Strong updrafts, downdrafts and eddies develop as the air flows over hills and down valleys. Strong updrafts, downdrafts, and eddies develop as the air flows over hills and down valleys. Because of the chaotic nature of updrafts and downdrafts, pressure perturbations are important. Downbursts are particularly strong downdrafts from thunderstorms. Atmospheric moisture is largely prevented from precipitating due to continuous downdrafts and other factors. They only occur when there are localized downdrafts striking through the stratocumuliform cloud. Contributing to the accident were the downdrafts, high density altitude, and mountainous terrain. Air spilling over the San Gabriels can cause violent up- and downdrafts. Downdrafts can occur on final to RWY 20. Therefore, in general terms, negative buoyancy is typically the major contributor to downdrafts. Rear-flank downdrafts have a well-established association with hook echoes. The descent of "Akron" was only temporarily halted, before downdrafts forced the airship down farther.