HOW IS ATMOSPHERIC PRESSURE MEASURED?

ATMOSPHERIC PRESSURE -

The blanket of air, dust, and moisture that surrounds our planet is called the 'atmosphere'. The air has weight and it exerts pressure. The instrument used to measure the atmospheric pressure is called a 'barometer'. The various changes in the atmospheric pressure are used to forecast the weather. For example, a sudden fall in the pressure indicates a storm. It is also used to measure the altitude or height of objects above sea level.

In 1643, an Italian, Evangelista Torricelli, demonstrated that air exerts pressure. He was able to prove that air pressure at sea level is normally, 76 centimeters of mercury barometer for measuring the atmospheric pressure.

A simple barometer consists of a glass tube, open at one end and closed at the other. It is filled with mercury. When turned upside down and immersed in a container of mercury, there is a drop in its level in the tube. The rest of the mercury is held up in the tube due to the air pressure. The height of the mercury column, in the tube, indicates, the atmospheric pressure. This simple barometer was later improved upon by a scientist called 'Fortin'.

Nowadays, two types of barometer are used to measure the atmospheric pressure: mercury barometers and aneroid barometers. In a mercury barometer, a printed scale, besides the tube containing mercury, indicates the pressure. They are heavy and are difficult to move from place to another. Aneroid barometer does not use any liquid. It consists of a cylindrical box, which is evacuated and closed with a thin elastic diaphragm. The variations in atmospheric pressure cause the diaphragm to move in and out. The change is indicated by a pointer, which moves on a scale by a system of levers. Aneroid barometers are smaller in size, and thus, easier to carry. Scientists now measure air pressure in a unit called 'millibar'. The millibar is equal to the column of mercury - 0.08 centimeter high in a tube of 1-centimeter diameter. Previously, it was being measured in the unit 'torr'. One torr is equal to 1 millimeters column of mercury. The atmospheric pressure equals 6.7 kilograms per square inch or 14.7 pounds per square inch.