Definition 1 of 2
Definition
A unit of pressure in the metric system equal to 100,000 pascals (100 kPa), which is approximately equal to one standard atmosphere of pressure at sea level (1 bar = 0.987 atmospheres = 14.5 psi). The millibar (one-thousandth of a bar) is commonly used in aviation weather reporting to express atmospheric pressure.
Plain English
A unit for measuring pressure. One bar is roughly the pressure the atmosphere puts on you at sea level. In aviation, you'll usually see it as the millibar -- a smaller version used on weather charts.
Context Anchor
Seen in some weather data, aircraft maintenance information, tire pressure values, and pressure system discussions, especially when metric units are used.
Derivation
From the Greek 'baros,' meaning weight. The same root gives us 'barometer' -- an instrument that measures the weight of the atmosphere pressing down on it.
Why Pilots Care
Pressure readings in bars or millibars affect altimeter calibration, density altitude calculations, and safe flight planning in changing weather.
Intuition Check
Bar does not mean a metal rod, a line on a chart, or a place to get a drink here. In this aviation context, bar is a unit for measuring pressure.
Example Sentence 1
The surface chart showed a low-pressure center reading 996 millibars, indicating an active weather system.
Example Sentence 2
Pilots set the altimeter to match the reported bar value so the instrument reads correctly at field elevation.