Barometers are essential tools for meteorology, used to measure the pressure of air. High air pressure causes nice weather. Low air pressure brings rain and storms. Air pressure is the reason winds blow. Air will move from high pressure areas to low pressure areas. The reading of air pressure a barometer gives will help a meteorologist predict if a storm is coming or not. There are three types of barometers which can measure air pressure.
Water-based barometers employ a glass container with a spout. Water fills the container to halfway up the spout. There is some air inside the body of the container. In areas of low pressure, the air pocket in the body forces some of the water up the spout. If the pressure is low enough, the water may leak out of the spout. If the pressure is high, the water will not move much, if at all.
Mercury barometers have a long glass tube sealed at one end. The other end is open and filled with mercury. The tube is inverted and placed into a bowl of mercury, or a cistern. The tube sits next to a scale of numbers which measure the amount of pressure in the air. As the pressure in the air increases, the mercury is pushed down the tube. The number corresponding to the mercury level indicates the amount of pressure in the air.
Aneroid barometers are the most recent type of barometers. The aneroid barometer has a clock-type face complete with numbers and a dial. However, instead of measuring time, the numbers measure the amount of pressure in the air. Inside the barometer is a metal cell filled with a tiny amount of air. As air pressure increases, the sides of the cell come together. One side of the cell is fastened to the base of the barometer. The other side is connected to a series of levers and pulleys which move the dial on the face. The dial moves to the right with high pressure and to the left with low pressure.