The high point of an ocean wave is called the crest, and the low point is the trough. The distance from crest to trough is of course the wave height, and the distance between successive crests is known as the wavelength. Waves are created by wind blowing on the oceans surface, which steadily adds energy to them. The size of waves thus depends on how hard and for how long the wind blows. Because waves can travel long distances without losing much energy, they may appear when there is no wind, having been produced by some distant storm.