The barometer is one of the basic instruments of meteorology. It is used to measure air pressure. The word comes from a combination of two Greek words: "baro," meaning weight, and "meter," meaning measure. Dating back to the middle 1600s, early mercury barometers consisted of a glass tube sealed at the top but open at the bottom. This tube is partially filled with mercury and is suspended in an open container also containing mercury. As the air pressure increases, mercury is forced upward inside the tube. Likewise, a lower air pressure allows the mercury to drop.
Historically, the changes in the height of the mercury were measured with a ruler, so barometric pressure used "inches of mercury" as a unit. Today, the metric units of the bar and the millibar are the preferred units of scientists. Changes in barometric pressure across a region are extremely useful in forecasting the movement of weather systems.