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No two thermals are exactly alike since the thermal sources are not the same. 

Whether considered a bubble or column, the air in the middle of the thermal rises faster than the air near the sides of the thermal. A horizontal slice through an idealized thermal provides a bull’s-eye pattern. Real thermals usually are not perfectly concentric; techniques for best using thermals are discussed in the next chapter.  [Figure 9-9] 

The diameter of a typical thermal cross-section is on the order of 500–1,000 feet, though the size varies considerably. Typically, due to mixing with the surrounding air, thermals expand as they rise. Thus, the thermal column may actually resemble a cone, with the narrowest part near the ground. Thermal plumes also tilt in a steady wind and can become quite distorted in the presence of vertical shear. If vertical shear is strong enough, thermals can become very turbulent or become completely broken apart. A schematic of a thermal lifecycle in wind shear is shown in Figure 9-10 . 

## Atmospheric Stability 

Stability in the atmosphere tends to hinder vertical motion, while instability tends to promote vertical motion. A certain 

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amount of instability is desirable for glider pilots; without it, thermals would not develop. If the air is moist enough and the atmospheric instability is deep enough, thunderstorms and associated hazards can form. Thus, an understanding of atmospheric 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, such as the evolution of large-scale cyclones, are not covered here. 

Generally, a stable dynamic system is one in which a displaced element returns to its original position. An unstable dynamic system is one in which a displaced element accelerates away from its original position. In a neutrally stable system, the displaced element 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 discussed 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. A parcel 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). 

