subatmospheric
/ˌsʌbatməsˈfɛrɪk/adjective
1 Geology. Taking place or existing in contact with the overlying atmosphere; subaerial.
2Chiefly Medicine and Engineering. Of pressure: less than that of the atmosphere (at sea level under normal conditions).
Origin
Mid 19th century; earliest use found in Quarterly Journal of the Geological Society of London. From sub- + atmospheric.