Stability and cloud base.

As a bubble or parcel of air ascends (rises), it moves into an area of lower pressure (pressure decreases with height). As this occurs, the parcel expands. This requires energy (or work), which takes heat away from the parcel, so the air cools as it rises (see Figure 12-1).

The lapse rate of a rising unsaturated parcel (air with relative humidity less than 100 percent) is approximately 3°C per 1,000 ft (9.8°C per km). This is called the dry adiabatic lapse rate.

Concurrently, the dewpoint decreases approximately 0.5°C per 1,000 ft (1.8°C per km). The parcel’s temperature-dewpoint spread decreases, while its relative humidity increases.

In Figure 12-2, at the surface the air parcel has a temperature of 18°C and a dewpoint of 13°C, indicating that it is unsaturated. As the parcel ascends, its temperature decreases at the dry adiabatic lapse rate of 3°C per 1,000 ft, while the dewpoint decreases at 0.5°C per 1,000 ft. The temperature-dewpoint spread decreases while relative humidity increases until the parcel achieves saturation at its LCL of 2,000 ft.

As moist, unstable air rises, clouds often form at the altitude where temperature and dew point reach the same value.

Further lifting of the saturated parcel results in condensation, cloud formation, and latent heat release. Because the heat added during condensation offsets some of the cooling due to expansion, the parcel now cools at the moist adiabatic lapse rate, which varies between approximately 1.2°C per 1,000 ft (4°C per km) for very warm saturated parcels to 3°C per 1,000 ft (9.8°C per km) for very cold saturated parcels.

For simplicity, examples shown in this handbook use a moist adiabatic lapse rate of 2°C per 1,000 ft.

A descending saturated air parcel quickly becomes unsaturated (see Figure 12-3). Its temperature increases at 3°C per 1,000 ft, while its dewpoint increases at 0.5°C per 1,000 ft (see Table 12-1).

The parcel and the surrounding environmental air temperatures are then compared. If the lifted parcel is colder than the surrounding air, it will be denser (heavier) and sink back to its original level. In this case, the parcel is stable because it resists upward displacement.

If the lifted parcel is the same temperature as the surrounding air, it will be the same density and remain at the same level. In this case, the parcel is neutrally stable.

A standard temperature lapse rate is one in which the temperature decreases at the rate of approximately 3.5 °F or 2 °C per thousand feet up to 36,000 feet.

But since this is an average, the exact value seldom exists. In fact, temperature in the troposphere sometimes remains constant or even increases with height.

Absolute stability (see Figure 13-1) is the state of a column of air in the atmosphere when its lapse rate of temperature is less than the moist adiabatic lapse rate. This includes both isothermal and inversion temperature profiles.

Absolute instability (see Figure 13-3) is the state of a column of air in the atmosphere when it has a superadiabatic lapse rate of temperature (i.e., greater than the dry adiabatic lapse rate). An air parcel displaced vertically would be accelerated in the direction of the displacement.

Conditional instability (see Figure 13-4) is the state of a column of unsaturated air in the atmosphere when its lapse rate of temperature is less than the dry adiabatic lapse rate but greater than the moist adiabatic lapse rate. An air parcel lifted upward would be initially stable, but at some point, above its LCL, it would become unstable. The term “conditional” means the parcel must be lifted to a particular level (altitude) before it becomes unstable and rises because of its own buoyancy.

The LFC is the level at which a parcel of saturated air becomes warmer than the surrounding air and begins to rise freely. This occurs most readily in a conditionally unstable atmosphere.

Virtually all showers and thunderstorms form in an air mass that is classified as conditionally unstable. A conditionally unstable air mass needs a lifting mechanism strong enough to release the instability.

Convective clouds and precipitation pose a distinctly different flying environment than stratiform clouds and precipitation. These sharply contrasting conditions result from the atmosphere either resisting or accelerating the vertical motion of air parcels.

A stable air mass can produce low stratus clouds and fog.

A warm, moist air mass moving over a cold surface (see Figure 11-2) produces stable air associated with stratiform clouds, fog, and drizzle.

As air moves upward, it cools by expansion. A convective current continues upward until it reaches a level where its temperature cools to the same as that of the surrounding air. If it cools to saturation, a cumuliform cloud forms.

Moist, unstable air causes cumulus clouds, showers, and turbulence to form.

When completely formed, the clouds have clear-cut horizontal bases and flattened or slightly rounded tops.

The cloud top usually marks the approximate upper limit of the convective current. A pilot can expect to encounter turbulence beneath or in the clouds, while above the clouds, air generally is smooth (see Figure 19-1).

The principal characteristic of an inversion layer is its marked stability, so that very little turbulence can occur within it.

Daytime heating of the surface increases temperature lapse rates and decreases stability. Conversely, nighttime cooling of the surface decreases temperature lapse rates and increases stability.