Barometers measure air pressure to help predict the weather and to determine the altitude of an aircraft. Mercury barometers measure pressure as air presses on a cistern of mercury, changing the height of a column of the mercury that extends through a vacuum in a glass tube. Aneroid barometers measure direct air pressure on a flexible diaphragm and are less accurate than mercury barometers.
Meteorologists use mercury barometers to watch for trends in air pressure changes. The barometer consists of a narrow, vacuum-filled test tube inverted into a wider container of mercury. The atmosphere pushes down on the exposed mercury in the wider dish. This force pushes mercury up into the vacuum in the narrow tube. As the force changes, the height of the mercury in the tube changes. Meteorologists measure the force of the pressure by reading a scale on the side of the glass tube.
Aneroid barometers are more portable than mercury barometers. This makes them the preferred tool for home weather forecasting and for measuring altitude in aircraft. The device consists of a cylinder containing a partial vacuum. The cylinder has a flexible top that moves up and down through the partial vacuum in response to air pressure changes. A needle rests on the flexible surface and transfers the pressure changes to a pointer that measures the pressure on an integrated dial.