LEE WAVE

vertical undulation of airstreams on the lee side of a mountain. (The lee side is the side that is sheltered from the wind.) Airstreams are often deep enough to flow directly over a mountain range; they sometimes form strong vertical currents, the speed of which commonly reaches 5 m (16 feet) per second. The first wave occurs above the mountain that causes it, but in most deep airstreams a series extends downstream; over 100 have been reported where they are not interfered with by other mountains, as over the sea. These waves are most important when their largest amplitude occurs in the middle levels of the atmosphere and they die away at greater heights; they may produce clouds, called wave clouds, and have an important effect on the weather. Lee waves occur most often when a deep airstream with stronger winds in the higher levels and stably stratified air in the lower levels flows across a long ridge having a steep lee slope. The strongest up current then occurs not over the wind-facing slope but at the front of the first lee wave. If the lee slope is very steep and high, the waves may be of sufficient amplitude for a rotor, a vortex with a horizontal axis of rotation perpendicular to the direction of flow, to occur. In a rotor, the wind at the ground blows toward the mountain. The regular spacing between the waves is usually 2 to 8 km (1 to 5 miles). If this spacing coincides approximately with the spacing of the hills, the waves become large; if not, the lee waves of one mountain may be annulled as the air passes over a second. In hilly country with a complicated topography, intense waves may be temporarily set up in one or two places; such strong winds may occur under the wave troughs that large stretches of forest are blown down. By day, the winds up a lee slope inhibit the formation of lee waves by inducing the flow to break away from the hilltops. At night, the shallow downslope (katabatic) wind induces the airflow to adhere to the slope so that waves are formed; a deep, strong wind then blows down the lee slope. One of the most fully explored and spectacular lee waves is the Sierra wave, which occurs in a westerly wind to the lee of the Sierra Nevada Range in California. It is best developed when a jet stream associated with a front blows across the range. In it, gliders have soared to elevations of more than 14,000 m.