Air pressure is the weight of the air over a specific area. At sea level this is about 1 kg/cm2. The pressure gradually decreases from the surface of the Earth at a rate of about 1 cm Hg/123 m (1 in/1,000 ft) in the first few kilometers. About half of the weight of air is found in the first 5.5 km of the atmosphere. The air gets very thin above that point. The layer of air continues until about the middle of the mesosphere. At this point, air is too thin to be measured.
Air pressure is measured using a barometer. The two types are mercury and aneroid. The mercury thermometer is a column of mercury in a tall tube. This tube is inverted into a bowl of mercury. The air puts a force on the mercury in the bowl, keeping the mercury in the tube from flowing out. The height of the mercury column is about 76 cm (30 in) tall. As the pressure increases, it forces mercury higher up into the tube. When the air pressure falls, the level in the tube falls as well. When you hear the barometric pressure reading on a weather report, this is the value that they are referring to. The units used are centimeters (or inches) of mercury. Millibars are used in the metric system and on station models. A millibar is about 1/1,000 of the pressure at sea level. Standard (average) sea‐level pressure is 1,013.2 mb (29.92 in of Hg). This is also called 1 atmosphere of pressure. Figure shows the pressure scales.
Figure 1 Scales showing pressure in millibars and inches of mercury.
An aneroid barometer is a sealed container with a pointer attached to it. As the pressure increases, it squeezes the container. This moves the pointer in one direction. The pointer moves in the reverse direction as the pressure falls. Air pressure can change for several reasons. The amount of moisture content in the air can change the pressure. As the amount of water in the air increases, the pressure decreases. This is because a molecule of water has less mass than a molecule of air. Cold air is denser than warm air and therefore has a higher pressure.