Barometric pressure is a measure of atmospheric pressure as measured by a barometer. Barometric pressure is commonly referenced in weather reports as either high or low. In the case of weather systems, the terms low and high are relative terms, meaning that system has either lower or higher barometric pressure than the surrounding areas. Many barometers use mercury to measure barometric pressure. Since the chemical symbol for mercury is Hg, readings of barometric pressure are often reported in inches of mercury (in/Hg) or millimeters of mercury (mmHg). One atmosphere of barometric pressure is equal to 760 millimeters of mercury.