National Geographic : 1967 Jan
Graceful search for "green air"-soaring's term for updrafts-leads a pilot over a town where dark roofs and warm streets give birth to a thermal. Banking into the column of rising air, he joins a hawk in a spiral ride up to a cloud base. In the distance a dark square of plowed earth promises another thermal, and the cumulus cloud above confirms the promise. Such clouds form when the air, cooling and expanding as it rises, releases excess moisture. Where next? On the downwind side of a hill, surface air has been sheltered from the cooling breeze; perhaps the ground has heated the sta tionary air long enough to start a thermal. The pilot glides over this "wind shadow" and indeed finds an updraft-too subtle to register on human senses, but strong enough to lift the sailplane in its free-floating course across the sky. The pilot watches his variometer (inset) to tell when his plane is rising or descending. The simple device made thermal soaring feasible, although success still rests on the pilot's skill.