Undular bore
Undular bores are usually formed when two air masses of different temperatures collide. When a storm (typically a thunderstorm) approaches a layer of cold, stable air, it creates a disturbance in the atmosphere producing a wave like motion. Although the undular bore waves appear as bands of clouds across the sky, they are transverse waves (a.k.a. gravity waves), and are propelled by the transfer of energy from an oncoming storm. The ripple like appearance of this wave is described as the disturbance in the water when a pebble is dropped into a pond or when a moving boat creates waves in the surrounding water. The object displaces the water or medium the wave is travelling through and the medium moves in an upward motion. However, because of gravity, the water or medium is pulled back down and the repetition of this cycle creates the transverse wave motion.
The undular bore's period can measure 5 miles peak to peak and can travel 10 to 50 mph. The medium it travels through is the atmosphere. There are several varying types of ‘‘bores’’ in different layers of the atmosphere, such as the mesospheric bore which occurs in the mesosphere.
Undular bores are believed to be catalysts for thunderstorms. Although a thunderstorm helps create an undular bore, an undular bore can in turn intensify a thunderstorm because it further disturbs the atmosphere.
http://en.wikipedia.org/wiki/Undular_bore
