Thermals are bubbles or columns of warm, moist air that rise from the ground. Sailplanes circle in these vertical air currents and are able to sustain their flight. Typically a vortex ring-like circulation is found in thermals and they are roughly 200-600 feet in diameter at 800-1000 feet above the ground. When we are thermal soaring we enter into our more normal visually expansive environment, but since most soaring is done with cloud bases less than 10,000 feet, we still have some important visual cues related to the ground. We can have considerable problems in judging distance. Since we are often circling, the banked attitude can lead to even more problems in judging distance, along with perceptions in general. Haze and precipitation are likely the worst problems we will encounter. Although the atmosphere can at times be turbulent, this is usually less problematic than with ridge lift. In North America, cloud flying is illegal and so most of the soaring is done in the dry thermal layer, from the ground up to a cloud base. For thermals to develop there is a need for a temperature difference between the air heated near the ground and the general airmass to ‘drive’ the thermal upwards. The air is not that good a conductor so mixing is not as big a factor as you might expect. As the temperature difference between the thermal and the surrounding air decreases the thermal’s rising air slows down and eventually stops. This height is usually limited by the height of some inversion layer of air. This inversion layer may develop on clear nights as the earth loses heat via radiation close to the ground while the air at altitude remains warmer. During daytime heating thermals do not develop right away but rather require a certain amount of time to build up enough energy to break away from the ground. The trigger temperature is the surface air temperature at which there is a sharp transition from slow to rapid development of thermals. The lowest level near the round produces both up and downdrafts due to intense heating at that level. This phenomenon occurs up to perhaps 1000 feet. Above this level the air cools at approximately the dry adiabatic lapse rate level.
Under standard conditions the dry adiabatic lapse rate is 3 degrees Celsius per 1000 feet. This represents the rate the air cooling by expansion only. If the dewpoint is reached as the thermal’s air cools then clouds form. The buoyancy of the thermal will tend to increase or decrease depending on the difference between the thermal’s lapse rate and that of the surrounding air. The sides of a cloudy thermal may be cooled by the surrounding air and by evaporation of the thermal’s relatively moist air into the drier air about it thus producing some sink. The thermal itself is given a boost at this point however due to the latent heat released by condensation in its core which maintains or increases the temperature difference between the thermal and the surrounding air. A typical thermal has a lifespan of 20 minutes. Some thermals may last longer by drawing in some of the surrounding air due to the driving force of the condensation. Uneven heating of the earth contributes to the strength and triggering of individual thermals. Some factors in this are the angle of incidence of the sun’s rays, the moisture content of the ground (more heat needed to evaporate the moisture before the air can be heated), the type of soil (those types that drain faster may produce thermals more quickly and others may be better heat conductors due to their composition), the nature of the vegetation. Some terrain produces thermals early in the day while other terrain is a better source of late thermal. Another factor in thermic condition is the recycling of thermals. Thermals over particular terrain, or conditions in general, may appear to dissipate or strengthen in a cyclic fashion in the course of a day. A general rule of thumb is that thermal spacing will be approximately 12 to 22 times the height to which they rise. The wind may dissipate incipient thermals or trigger them depending on the strength. The wind shear will generally cause a bending of thermals with height. When the convective layer is capped by a stable layer of air and the wind is moderate to strong cloud streets may form with the clouds and associated lift lined up in rows with the wind.