Wind direction and speed above and downwind from an erupting volcano affect the dispersal pattern of volcanic ash in the atmosphere and ash deposited on the ground. Both direction and speed typically varies with increasing altitude above the ground and distance from an erupting volcano. Significant change in wind direction and speed may occur during the course of a single long eruption (12-36 hours), which can result in a complex ash-dispersal pattern, especially during cyclones or hurricanes (see Mt. Pinatubo, left). During the course of a prolonged series of eruptions that last weeks to months, changing wind patterns will typically blow ash in widely different directions.
Wind typically varies significantly across the boundary between the troposphere and stratosphere (about 10 km (6 mi) above sea level). Air temperature decreases rapidly from the earth's surface up through the troposphere, but remains relatively constant from the base of the stratosphere (10 km (6 mi)) to about 25 km (16 mi), above which it increases upwards. The troposphere is dominated by vigorous convection and circulation, resulting in complex wind patterns. Above, the stratosphere has a non-convective, sometimes slightly turbulent, circulation that is not directly connected to wind patterns in the troposphere.