Parallel lines of small cumulus: irregular over land, downwind from a sunny slope, and regular over warm seas. The cloud-free areas come from the mixing of saturated moist cloud air with overlying warmer, drier air: ‘an individual cloud is capable of drying out a large area of the surrounding layer through cloud-top entrainment and thus is able to produce significant gaps between the clouds’ (Chlond (1992) Boundary-Layer Met. 58, 1–2).