Wind is the movement of air over the Earth's surface. Wind speed can be measured using an anemometer or estimated using the Beaufort scale by observing the effects of wind on the landscape. The Beaufort scale ranges from 0 to 12, with 0 being no wind and 12 being hurricane conditions. Wind direction is shown by a wind vane, which is placed in an open, high location like a rooftop without obstructions. A wind rose is a diagram showing the frequency of winds between different directions over a period of time, with the longest arm indicating the prevailing wind direction.