Mountain Waves
Moderate to strong winds in a stably stratified atmosphere blowing across high mountain ranges will cause large-scale mountain waves for many miles downwind. The stable air, lifted by the wind over the mountain range, is pulled downward by gravity on the lee side. Inertia carries the air past its equilibrium level, so it rises again farther downslope. This oscillatory motion forms a series of lesser waves downstream until the oscillation finally ceases. Waves may extend as high as 40,000 feet or more in the well-known Bishop wave in California. Large-scale waves occur in the Rocky Mountains, and waves on a lesser scale appear in the Appalachians and elsewhere.
The lee slope of the mountains may experience strong downslope winds or many eddies of various sizes which roll down the slope. Within each wave downstream from the mountain range, a large roll eddy may be found with its axis parallel to the mountain range. Roll eddies tend to be smaller in each succeeding wave downstream. The waves downwind of the mountains are referred to as lee waves or standing waves.
If sufficient moisture is present, cap clouds will form over the crest of the mountains, roll clouds will be found in the tops of the roll eddies downstream, and wave clouds will be located in the tops of the waves.
Encyclopedia ID: p423
