Ordnance Survey – Great Britain's national mapping agency

Frequently asked Questions about Barometers

30 January 2007

Q. What is a barometer?

A. Barometers are instruments used to measure atmospheric pressure. Atmospheric pressure fluctuates with atmospheric conditions and height above sea level. Barometers measure the weight of the column of air that extends from the instrument to the top of the atmosphere. Invisible gases found in the earth's atmosphere have weight and exert a pressure on all of us. It is this pressure that barometers calculate. Mercury and aneroid are the main two types of barometer.

Q. How does a barometer work?

A. Barometers do not use liquid. A barometer consists of a ridged metal capsule where most of the air is taken out, which is prevented from collapsing by a spring. As the air pressure varies the capsule makes very slight movements. This minor movement is shown on the dial on the face of the barometer.

Q. How do I set my barometer?

A barometer will include one dial with two pointers. One dial can be moved by turning the screw on the face, while the other moves by itself and shows the actual pressure. You must set the milled screw on the reverse of the barometer to your height above sea level. You need to deduct one millibar for every nine meters above sea level.

Q. Where can I obtain my height above sea level?

A. Contour information is available from Ordnance Survey's free Get-a-map service. Alternatively, you can obtain this information by purchasing an OS Explorer Map.

Q. Where can I obtain my barometric pressure?

A. Providing you have a valid post code, the Meteorological Office can provide you with barometric pressure by calling 0870 900 0100.

Q. When is the best time to set my barometer?

A. It is wise to choose a day when pressure remains reasonably consistent. Settled, clear and sunny days make the perfect condition.

Source: Standard definition and Meteorological Office

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