PLT494: Thermals: Formation, Structure, And Locating

What this means for your checkride

Thermals are rising currents produced by convection and are the most common lift source used to sustain glider flight. The handbook describes two models, the bubble or vortex-ring model and the column or plume model, and notes that air in the middle of a thermal rises faster than air near the edges, giving an idealized bulls-eye cross section that is roughly 500 to 1,000 feet across and expands as it rises. The best air masses for thermals have cool air aloft with enough surface heating; cumulus clouds often mark thermal tops, while dry or blue thermals form when there is too little moisture for cloud. Surface markers such as dust devils show where a thermal is leaving the ground.

Recognizing how thermals form, what they look like, and how they mark themselves is what lets a pilot find and use lift.

Where to study it