All About the Barometer: What It is and How it Works
Meteorologists require information about the atmospheric pressure to predict the forthcoming weather. A difference in atmospheric pressure between two locations causes winds to blow from the high towards a low pressure area with a high pressure area indicating good weather and a low one indicating storms.
Meteorologists record the atmospheric pressure, adding this to weather properties data gathered from other instruments to form their weather forecasts. The pressure can be in millibars (mB) or inches mercury (inches Hg)
A weather map showing high and low pressure areas is shown below;
Barometers used in our homes give an indication of the weather. The dials are calibrated in mB, showing low pressure through to high pressure clockwise, showing different conditions.
A barometer dial is shown below;
The four types of barometers
The water barometer consisted of a 30’ long vertical glass tube that was sealed at the top; the open bottom was immersed in a cistern of water. As the atmospheric pressure increased, the level of the water in the column rose, falling again as the atmospheric pressure decreased.
The standard mercury barometer design is the same as the water one only the glass tube is 3’ high.
As a storm approaches, the atmospheric pressure drops and the level of the column of mercury also falls. Once the storm passes and high pressure returns, the height of the column of mercury rises with the increasing atmospheric pressure.
The aneroid barometer succeeded the water and mercury barometers and gives a very accurate measurement of atmospheric pressure. It consists of a small evacuated container with a corrugated top. As the atmospheric pressure rises, the corrugated top is pushed inwards and as the pressure decreases the top pops back up again. A system of levers connects the corrugated top to a dial that indicates atmospheric pressure in millibars.
The barograph is an automated version of the aneroid barometer, except there is a pen connected by levers to the corrugated top. This pen traces atmospheric pressures onto a barogram located on a revolving drum. Examples of barographs and barograms are shown below;