The blast wind at sea level may exceed 1,000 km/h, or ~300 m/s, approaching the speed of sound in air. The range for blast effects increases with the explosive yield of the weapon and also depends on the burst altitude. Contrary to what might be expected from geometry, the blast range is not maximal for surface or low altitude blasts but increases with altitude up to an "optimum burst altitude" and then decreases rapidly for higher altitudes. This is caused by the nonlinear behavior of shock waves. When the blast wave from an air burst reaches the ground it is reflected. Below a certain reflection angle, the reflected wave and the direct wave merge and form a reinforced horizontal wave, known as the '" Mach stem " and is a form of constructive interference. This phenomenon is responsible for the bumps or 'knees' in the above overpressure range graph.