Figure 9-9.  Cross-section through a thermal. Darker green is  stronger lift; red is sink. Figure 9-8.  The column or plume model of a thermal. 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. 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] 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. 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 . 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).  Stability in the atmosphere tends to hinder vertical motion,  while instability tends to promote vertical motion. A certain  Atmospheric Stability 9-7