Lee waves

Under certain conditions of atmospheric stability, standing waves may form in the lee of mountains. This wave motion is an oscillating exchange of kinetic and potential energy, excited by normal winds flowing over the mountain range, which produces alternately accelerated and retarded flow near the ground. Sustained lee waves at the maximum amplitude are obtained when the shape of the mountain matches their wavelength, or when a second range occurs at one wavelength downstream. Unusual cloud formations often indicate the existence of lee waves, in that they remain stationary with respect to the ground instead of moving with the wind. These clouds are continuously forming at their upwind edge as the air rises above the condensation level in the wave and dissipating at the downward edge as the air falls again.

Conditions are frequently suitable for the formation of lee waves over the mountainous regions of the US, an effect that is routinely exploited by glider pilots to obtain exceptionally high altitudes. The combination of lee waves with strong winds that are sufficient to produce damage to structures is fortunately rare, but do occur in hazardous mountainous regions.

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