If you see this image next to a definition, click on it to get an interactive calculator for computing that weather factor. Barometer Definition A barometer is used to measure atmospheric pressure . A typical barometer employs a mercury-filled glass tube, sealed at one end with a mercury reservoir at the other end. The mercury then adjusts between the tube and the reservoir to reflect atmospheric pressure. Low pressure will cause the mercury to drop in the tube.
High pressure forces the mercury to rise in the column.
Another type of barometer is an aneroid barometer . Aneroid barometers use a tightly sealed box with a partial vacuum inside. Small changes in external air pressure cause the cell to expand or contract, which reflects outside atmospheric pressure. Okay, so it's not the actual barometer you're interested in, but barometric pressure . Barometric pressure is the amount of pressure the air is exerting.