How to set your barometer

Atmospheric pressure varies with height above sea level as well as with atmospheric conditions. As the altitude at which a barometer operates is usually constant (the station height), a correction is made to make the reading of a barometer seem as if it were made at sea level. This means that the pressure reading has to be slightly increased from the value read on a barometer situated above sea level (often know as the station pressure).

Pressure maps shown on the television and in newspapers show pressures which are corrected to the values which would be measured at sea level. If a barometer's pressure reading is to be compared with pressure maps distributed by the Media, then it first needs to be set to read sea level pressure.

The easiest way to set a barometer is to adjust it to give the same reading as that of a weather chart for in your area at the same time. This is best done in atmospheric conditions when the pressure varying only very slowly. Anticyclones are ideal for this, often associated with established clear and sunny (but sometimes foggy in winter) conditions. Don't adjust it when conditions are unsettled or windy.

You can only make the adjustment to the barometer if you have an aneroid barometer, usually by a small screw on the back of the barometer, but the instructions will need to be consulted. Mercury barometers (these contain mecury and are about a metre long) should not be tampered with as a mecury spillage presents a safety hazard.

After you have made the adjustment, the barometer should give a reading of pressure which is the same as that on a weather chart for the same time. Note that you need to use a chart of the analysis (i.e. what the values actually were), rather than a forecast.