Atmospheric pressure is an important characteristic of the atmosphere. Wind and other weather phenomena occur due to changes in atmospheric pressure. In physics, the pressure is the perpendicular force on the object.
What is atmospheric pressure?
Our atmosphere is composed of air. The molecules of gases and water vapor constitute the air. Due to gravitational force, the air has weight and it exerts pressure on the earth. At any surface, Atmospheric pressure is the force exerted by the weight of the overlying air above the surface.
Atmospheric pressure varies with temperature and composition of the air. The atmospheric pressure gradually decreases with altitude.
How atmospheric pressure is measured?
Air pressure is measured by a device called a barometer. Atmospheric pressure is also called barometric pressure. There are two types of barometers: mercury barometer and aneroid barometer.
The mercury barometer was invented by Italian scientist Evangelista Torricelli. The barometer consists of a glass tube which closed at one end. This glass tube is filled with mercury and inverted it in an open dish of mercury. When there is the atmospheric pressure exerted on the mercury, then mercury rises up in the glass.
The atmospheric pressure is mostly measured in millibar (mb). Standard sea-level pressure is 1013.2 millibars and it is the same as the 76-centimeter height of mercury barometer.
Variation in atmospheric pressure with altitude
At any given altitude, atmospheric pressure is caused by the weight of the air above. Air pressure gradually decreases with altitude. This is because as we go up in height, the air density decreases. Most of the mass of the atmosphere lies in the troposphere.
Horizontal variation in air pressure
The air pressure varies from place to place. the horizontal variation in atmospheric pressure is mainly caused by temperature. Air density is directly affected by temperature.
When the temperature rises, the air expands. As the air expands, the density of air decreases and it puts less pressure on the surface of the earth. Thus, the equatorial region often at low pressure.
When the temperature of the atmosphere falls, the density of the air increases while the volume of the air decreases. This results in an increment of air pressure. For this reason, the air pressure in the polar region is often higher.