Thermal Soaring

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 can be found in thermals and they are roughly 300-600 feet in diameter at 1000 feet above the ground.

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.  When the temperature difference between the thermal and the surrounding air lessens then 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.  At lowest level near the ground produces both up and downdrafts due to intense heating at that level.  This phenomena occurs up to perhaps 1000 feet.  Above this level the air cools at approximately the dry adiabatic lapse rate level.

A general rule of thumb is that thermal spacing will be approximately 1 1/2 to 2 1/2 times the height to which they rise.  The wind may dissipate incipient thermals or alternatively kick them off 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.