Wind measurements from the ACARS weather reporting system. Winds are displayed as wind barbs, which indicate direction and speed of the wind, at altitudes from 10,000 metres to 13,500 metres. ACARS is a system for transmitting data between aircraft and receiving stations. Weather measurement equipment is attached to commercial aircraft, and the data relayed and interpreted to provide accurate and up_to_date information about the conditions. Some 140,000 observations are collected each day, with around 4000 aircraft involved.