Wind blowing over the surface of a body of water forms waves. The friction between air and water caused by a gentle breeze on a pond causes ripples to form. A strong blow over the ocean causes larger waves as the moving air pushes against the raised ridges of water. The waves reach their greatest height when the rate at which they travel nearly matches the speed of the wind. The waves form at right angles to the direction from which the wind blows. In open water, if the wind continues to blow, as happens in the Roaring Forties in the southern hemisphere, long, organized masses of water called swell roll across the ocean. If the wind dies down, the wave formation is reduced but waves already formed continue to travel in their original direction until they meet land. Small waves form in small areas of water with islands and other landmasses but large waves form in open stretches of sea where the wind blows steadily and strongly. When waves meet other waves coming from different directions, interference between the two can produce broken, irregular seas.