The longitudinal (or linear) dunes were initially presumed to be formed by moderately variable winds that either follow one mean direction or alternate between two different directions. Subsequent observations indicate that the dunes point to the east although climate simulations indicate Titan's surface winds blow toward the west. At less than 1 meter per second, they are not powerful enough to lift and transport surface material. Recent computer simulations indicate that the dunes may be the result of rare storm winds that happen only every fifteen years when Titan is in equinox. These storms produce strong downdrafts, flowing eastward at up to 10 meters per second when they reach the surface.