Wind refers to the movement of air over the Earth's surface. Wind speed is measured using an anemometer and can be estimated using the Beaufort scale by observing the effect of wind on the landscape. Wind direction is shown by a freely moving pointer on a wind vane, which is usually placed on a high, open location like a rooftop with few obstructions. A wind rose is a graphical tool used to display wind speed and direction data, with longer arms indicating more frequent wind from that direction over the period observed.