, Philippines. As air travels along the surface of the Earth, it often encounters obstructions, including natural features, such as mountains or hills, and artificial structures, such as buildings and other constructions, which disrupt the flow of air into "eddies", or areas of turbulence. When moist, stable air flows over a larger eddy, such as those caused by mountains, a series of large-scale
standing waves form on the
leeward side of the mountain. If the temperature at the crest of the wave drops below the
dew point, moisture in the air may condense to form lenticular clouds. Under certain conditions, long strings of lenticular clouds may form near the crest of each successive wave, creating a formation known as a "
wave cloud". Those wave systems can produce large
updrafts, occasionally enough for water vapour to condense and produce
precipitation. Lenticular clouds have been mistaken for
UFOs, because many of them have the shape of a "flying saucer", with a characteristic "lens" or smooth, saucer-like shape. Lenticular clouds generally do not form over low-lying or flat terrain, so many people may have never seen one and don't know that they can exist. Bright colours (called
iridescence) are sometimes seen along the edge of lenticular clouds. ==Flight==