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.
Plain English
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.
Context Anchor
Seen in night flying and human-factors discussions, especially when explaining why altitude can reduce night vision.
Derivation
Barometric' comes from the Greek 'baros' meaning 'weight' — a barometer literally measures the weight of the air above you. So 'low barometric pressure' means the column of air pressing down has less weight than normal, which is another way of saying the air is thinner.
Why Pilots Care
An uncorrected altimeter will show the airplane higher than it really is, raising the chance of controlled flight into terrain when visual references are poor.
Grounding Statement
As the airplane climbs, the air becomes less compressed, so each breath delivers less usable oxygen and night vision can fade sooner.
Intuition Check
Low barometric pressure does not only mean “bad weather” or a low pressure system. In this context, it means the surrounding air pressure is lower, especially at altitude, which reduces oxygen available to the body.
Example Sentence 1
The pilot noted the low barometric pressure on the evening briefing and decided to use supplemental oxygen earlier than usual on the night cross-country.
Example Sentence 2
During the cross-country flight the crew watched for areas of low barometric pressure because those regions required higher minimum altitudes at night.