Next, let's try half submerging a round stick held horizontally in the water, and moving it from right to left. Near the water's surface, the waves are almost vertical. In other words, waves that have a cycle close the Brunt-Väisälä frequency occur. The group velocity of these waves is zero, so they don't travel far from where the "mountain" (round stick) passes. Waves that transmit to the depths have a much longer cycle. For the sake of simplicity in this experiment, we moved the half-submerged round stick on the surface of the water, however we would have obtained the same result if we had taken a half-cylindrical stick and moved it back and forth on the bottom of the water tank. In other words, we could turn the whole experiment upside-down and map the movement of the water with a coordinate map that moves with the half-cylindrical stick to see the waves that occur on the leeward side of mountains in the atmosphere. These waves are called lee waves.