 The sign is an imaging finding using a 3.5–7.5 MHz ultrasound probe in the 4th and 5th intercostal spaces in the anterior clavicular line using the M-Mode of the machine.This finding is seen in the M-mode tracing as pleura and lung being indistinguishable as linear hyperechogenic lines and is fairly reliable for diagnosis of a pneumothorax.Even though the stratospheric sign can be an indication of pneumothorax its absence is not at all reliable to rule out pneumothorax as definitive diagnosis usually requires X-ray or CT of thorax.