A barometer is a tool used to measure the pressure of the atmosphere in meteorology. The pressure is measured with water, mercury or air, and it can help predict upcoming changes in weather. Air pressure is also used in surface weather analysis to identify surface troughs, high pressure systems and frontal boundaries.
Water barometers are made of a sealed glass container filled halfway with water. A narrow spout allows the atmospheric pressure to move the water up and down. A mercury barometer also uses a glass container, this one closed at one end, with a mercury-filled area at the base. The mercury creates a vacuum in the top of the tube, and is driven up and down by the changes in atmospheric pressure. It is similar to the water barometer.