A barometer is an instrument which is used for measuring the pressure of air around us or in other words, a barometer helps measure atmospheric pressure. When there is a change in the atmospheric pressure, our weather too changes. Thus a barometer can be used to forecast weather too.

The first was made by Evangelista Torricelli, an Italian physicist, during 1643. He initially filled a 36 inch long glass tube with mercury which was closed at one end. He then placed the open end of the tube into a basin filled with mercury. He found that some of the mercury in the tube would fall into the basin and leave a gap in the tube of about 6 inches in the top. The gap which was created was a vacuum and contained nothing. Torricelli concluded that the remaining mercury in the tube was being balanced by the atmospheric pressure on the surface of the mercury in the basin. The higher the mercury rose in the tube, the greater was the atmospheric pressure. During 1648, Blaise Pascal who was a French scientist discovered that the mercury level in a barometer gradually dropped as it was carried up a mountain. A similar barometer at the foothill of the mountain showed the same reading throughout the day. This was due to the fact that atmospheric pressure drops as one goes higher above sea level. Thus it was found that a barometer can be used as an altimeter which can measure altitudes or height above sea level.

At a fixed altitude and consistent temperature, any change in the reading of a barometer can be associated with change in atmospheric pressure. If the pressure drops suddenly, it is associated with bad weather and if the pressure rises gradually, this indicates probable improvement in the weather.

An aneroid barometer is another type of barometer which has no liquid in it. The word aneroid means ‘no liquid’ and is not made out of mercury. Instead, it contains a vacuumed thin round metal box which contracts and expands depending on the changes in the atmospheric pressure. A leaver is attached to one side of the box which shows how far in or out the side moves ad thereby measures what the atmospheric pressure is on a scale. A barograph is an aneroid barometer with a pen being attached to the lever which provides a continual record of changing pressure. The pen continuously draws a line on a revolving drum and depending on the pressure level, the line goes up and down on the drum.

Barometers play a vital role in determining the atmospheric pressure and help forecast weather accurately. Thus they form an integral part of the meteorological departments throughout the world.