The measurement and computation of wind speeds and directions at various levels above the surface of the earth. Among the methods employed are 1) the visual tracking, by theodolite, of ascending balloons (pilot-balloon observation or rabal); 2) the use of a radio direction finder to track the radio signals emitted by an ascending radiosonde or other type of transponder (rawinsonde observation or rawin observation); 3) the use of radar to track a balloon-borne radar target, sometimes in combination with a radiosonde (rawin observation). The tracking of high-altitude, constant- level balloons (transosonde) may be considered to fall within this group. Winds-aloft data are included in many aircraft observations, particularly in aircraft weather reconnaissance flights.