Amateur meteorologists love barometers: the instrument that makes us all expert weather forecasters. For with a barometer you can not only measure the current air pressure, but also see which direction the needle is going, and therefore have a fair chance of predicting the weather to come Put simply, when the pressure is falling, unsettled weather is usually on the way, with low pressure systems bringing cloud, wind and rain. Conversely, when the pressure is rising, you can expect a high pressure system, which usually bring clear, cloudless skies and settled weather. Instruments to measure atmospheric pressure were first invented in the 17th century, with scientists such as Galileo and Descartes contributing to their development.

Barometers We live at the bottom of a great ocean of air. We can't see it but we can feel it every time the wind blows. This air presses on us from all directions and we walk around with a great weight sitting on our heads. We have, however, evolved on this planet with this atmosphere and this gravity so we don't notice the pressure on a day to day basis. If we take off in an aeroplane, however, the changing air pressure makes our ears "pop". Many early scientists had seen the effects of this air pressure but it was Toricelli (1608-1647) [pictured left] who invented the first barometer - an instrument for measuring air pressure - in 1643.
	He inverted a glass tube, closed at one end and full of mercury, into a reservoir of mercury. He saw that the column of mercury in the tube dropped a little, leaving a vacuum above it in the tube. He then noticed that the level of the mercury rose and fell regularly every day and had rises and falls over longer periods as well. Toricelli did not call his invention a barometer. Boyle did that in 1665. The word comes from the Greek baros meaning "weight". Mercury is the favoured liquid for a barometer as it is so heavy. The barometer tube only needs to be a metre long. This makes it a convenient size for everyday use. Any liquid can be used in a barometer if you have a tube long enough. Pascal, the French scientist, made one with a tube 14 metres long.

He used red wine as his liquid. Water barometers have often been made and they are popular in schools. Water is 13 times less dense than mercury so you will need a tube about 13 metres long. A barometer this size, however, is not practical.

Mercury in glass barometers are handsome instruments and very accurate but they are difficult to transport. Mercury can be very nasty in contact with other metals. Aluminium is particularly vulnerable. A broken barometer on an aeroplane can have devastating consequences which is why very strict safety procedures have to be followed if moving a barometer by air.

The most common type of barometer is the aneroid barometer. Many people have one in the home. The barometer in the picture with this column is an aneroid barometer. These barometers do not use liquid in a tube but a metal box from which some of the air has been extracted. As the air pressure rises, the box is compressed; as the pressure falls, the box expands. A pointer is attached to the box such that it swings right and left as the box contracts and expands. A calibrated card behind the pointer lets us read the pressure.

Setting a barometer
Many people have them and if they are to be any use, they must be set correctly. The air pressure decreases with altitude so if we measure the pressure at Lyneham and Snowdonia at the same time, Snowdonia will have a lower pressure than Lyneham. We could set our barometers to the correct local pressure and this is called Station Level Pressure (SLP). It cannot, however, be compared with the weather maps on TV or in the newspapers as the pressures on those are all Mean Sea Level Pressures (MSLP). The Met Office reads the barometers at SLP and then makes a correction to bring the reading to MSLP. You can bypass the correction by setting your barometer to MSLP. You can then make direct comparisons with the reports on TV or in the papers.

Go to the BBC Weather website link for Lyneham. Check the pressure given and set your barometer to that. There is a screw in the back of the barometer that allows you to set the pointer. Don't worry about the pointer controlled from the front at this stage. Let the barometer settle down and the next day check the pressure again just after the hour when the new observation is posted. Reset your barometer - it will need only a fine adjustment. Do this once more and you will be ready. Check the setting every two months or so to keep it accurate.

Reading the Barometer
The barometer should be read at the same time every day. When you read it, move the pointer control on the front until the needle sits over the barometer pointer. When you look at the barometer the next day you will be able to see if the pressure has risen or fallen. 9am is the traditional time for reading barometers but pick a time that suits you. There are tides in the atmosphere and the pressure falls from about 9am to about 3pm, rises to 9pm, falls to 3am and then rises again to 9am. Toricelli noticed this with his barometer. The movement is called the "diurnal variation of pressure" and this is why the barometer must be read at the same time every day.

The units we use to measure air pressure are hectoPascals (hPa). This is the same figure as the earlier millibars (mb). Even earlier, and still in the USA, the pressure was given by the height of the mercury column in inches of mercury. The liquid must be specified as it could just as easily be a lot more inches of red wine.

Can we forecast the weather with a barometer?
Many aneroid barometers have words like "fair", "change", "rain", "stormy" on the front. Are these accurate forecasts? No they are not. Most of the barometers we see get their traditional faces from northern Europe where there are wide variations in pressure. Rapidly falling pressures can indicate the approach of a trough or a front or, on the coast, the development of a low. You should look at the weather map to find out what is happening. Any of these can bring bad weather so the barometer can be useful there. High pressure, 1020hpa or higher, should bring fine days with light winds. Most of the time, though, in our area, the barometer needle will point to "change". That is just because there is little variation in pressure from day to day.

Use the barometer well and you will find it a useful addition to your knowledge and understanding of the weather.