is known as the lapse rate. In standard conditions, the lapse rate is about 3.5F for every 1,000 feet. The actual lapse rate varies from day to day. When the temperature decreases rapidly with increased altitude, the actual lapse rate is high and the atmosphere unstable. Conversely, a stable atmosphere reflects a low lapse rate.

Another reason for cooler temperatures aloft is adiabatic cooling, which occurs when air is forced upward. A parcel of air forced upward cools because the reduced atmospheric pressure allows it to expand. We're aware from daily experience that gases heat up when compressed and expand as they cool. For example, an air compressor or tire pump gets hot, but gas escaping from an aerosol can is cool. The degree of cooling depends on whether the air is dry or moist. Moist saturated air tends to hold its temperature because the condensing moisture releases heat, offsetting the temperature drop. If the actual lapse rate is high and the adiabatic cooling rate is low, the upward-moving parcel of air tends to be warmer than the air around it. Because warm air rises, a parcel of air warmer than its surroundings tends to continue to rise once displaced upward. This condition represents instability. If the actual lapse rate is low, however, the parcel's upward motion is stifled, resulting in a stable atmosphere.

So, are you asking yourself what could push this parcel of air upward in the first place? Three entities have that ability -- terrain, fronts, and convection. Air that is forced over rising terrain, such as a mountain range, has nowhere to go but up. The same upward force can be created by the blunt edge of an approaching cold front. Sometimes just the heating of the earth creates the upward force. As the hot surface warms the surrounding air, the hot air can break away and bubble upward, much like a pot of boiling water.

Smooth air, stratus clouds, and gentle continuous precipitation are characteristic of a stable atmosphere. The unstable atmosphere tends to be more turbulent and filled with rain showers and thunderstorms. Turbulence dissipates fog, smoke, and haze, however, leaving the air clearer immediately after it occurs. This increased visibility is good for flying. Since severe thunderstorms are associated with unstable air, weather forecasters are always on the lookout for warm, moist air at the surface with colder, dry air aloft.

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