Swells coming from a great distance and forming large breakers on exposed coasts. They are best known on the islands of St. Helena and Ascension in the South Atlantic Ocean during the months from December to April, when they come from the northwest. = They arrive, often in calm weather, with practically no warning, and are dangerous to shipping. Rollers also occur at Fernando do Noronha, Tristan da Cunha, and on the coasts of West Africa, Peru, and the East Indies.