What Barometers Do
A barometer simply measures air pressure – expressed in millibars or inches. An aneroid barometer displays the reading by means of dial and a needle, whereas a mercury barometer uses the height of liquid mercury within a glass tube. Since changes in air pressure accompany changing weather, these instruments can be used to help forecast local weather some 24-48 hours in advance.
Set up for Altitude
In addition to changes associated with weather patterns, barometric pressure also changes with altitude – decreasing the higher you are. All our aneroid barometer movements are set at the factory to give the correct reading at sea level (the standard measurement point). If the barometer is to be mounted above sea level – i.e for most domestic use – it is necessary to adjust your barometer accordingly.
An accurate pressure reading should be obtained from the nearest weather or meteorological station. In the UK this can be obtained by dialling 0870 900 0100 (local call rate), or +44 1392 885 680 from overseas. This is the main Met Office Headquarters in Exeter, which has a very helpful customer care centre. Alternatively you can visit their website at www.metoffice.gov.uk Then, using a small screwdriver, the screw on the back of the movement should be turned until the adjusted needle indicates correctly.
Making a Weather Forecast
The notes on the barometer face (RAIN, CHANGE etc) are broad indications and less important than the actual pressure change. Regular observation will quickly lead to a greater understanding of weather patterns. Generally:
- Increasing Pressure (movement of the needle in a clockwise direction) suggests improving weather, associated with a HIGH or "anti-cyclonic" pressure system.
- Decreasing Pressure (movement of the needle in an anti-clockwise direction) suggests deteriorating weather, associated with a LOW or “cyclonic” pressure system.
- Steady Pressure, typical of extended fine weather periods, suggests more of the same.
To observe the trend, the movable pointer is turned so that it sits directly above the barometer needle. Later (perhaps 6 hours for an easily-identifiable change), the barometer glass is tapped gently and the direction of movement of the needle noted.
For the UK, our handy weather forecaster (4642 – see below) helps interpret your readings and, by allowing for time of year and wind direction, provides a surprisingly accurate local forecast.
IMPORTANT: Aneroid barometer movements should not be relied upon for serious scientific or industrial use.