Further, the angle at which the waves are transmitted depends on their cycle. If the cycle is short, the angle at which the waves are transmitted becomes larger, and they spread up and down at a sharp angle. Then, when the angle at which the waves spread exceeds 90 degrees, no waves occurs. The cycle at which the waves disappear is called the Brunt-Väisälä frequency. In the sea, this cycle lies between a couple of minutes to tens of minutes. In other words, the cycle of almost all of the waves that occur on the sea's surface is too short for the wave to be transmitted as an internal gravity wave to the deep sea.