Air is composed of molecules. Air is matter. It has mass and takes up space. Air is composed of different gases such as nitrogen, oxygen, carbon dioxide, water vapor, and other gases. Air molecules are in constant motion. As they move, they come in contact with surfaces.
Air molecules push and press on the surfaces they contact. The amount of force per unit area that air molecules exert on a surface is called air pressure. (What is Air Pressure 6) Air pressure is caused by all of the air molecules in the Earth's atmosphere pressing down on the Earth's surfaces. We can measure air pressure to help us predict weather conditions around the world.
Temperature also affects air pressure because air contracts when it cools and expands when it is heated. So if air above a region of Earth cools, it does not extend to as high an altitude as the surrounding air. In this case, its pressure at higher temperature is lower than in the surroundings even when the pressure at the surface is the same as in surrounding areas. Then air flows into the cooler region at high altitude, making the total weight of air above the region greater than in the surroundings.
This is a "high'. The cool air descends to the earth's surface. Near the surface, the falling air spreads out, spiraling clockwise in the northern hemisphere. The opposite happens where air is warmed by the sun or by the Earth's surface temperature. The resulting rising air is above a "low.' Near the surface, air flows into the "low' to replace the rising air, spiraling counter-clockwise (Atmosphere 26). Highs and lows react to each other causing a variety of conditions.
Driving up or down a mountain leads to a reduction or increase of air pressure in the outer part of the ear, creating a pressure difference across the eardrum, which separates the outer ear from the middle ear. The difference distorts the eardrum, so that sounds are muffled (What is Air Pressure 9). However, this can be taken care of by swallowing air and opening the Eustachian tube between the middle ear and the nasal cavity, which in turn is joined to the mouth. The air along the tube suddenly equalizes pressures across the eardrum, which consequently pops back to its normal shape, quite noticeably.
The same pressure differential builds up, but quicker, when taking off from near sea level in a pressurized jet aircraft, because the pressure inside the cabin is held at about 780 hPa. Air pressure is most often measured with a barometer. There are several different types of barometers. One type is the mercury barometer.