In 1943, Munk and Sverdrup began looking for a way to predict the heights of ocean surface waves. The Allies were preparing for a landing in North Africa, where two out of every three days the waves are above six feet. Practice beach landings in the Carolinas were suspended when waves reached this height because they were dangerous to people and landing craft. Munk and Sverdrup found an empirical law that related wave height and period to the speed and duration of the wind and the distance over which it blows. The Allies applied this method in the Pacific theater of war and the Normandy invasion on D-Day.