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Barometers

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Weather chart showing pressure systems over Europe
The barometer is one of the most important instruments in weather forecasting.

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Most of us are used to seeing one hanging in the hallway that gets tapped from time to time with some comment about the pressure falling or rising. Used correctly it can give a good indication of the short-term weather trends.

The barometer measures air pressure.
The barometer measures air pressure. The invisible gases in the earth's atmosphere have weight and exert a pressure on all of us. It is this pressure that the barometer measures.

Evangelista Torricelli, born in Italy in 1608, was the first person to discover the principles of the barometer by inverting a long glass tube filled with mercury into a dish. Some of the mercury stayed in the tube, and he discovered that the height of the column of mercury varied due to changes in atmospheric pressure.

This mercury barometer is still one of the most accurate ways of measuring atmospheric pressure, but has some drawbacks in that it is not easily moved and the mercury's volume changes as the temperature changes.

The aneroid barometer... is the type most commonly found in houses...
The aneroid barometer, which is the type most commonly found in houses, was talked about in the last half of the 18th century. It wasn't until the mid 19th century that one was made by Lucien Vidie. This type of barometer uses no liquid. It consists of a corrugated metal capsule where most of the air is taken out, which is prevented from collapsing by an internal spring. As the air pressure goes up and down, so the capsule sides move in and out. This slight movement is then shown on the dial on the face of the barometer.

Pressure can be measured through a number of scales, but the most common, and that used by the BBC, is millibars. However, pressure can also be calculated in Pascals or inches of mercury and you can convert those into millibars by using our Pressure Calculator.

Setting a barometer
On the face of the normal household barometer there are is one dial with two pointers. One of them can be moved by turning the screw on the face, while the other moves on its own and shows the actual pressure. To set the latter for the correct pressure you use the milled screw on the back of the barometer and normally set this to 'mean sea level'. This is the pressure shown on TV weather maps. Pressure decreases with height so if you live above 1000 feet, you can set the pressure for your altitude by taking off 34 millibars for every 1000 feet (or one inch if your barometer is calibrated that way).

For most places in the UK,if you want to compare your readings to those used by professional meteorologists, it's probably best to set mean sea level. This is because it is not the actual pressure that will give a clue to the forecast, but the way that it is changing. You can get the pressure setting for your area through our five day forecasts. So once a day, set the two hands together. Every day check to see what the pressure has done. If the atmospheric pressure is falling, then the weather is becoming more unsettled. If it is rising, it indicates more settled weather.

The best day to set your barometer is on a day with anticyclonic conditions when the pressure won't change too much. A typical day would be when there is little wind and conditions are perhaps clear or sunny. On such a day, phone your local weather centre and ask for a current pressure reading. If you can't track the weather centre down, phone the Met Office's customer centre (0870 900 0100) and they should be able to assist.

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Key Points

• Barometers measure air pressure.
• Aneroid barometers are most commonly used.
• Pressure decreases by 34 millibars every 1000 feet in altitude.

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