Terrific turbulent action going up on the left hand side, then slowing down completely into the deep blue below, which is almost completely static, like a threatening thunder cloud. A glider pilot needs thermal to gain height. The rising hot air lifts him up in a great spiral, sometimes meeting the cold layers of air with an impact as sharp as being hit by a stone.
Peter Lanyon (on Thermal, 1960), 1963 British Council Lecture quoted in: Stephens, Chris. Peter Lanyon: At the edge of landscape. London: 21 Publishing, 2000, 159.