The height of the waves is about one-half the speed of the wind. The depth of the water is four-thirds the height of a breaking wave. (Example: Height of wave is equal to 6 feet, 1/3 of 6 = 2, 2 × 4 = 8, depth of water is about 8 feet.) The likely existence of a sandbar or reef just under the water when waves are observed breaking offshore and again onshore. The wind must blow across the water about 12 hours to generate maximum wave activity.