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A bar is a unit of measurement used to describe pressure, though it's not part of the official international system of units (SI). It's commonly used in everyday applications like weather forecasting and tire pressure gauges because it's close to atmospheric pressure at sea level.
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The bar (symbol bar) is a metric unit of pressure defined as 100000 Pa (100 kPa or 1000 hPa), though not part of the International System of Units (SI). A pressure of 1 bar is slightly less than the average atmospheric pressure on Earth at sea level (approximately 1.013 bar). By the barometric formula, 1 bar is roughly the atmospheric pressure on Earth at an altitude of 111 metres at 15 °C.
The bar and the millibar (symbol mbar) were introduced by the Norwegian meteorologist Vilhelm Bjerknes, a founder of the modern practice of weather forecasting, with the bar defined as one megadyne per square centimetre.