Paragliders as probes of the Alpine boundary layer: sink polars from 86 000 flight logs and the flow field of a valley

Jörg Korner

Paraglider flight logs are a dense, free and continuous record of the air over mountains: every thermal day, hundreds of pilots fly for hours along the slopes and across the valleys and record their vertical speed once per second. The obstacle is that a vario measures the air plus the sink of the glider, and the sink polars of paragliders are unknown. We obtain them from the logs themselves. Pilots choose their speed by the air: they slow down in lift and push full speed in strong sink and when leaving a thermal. But at some airspeeds, the ones they fly most, glider and pilot together are nearly a constant and only the air varies, so the most frequent vertical speed in straight flight there is the sink of the glider. The physical form of the polar shows where this holds: in a window around the airspeeds flown most, the measured points follow the physical curve; outside it they do not, and the logs show that these points lie in special air. We show this for each class of gliders and use only the window. What the choice of speed still carries into the window is measured with the air found by neighbouring pilots, whose airspeeds do not depend on the airspeed of the glide, and removed. Polars for 73 glider types follow, precise to ±0.03 m/s at the airspeeds flown most, enough to see a new race harness arrive, and to find no difference between one wing in two cloths. With the polars removed, 2.0 million glides from 86 190 flights in the Valais, 2007–2026, 153 million seconds of straight flight in all, become measurements of the air and yield a flow field of the whole valley system on a 0.5 km grid, by height and hour of day, which shows the slope and valley circulation and holds in years not used to build it. It measures what climbs cannot: the air between the thermals, free of the glider and pilot effects that climb rates carry.