Altocumulus Wave Clouds Over Melkbosstrand, South Africa
January 23, 2020
Photographer: Amanda Pollard
Summary Authors: Amanda Pollard; Jim Foster
These altocumulus wave clouds were observed over Melkbosstrand, Cape Town, South Africa on January 6, 2020. Wave clouds take shape when stable air is forced to rise. Most often a mountain range is the forcing mechanism, but even a series of hills may be sufficient to initiate lifting. Oscillation of winds aloft caused by a topographic barrier sets up a train of lee waves, known as stationary gravity waves. When air moving over such a barrier moves upward and crests, adiabatic expansion cools and condenses the water vapor within the air, forming a visible cloud. The oscillating air then sinks downward to an altitude (temperature and barometric pressure) where the vapor evaporates, so the cloud dissipates. This cycle is repeated as long as the air moves in a wave-like fashion, often resulting in eye-catching cloud trains or in this case, cloud fingers. The wavelength (distance from one crest to another or one cloud to another) is determined by a number of factors including wind speed, altitude of the waves, and stability of the atmosphere.
Photo Details: Camera: HUAWEI ANE-LX1; Exposure Time: 0.0004s (1/2604); Aperture: ƒ/2.2; ISO equivalent: 50; Focal Length (35mm): 26.