Differences in air pressure, though not noticeable to humans, cause the atmosphere to move in the form of wind and are a significant factor in weather forecasting. As air pressure changes, the height of the column of mercury in the tube changes. Measuring wind - Wind direction is determined with a wind vane . The wind direction is always stated in terms of the direction from which it is coming. Prevailing wind is the general direction from which the wind usually comes.
Wind speed is measured with a cup anemometer .
The spinning cups are attached to a rotating cable, which controls a dial on which the speed is read, much like a car speedometer. Recording variations in air pressure on a map - Air pressure measurements for many locations are recorded on a map. Points of equal air pressure are connected with lines called isobars . Without further modification the wind direction would be perpendicular to the isobars. Coriolis effect - Coriolis effect occurs because the Earth is spherical and is rotating.