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vertical undulation of airstreams on the lee side of a mountain. (The lee side is the side that is downstream from the wind.) The first wave occurs above the mountain that causes it, with a series of waves of equal horizontal wavelength extending downstream. Numerous equally spaced lee waves are often seen where they are not interfered with by other mountains, such as over the sea. They may produce clouds, called wave clouds, when the air becomes saturated with water vapour at the top of the wave