MEAN WIND

The atmosphere in motion near the surface of the earth (the 'wind') exhibits marked variations over very short periods of time, depending on such variables as low level thermal stability, ground-induced friction effects, vertical shear etc. Average or 'mean' winds are defined over periods such as 1, 2 or 10 minutes for reporting in meteorological bulletins. (60 minute periods are often used for climatological purposes). For SYNOP and METAR reports, the period is 10 minutes in most countries, though in tropical storm advisories and measurements associated with US-based organisations (e.g. NHC, JTWC), 1 minute averages are used, and often referred to as 'sustained' winds.