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Atmospheric Stability

 

Introduction to Glider Flying > Soaring Weather > The Atmosphere > Atmospheric Stability

Stability in the atmosphere tends to hinder vertical motion, while instability tends to promote vertical motion. A certain amount of instability is desirable for glider pilots, since without it, thermals would not develop. If the air is moist enough, and the atmospheric instability deep enough, thunderstorms and associated hazards can form. Thus, an understanding of atmos-pheric stability and its determination from available weather data is important for soaring flight and safety. As a note, the following discussion is concerned with vertical stability of the atmosphere. Other horizontal atmospheric instabilities, for instance, in the evolution of large-scale cyclones, are not covered here.

Generally, a stable dynamic system is one in which a displaced element will return to its original position. An unstable dynamic system is one in which a dis-placed element will accelerate further from its original position. In a neutrally stable system, the displaced ele-ment neither returns to nor accelerates from its original position. In the atmosphere, it is easiest to use a parcel of air as the displaced element. The behavior of a stable or unstable system is analogous to aircraft stability dis-cussed in Chapter 3–Aerodynamics of Flight.

For simplicity, assume first that the air is completely dry. Effects of moisture in atmospheric stability are considered later. Aparcel of dry air that is forced to rise expands due to decreasing pressure and cools in the process. By contrast, a parcel of dry air that is forced to descend is compressed due to increasing pressure and warms. If there is no transfer of heat between the surrounding, ambient air, and the displaced parcel, the process is called adiabatic. Assuming adiabatic motion, a rising parcel cools at a lapse rate of 3°C (5.4°F) per 1,000 feet, known as the dry adiabatic lapse rate (DALR). As discussed below, on a ther-modynamic chart, parcels cooling at the DALR are said to follow a dry adiabat. A parcel warms at the DALR as it descends. In reality, heat transfer often occurs. For instance, as a thermal rises, the circulation in the thermal itself (recall the bubble model) mixes in surrounding air. Nonetheless, the DALR is a good approximation.

The DALR represents the lapse rate of the atmosphere when it is neutrally stable. If the ambient lapse rate in some layer of air is less than the DALR (for instance, 1°C per 1,000 feet), then that layer is stable. If the lapse rate is greater than the DALR, it is unstable. An unsta-ble lapse rate usually only occurs within a few hundred feet of the heated ground. When an unstable layer develops aloft, the air quickly mixes and reduces the lapse rate back to DALR. It is important to note that the DALR is not the same as the standard atmospheric lapse rate of 2°C per 1,000 feet. The standard atmos-phere is a stable one.

Another way to understand stability is to imagine two scenarios, each with a different temperature at 3,000 feet above ground level (AGL), but the same tempera-ture at the surface, nominally 20°C. In both scenarios, a parcel of air that started at 20°C at the surface has cooled to 11°C by the time it has risen to 3,000 feet at the DALR. In the first scenario, the parcel is still warmer than the surrounding air, so it is unstable and the parcel keeps rising—a good thermal day. In the second scenario, the parcel is cooler than the sur-rounding air, so it is stable and will sink. The parcel in the second scenario would need to be forced to 3,000 feet AGL by a mechanism other than convection,being lifted up a mountainside or a front for instance. [Figure 9-9]

Figure 9-8. Life cycle of a typical thermal with cumulus cloud.

Figure 9-9. Stable and unstable parcels of air.

Figure 9-9 also illustrates factors leading to instability. A stable atmosphere can turn unstable in one of two ways. First, if the surface parcel warmed by more than 2°C (greater than 22°C), the layer to 3,000 feet would then become unstable in the second scenario. Thus, if the temperature of the air aloft remains the same, warming the lower layers causes instability and better thermal soaring. Second, if the air at 3,000 feet is cooler, as in the first scenario, the layer becomes unstable. Thus, if the temperature on the ground remains the same, cooling aloft causes instability and better thermal soaring. If the temperature aloft and at the surface warm or cool by the same amount, then the stability of the layer remains unchanged. Finally, if the air aloft remains the same, but the surface air-cools (for instance due to a very shallow front) then the layer becomes even more stable.

An inversion is a layer in which the temperature warmsas altitude increases. Inversions can occur at any altitude and vary in strength. In strong inversions, the temperature can rise as much as 10°C over just a few hundred feet of altitude gain. The most notable effect of an inversion is to cap any unstable layer below. Along with trapping haze or pollution below, they also effectively provide a cap to any thermal activity.

So far, only completely dry air parcels have been con-sidered. However, moisture in the form of water vapor is always present in the atmosphere. As a moist parcel of air rises, it cools at the DALR until it reaches its dew point, at which time the air in the parcel begins to con-dense. During the process of condensation, heat (referred to as latent heat) is released to the surround-ing air. Once saturated, the parcel continues to cool, but since heat is now added, it cools at a rate slower than the DALR. The rate at which saturated air-cools with height is known as the saturated adiabatic lapse rate (SALR). Unlike the DALR, the SALR varies substan-tially with altitude. At lower altitudes, it is on the orderof 1.2°C per 1,000 feet, whereas in mid levels it increases to 2.2°C per 1,000 feet. Very high up, above about 30,000 feet, little water vapor exists to condense, and the SALR approaches the DALR.

Thermal Shape and Structure
Understanding Soundings
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