However, in some countries, such as Norway for example,[7] the transition level is determined by adding a buffer of minimum 1,000 ft (300 m) (depending on QNH) to the transition altitude. Therefore, aircraft may be flying at both transition level and transition altitude, and still be vertically separated by at least 1,000 ft (300 m). In those areas the transition layer will be 1,000–1,500 ft (300–460 m) thick, depending on QNH.