Plain English Meaning

Air pressure that is lower than the normal baseline. Lower pressure means the air is thinner, so each breath delivers less oxygen to the body — and the eyes are especially sensitive to that drop at night.

Full Definition

An atmospheric pressure reading below standard sea-level pressure (29.92 inches of mercury or 1013.2 hectopascals), indicating thinner air with fewer oxygen molecules per given volume. In the context of night vision, low barometric pressure contributes to a higher effective altitude for the body, accelerating the onset of hypoxia and degrading the eyes' ability to function in low light.