Normally taken to be the levels between 850 mbar (about 5000 ft), and 200 mbar (39000 ft). The most common heights used being 500 mbar (18000 ft) and 300 mbar (30000 ft).
UPPER AIR
Meaning of UPPER AIR in English
Weather and meteorology English vocabulary. Английский словарь погоды и метеорологии . 2012