layer of winds that encircle the Earth in the lower stratosphere, at altitudes from 20 to 40 kilometres (about 12 to 25 miles), between latitudes 15° N and 15° S. They blow at velocities of 25 to 50 metres per second (about 55 to 110 miles per hour). They are alternately easterly and westerly, reversing about every 13 months. The quasi-biennial oscillation was originally known as the Krakatoa winds. This name was derived from the role the winds played in transporting dust thrown into the atmosphere by the explosion (1883) of the volcanic island of Krakatoa in present-day Indonesia.