A unit of pressure in the metric system. One atmosphere equals 1013 millibars. The 500 millibar level, a standard level used in meteorology and in weather forecast maps, is located about 18000 feet above sea level; approximately half of the earth's atmosphere is located below this height
The thousandth part of a bar, a bar being a unit of barometric pressure equal to 750·1mm (29·531in) of mercury at 0°C in latitude 45°.
A unit of atmospheric pressure equal to 10 -3 bar, 100 newtons per square metre or 0.7500617 millimetres of mercury. Used to describe the sensitivity of microphones.