If you take the total weight of all the atmospheric gases and divide it by the surface area of Earth, we get the atmospheric pressure. This average pressure due to the atmosphere on the surface of earth has been found to be 10,000 kg- force acting on every square meter area on the surface of the Earth and is defined as one bar.
One bar is equal to 9.8 x 104 N.m-2 or approximately 105 N.m-2 (or 105 Pascal). It will be interesting to find the force, due to atmospheric pressure, acting on a person's head. Let us say the surface area of the head of an average human being is about 100 cm2 or 0.01 m2. The weight acting on the head of the person due to the atmosphere is of the order of 10 3 N or approximately 100 kg weight. However, human beings do not feel this weight because the blood pressure in the body is slightly more than the atmospheric pressure and they balance each other.
1 bar = 105 N.m-2 or 105 Pascal (SI unit)
1 millibar = 102 N.m-2 or 102 Pascal.
The atmospheric pressure can be measured using a simple device known as a barometer. In a barometer, the pressure is expressed in terms of height of column of mercury in a narrow tube. One bar is equal to 760 mm of mercury or 760 tarr. One tarr refers to one millimeter of mercury.
Measurement of atmospheric pressure:
To find the atmospheric pressure at a place one need not go on calculating the volume and weight of atmosphere in an imaginary tube of 1m2 area that extends to, say, 100km above the Earth. In 1643, one scientist, Torricelli constructed a barometer to measure the atmospheric pressure.
Simple barometer comprises of a glass tube of uniform cross section and about 1m in length. The tube is open on one side and closed at the other end. The tube is graduated and completely filled with mercury. This tube is inverted by holding at the open end with the hand and is lowered into a large trough that is also filled with mercury to half its height.
When the hand is removed, some mercury from the tube falls into the trough as a result the mercury level in the vertical tube falls, the mercury level in the trough slightly rises, and both levels remain steady after a while. An empty space is created in the tube, above the mercury level. This vacuum is named after the inventor of the barometer and is called torricellian vacuum.
The principle of the barometer is simple. The atmospheric pressure acting on the mercury in the open trough would push the mercury level in the tube up to a certain height compared to the level in the trough. Therefore, the height of the column of mercury in the vertical tube gives a direct measure of the magnitude of the atmospheric pressure.
At sea level, the height of the mercury column has been found to be 76 cm. Meteorologists keep looking at the barometer and make forecast about the weather conditions. If there is a sudden drop in the level of mercury in the tube, they might predict rain due to low pressure or dry weather if the level rises.
The atmospheric pressure at sea level is called 1 bar which is equivalent to a rise of 76 cm of mercury in the barometer and the same is expressed in S.I units as 10 5 Pascal.