Tom, one of the world's leading meteorologists, has written a two part feature on the most elusive of cloud systems, wave

Waves can occur when the wind blows across a range of mountains. They are often called Lee Waves because the effects are most noticeable on the downstream (lee side) of the mountain ridge. These waves range from gentle undulations, where the air rises and falls smoothly at less than a hundred feet per minute, to powerful systems where the air surges up and down at several thousand feet per minute. Photo A. Low level wave bar which formed in the evening as the lower air became more stable. Photos by Tom. Lesser waves may only affect a layer a few thousand feet thick but the great waves can extend from ground level up into the stratosphere. Aircraft such as the U-2 (the Lockheed high altitude spy plane) have found waves at 60 000ft. Radar has found waves in reflective chaff at 100 000ft. Noctilucent clouds show wave-like characteristics at heights of 80-85km (around 270 000ft). Sailplanes have soared in waves since 1933 and in recent years have reached heights of 49 000ft and flown distances in excess of 1000km in wave. Conditions For Waves Waves need stable air. If stable air is displaced up or down it tends to return to its original level. However, it generally overshoots this equilibrium level and bobs up and down like a softly sprung car with no shock absorbers.   The condition is shown in Fig 1. The air is pushed upwards, expanding as it rises. Expansion makes it cooler than the environment so it sinks back. Its momentum then makes it overshoot the equilibrium level. Descent compresses and warms it so it rises again. This results in an oscillation. The greater the stability the more rapidly it undulates. The time taken is known as the Brunt-Vaisala period. If the layer includes an inversion one cycle takes about five minutes. If the air becomes less stable the oscillation takes longer; unstable air has no restoring force so no such waves occur. Wavelength The wind blows through this undulation. If the speed was 10m/sec (nearly 20kts) and the oscillation period was 300sec (5mins) the air would travel 3000 metres during one cycle. This can be called the "natural wavelength" of the layer. This wavelength grows longer if the wind speed is increased or the stability is reduced. Stability and wind speed vary with height so there are many "natural" wavelengths. When the air flows across a mountain ridge the jolt sets off many different frequencies but the atmosphere only resonates over a small range. Only the resonant waves are amplified, the others die out. Wave Speed Waves which move through the air can be halted by an opposing wind. This makes them "Standing Waves" which remain stationary above the ground. Most lee waves are standing waves and the wind blows through them. Fig 1. Wave caused by up and down oscillations in stable air: the Brunt-Vaisala period. Tom's drawing have been enhanced by Steve Longland.