A cloud in the shape of a lens. Lenticular clouds form in mountainous regions when moist air flows over a mountain creating waves. The cloud forms in the wave crest giving it the lens shape. Occasionally, lenticular clouds will stack up one above the other extending well into the stratisphere and sometimes look like hovering UFOs. In fact, the first modern sighting of a UFO was made over Mt. Rainier in Washington, an area where lenticular clouds often form.