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Wind is the term used to describe the large-scale flow of atmospheric air.
In global terms, the main cause of wind is the difference in air pressure between two regions within the atmosphere. Initially, wind flows from high to low pressure, but as the wind speed increases, the rotation of the earth takes effect, causing the wind to travel along the direction of the isobars.
Due to the warming of air during the day and the cooling at night, wind direction and strength changes between day and night. Typically, the wind increases in strength and veers by day, and reduces in strength and backs at night. This phenomenon, known as diurnal variation, is particularly marked in sea and land breezes.
In tropical areas, very low pressure areas can develop over the sea giving rise to tropical revolving storms, which feature extremely powerful winds.
Strong cross winds may pose a hazard on landing and take off, especially in gusty conditions; most aircraft types have a cross-wind component limit listed in their Aircraft Flight Manual (AFM) or Operations Manual.
Strong winds may cause turbulence, for example, at low altitude where the wind is blowing over hilly terrain. Turbulence may also occur in clear air, for example severe clear air turbulence is frequently associated with jet streams.
Violent up and down-drafts caused by thunderstorms cause the local wind strength and direction to change rapidly giving rise to wind shear, which can pose a serious safety hazard close to an airfield.
Wind direction is conventionally reported in written forecasts, Automatic Terminal Information Service (ATIS) or weather broadcasts using three figures, rounded to the nearest 10° true; this is followed by the wind speed in kilometres per hour or knots. For example, a wind from a direction of 273° true, strength 18 knots would be reported as 270/18 kt. The surface wind reported by the air traffic controller to a pilot follows the same convention, except that magnetic direction is used instead of true. Significant changes to forecast or actual wind speed and direction are also reported.
For details, see separate article Wind Velocity Reporting