Soaring: gliders climbing in circles

Most gliders have no engine for sustained flight, so they climb by finding rising air. On sunny days, columns of warm air called thermals lift a glider that circles tightly inside them, often at several hundred feet per minute. The pilot then glides toward the next thermal, trading the height for distance.

The site looks for verified circling by an aircraft that looks like a glider: an emitter category for gliders, the 1202 squawk code that gliders in the United States use when they are not talking to air traffic control, or a glider type code. It wants a gain of at least 500 ft at 200 to 1,000 ft per minute, with a median ground speed of 40 to 80 knots, so that a powered aircraft circling at a steady height does not count.

Other aircraft circle too, so the site needs the glider evidence, and a towplane climbing with a glider is filed as a formation instead. The data records the climb. The rising air itself is inferred from the track, since nothing in the broadcast measures it, and a glider that turns in lift without gaining height leaves no story at all.