Barometer is an instrument that measures the pressure of the atmosphere. Weather forecasters use barometers to detect changes in air pressure. In most cases, such changes indicate that the weather will soon change. Barometers can also be used to measure altitude because air pressure decreases as altitude increases.

The barometer was invented by Evangelista Torricelli, an Italian physicist, who wrote a detailed description of the device in 1644. Torricelli's device consisted of a long glass tube filled with mercury and inverted in a cup of mercury. The column of mercury in the tube fell until its top was about 30 inches (76 centimeters) above the surface of the mercury in the cup. The pressure of the air on the surface of the liquid in the cup held the mercury in the tube. Torricelli thus showed that the pressure of the atmosphere roughly equals the weight of a 30-inch column of mercury.

Modern barometers measure air pressure in inches or millimeters of mercury or in units called bars and millibars. The bar is a unit of pressure in the metric system, and a millibar equals 1/1,000 bar. Scientists record most pressure measurements in millibars. The atmospheric pressure at sea level averages 1,013 millibars, which equals 29.92 inches (760 millimeters) of mercury. To compare measurements taken at various altitudes and temperatures, scientists first adjust them to the values that would be measured at sea level at 32 °F (0 °C).