The earth we live is surrounded by a blanket of air. Air is a mixture of gases. Air surrounding the earth is called as atmosphere. This gases have some weight, so they experience some gravitational force. These mixture of gases exert pressure on us and is called air pressure or atmospheric pressure. Atmospheric pressure is a technical term used to represent the pressure exerted by the air at a place.“A barometer is an instrument used for measuring atmospheric pressure.”
Air flows only when there is a difference in air pressure. Air flows from a high pressure to low pressure. The larger the difference in pressure between two places the faster it flows. Depending on its velocity, if air travels at high velocity it forms eddies and the sir flow is more chaotic and at low velocities, the air is more straight lined and it is called laminar flow.
Air exerts pressure in all directions can be proved by doing these simple experiments,
Experiment 1 : Take an empty soda bottle with minimum thickness open the cap. Hold it in the mouth and suck all the air with a vacuum pump. What happens to the bottle? The shape of the bottle will be altered. Why? When we suck the air out of it, the air present out side the bottle will exert pressure on the bottle in all possible directions. Therefore the bottle is crushed.
Experiment 2 : Take a thin tin box with a small opening at one end and heat it. When it is heated what happens? We observe that shape of the box is altered. Why? On heating the air in the box will reduce and the walls of the box experiences pressure by the surrounding air, altering the shape of the box.
The air pressure on earth is not same at all the places. There are three factors effecting air pressure at a place, they are temperature at a place, gravity at a place and moisture in the air over the place.
Air pressure will change depending up on the temperature. The temperature changes through out the day. When the temperature rises the water vapor content in the air increases which in turn displaces other molecules suspended in air (Avogadro's principle),thus causing a lower atmospheric pressure since water vapor is less denser than dry air. The temperature on the earth is not uniform creating high and low pressure regions. High and low pressure systems have an important effect on our weather.
The earth gravitational force decreases with distance from the center of the earth, so air weighs less at altitude than at sea level. Literally it means air at altitude is less dense compared to sea level, and air becomes progressively less dense with increasing altitude.
Interestingly, the moist or damp air is lighter than dry air. According to Avogadro's Principle, "Equal volumes of gases at the same temperature and pressure will have equal number of molecules " . Water molecules are lighter than dry air molecules, therefore moisture in the air leads to lower air pressures than dry air as water vapor is of lesser molecular weight then dry air molecules (which have oxygen and nitrogen). This is why lower atmospheric pressure leads to rain because of more number of water molecules and higher pressure means clear weather because the air has lesser water molecules which translates to lesser moisture because dry air has more oxygen and nitrogen molecules.