Internal gravity waves also have another strange characteristic. Their phase velocity and group velocity intersect at right angles. A phase velocity is the velocity at which the troughs and peaks of the waves travel, at a right angle to the face of the wave. Their group velocity is the speed at which a group of waves proceed, or the speed at which the energy of the waves is transmitted. The group velocity and phase velocity of normal waves that occur on the sea's surface head in the same direction. That's why ripples on the surface of water spread concentrically. However, there is no guarantee that the phase velocity and group velocity of all waves will head in the same direction. It is probably more accurate to say that the two speeds only face the same direction by chance.