Tracks of flights taking wind measurements for the ACARS weather reporting system. Wind direction and speed are measured 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.