The winds of a tropical cyclone produce wind waves which move outward from its center. The size and speed of the waves depends upon the wind velocity and the length of the over-water trajectory of the winds. As the wind waves move away from the center, their height diminishes, their length is reduced, and they become low undulating waves known as swell. The wind waves and swell move outward from the storm center at a rate that nearly always exceeds the forward speed of the storm. Since these waves precede the storm, they are one of the first signs of its approach.