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Barometers are used to measure atmospheric pressure--the pressure of the air. There are three different types of barometers: water-based, mercury and aneroid. Changes in atmospheric pressure, measured by a barometer, indicate a change in weather.
Altitude affects atmospheric pressure. Unless you are reading your barometer at sea level, you will need to know what your altitude is to know what the actual atmospheric pressure. A barometer is very useful even if you don't calculate actual atmospheric pressure because still indicates changes in weather.
Atmospheric pressure rises when air moves across large cool masses, such as cool water or land. Air molecules condense as the are cooled and the surrounding air in the upper atmosphere flows into the area.
Atmospheric pressure falls when air moves across large warm masses, such as warm water or land. When the air molecules warm up, they move around more taking up more space in the atmosphere. This causes some of the air molecules in the upper atmosphere to flow out of the area.
A water-based barometer is a sealed glass container filled halfway with water. The container has a spout connected to it below the water line. The spout extends up above the water line and is open at the end. The water is usually coloured so that its levels can be easily read. When the atmospheric pressure is higher than it was when the barometer was sealed, the water level in the spout will drop below the water level in the container. This is an indication that cooler temperatures are coming. Conversely, when the atmospheric pressure is lower than it was when the barometer was sealed, the water level in the spout will rise above the water level in the container. This is an indication that warmer temperatures, storms and rain are coming. You can also read the water level in the container. If the water in the container is higher than in the spout, the atmospheric pressure is high. If the water in the container is lower than in the spout, the atmospheric pressure is low.
Mercury barometers consist of a reservoir and a 30-inch tube, sealed at the top, rising up out of the centre of the reservoir. The reservoir is filled with mercury and is open to the atmosphere. This type of barometer works similarly to the water-based barometer. When atmospheric pressure is high, it presses down on the mercury in the reservoir, forcing mercury up into the tube. When atmospheric pressure is low, it allows mercury to flow out of the tube and back into the reservoir.
An aneroid barometer uses alloy coils, made of beryllium and copper instead of a liquid, to measure changes in atmospheric pressure. The coils are attached to a dial. Changes in atmospheric pressure cause the coils to expand or contract. Higher atmospheric pressure causes the coils to compress. Lower atmospheric pressure allows the coils to expand.