Mechanical Properties of Fluids

Barometric Pressure – Definition and Formula

Barometric Pressure

The barometric pressure is the pressure that occurs because of the weight of the air above us. The atmosphere of the earth above us contains air.

Moreover, it is relatively light, containing that much of it. It begins to have some weight as the gravity attracts the air molecules. Moreover, the air that surrounds us comprises 78% of nitrogen, 21% of oxygen, under 1% argon and also a small amount of various other gases.

In addition, the air gets much thinner as we go higher because there are lesser molecules. Approximately 75% of the mass of the atmosphere is below the altitude of around 11 KM that is 6.8 miles or 36,000 feet thick layer on the surface of the earth.

However, the border where the atmosphere turns into outer space is generally considered to be around 100 KM i.e. 62 miles above the surface of the earth.

Definition of Barometric Pressure

We also call the barometric pressure as the atmospheric pressure. We define it as the pressure within the atmosphere of the Earth.

Moreover, the standard atmosphere is a unit of pressure, we define it as 1,013.25 mbar (101,325 Pa; 1,013.25 hPa). It is equivalent to 760 mm Hg (torr), 29.9212 inches Hg, or 14.696 psi.

The atm unit is nearly equivalent to the mean sea-level atmospheric pressure on the Earth. The atmospheric pressure of the earth at the sea level is around 1 atm.

Moreover, in most of the situations, atmospheric pressure is closely approximated by the hydrostatic pressure, the weight of the air above the measurement point causes this pressure.

barometric pressure

As the elevation increases, there is a less overlying atmospheric mass that results in the decrease of the atmospheric pressure with the increase in the elevation.

The pressure measures the force as per unit area, with SI units of Pascal i.e. 1 Pascal equals 1 newton per square meter, 1 N/m2. On average, a column of air with a cross-sectional area of 1 square centimeter i.e. cm2.

We measure it from the mean sea level to the top of the earth’s atmosphere, has a mass of around 1.03 KG. It also exerts a force or weight of about 10.1 newton that results in a pressure of 10.1 N/cm2 or 101kN/m2. A column of air with a cross-sectional area of 1 in 2 would have a weight of around 14.7 lbf that results in a pressure of 14.7 lbf/n2.

Formula of Barometric Pressure

The barometric formula, sometimes we call it an exponential atmosphere or isothermal atmosphere. It is a formula that we use to model how the pressure or density of the air changes with respect to the altitude. The pressure drops approximately by 11.3 Paper meter in the first 1000 meters above the level of the sea.

There are 2 different equations for calculating the pressure at various height regimes below 86 km. the first equation is useful when the value of the standard temperature lapse rate is not equal to 0:

We use the second equation when the standard temperature lapse rate is equal to 0:


The gravitational attraction of the planet on the atmospheric gases above the surface is the cause of the atmospheric pressure.

Moreover, it is also a function of the mass of the planet, the radius of the surface and the amount and the composition of the gases and their vertical distribution within the atmosphere.

Solved Question For You

Ques. An instrument that we commonly use for measuring the atmospheric pressure is:

(A). Manometer.
(B). Barometer.
(C). Calorimeter.
(D). Potentiometer.

Ans. (B). Barometer.

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