How does a Barometer Work

A barometer is an instrument used to measure the atmospheric pressure (weight of the air) by using water, air or mercury. Data collected about the air pressure conditions can help to forecast sudden changes in the weather. Measuring air pressure also helps to determine the surface troughs and high pressure systems on the planet.

Water-based Barometers
Lucien Vidie's law stating, "decreasing atmospheric pressure predicts stormy weather", formed the basis of basic water prediction appliances known as the 'weather glass' or the 'Goethe barometer'. This device consists of a sealed glass container, half filled with water. A narrow spout is connected to this container at a point below the water level, and it goes up above the water level, where it is open to the atmosphere. Preferably colored water is used, as it becomes easy to collect the readings. When the atmospheric pressure goes higher than it was when the glass container was sealed, then the water level in the spout drops below the level of water present in the container, indicating that the temperature is getting colder. On the other hand, when the atmospheric pressure is lower than it was when the container was sealed, then the level of water in the spout will exceed the water level in the container, indicating that temperature is getting warmer.

A simple way of interpreting this is - when the level of water in the container is higher than the level of water in the spout, it means that the atmospheric pressure is high; whereas, when the level of water in the container is lower than the level of water in the spout, it means that the atmospheric pressure is low.

Mercury Barometers
A mercury barometer is a glass tube, 30 inches tall, sealed at one end and open at the other. The tube is filled with mercury at the base, and placed upside down in a container called the 'reservoir'. The mercury barometer works by balancing the weight of mercury within the glass tube with the atmospheric pressure. When the weight of mercury is lesser than the atmospheric pressure, there is a significant rise in the mercury level in the glass tube and when the the weight of mercury exceeds the weight of atmospheric pressure, there is a fall in the mercury level. The level of mercury continues to change till the weight of the mercury in the glass tube is equal to the weight of the air above the reservoir. In low pressure conditions, the air rises from the surface of the Earth before it is replaced by air from the surrounding areas, therefore the weight of air above the reservoir is reduced, and the mercury drops to a lower level. In high pressure conditions, air sinks towards the Earth's surface faster than its ability to move out to the surrounding areas, thus more air gets accumulated above the reservoir and as the weight of the air is higher, mercury soars up to higher level.

Aneroid Barometers
An aneroid barometer consists of a small flexible metal box called the aneroid cell. This aneroid cell is made from an alloy of copper and beryllium. This cell is supported by a strong spring to prevent it from collapsing. Even minute changes in the external air pressure causes expansion or contraction of the cell. This expansion and contraction triggers the mechanical levers as a result of which tiny movements of the cell are amplified and displayed on the face of the aneroid barometer. A manually set needle is used to mark the recent measurement, so that the change doesn't go unobserved.

Today technologically advanced digital barometers are fast replacing their older counterparts. These digital barometers use electrical charges to calculate air pressure. This sophisticated technology helps to record accurate data about the pressure conditions, and thus helps in more accurate weather forecasting.