Air Pressure

Air Pressure

The weight of the air above a given level. Thisweight produces a force in all directions caused by constantly moving air molecules bumping into each other and objects in the atmosphere. The air molecules in the atmosphere are constantly moving and bumping into each other with each air molecule averaging a remarkable 10 billion collisions per second with other air molecules near the Earth’s surface. The density of air molecules is highest near the surface, decreases rapidly upward in the lower 62 miles (100 km) of the atmosphere, then decreases slowly upward to above 310 miles (500 km). Air molecules are pulled toward the Earth by gravity and are therefore more abundant closer to the surface. Pressure, including air pressure, is measured as the force divided by the area over which it acts. The air pressure is greatest near the
Earth’s surface and decreases with height, because there is a greater number of air molecules near the Earth’s surface (the air pressure represents the sum of the total mass of air above a certain point). A one-square-inch column of air extending from sea level to the top of the atmosphere weighs about 14.7
pounds. The typical air pressure at sea level is therefore 14.7 pounds per square inch. It is commonly measured in units of millibars (mb) or hectopascals (hPa), and also in inches of mercury. Standard air pressure in these units equals 1,013.25 mb, 1,013.25 hPa, and 29.92 in of mercury. Air pressure is equal in all directions, unlike some pressures (such as a weight on one’s head) that act in one direction. This explains why objects and people are not crushed or deformed by the pressure of the overlying atmosphere. Air pressure also changes in response to temperature and density, as expressed by the gas law: Pressure = temperature × density × constant (gas constant, equal to 2.87 × 106 erg/g K). From this gas law, it is apparent that at the same temperature, air at a higher pressure is denser than air at a lower pressure. Therefore, high-pressure regions of the atmosphere are characterized by denser air, with more molecules of air than areas of low pressure. These pressure changes are caused by wind that moves air molecules into and out of a region. When more air molecules move into an area than move out, the area is called an area of net convergence. Conversely, in areas of low pressure, more air molecules are moving out than in, and the area is one of divergence. If the air density is constant and the temperature changes, the gas law states that at a given atmospheric level, as the temperature increases, the air pressure decreases. Using these relationships, if either the temperature or pressure is known, the other can be calculated. If the air above a location is heated, it will expand and rise; if air is cooled, it will contract, become denser, and sink closer to the surface. Therefore, the air pressure decreases rapidly with height in the cold column of air because the molecules are packed closely to the surface. In the warm column of air, the air pressure will be higher at any height than
in the cold column of air, because the air has expanded and more of the original air molecules are above the specific height than in the cold column. Therefore, warm air masses at height are generally associated with high-pressure systems, whereas cold air aloft is generally associated with low pressure. Heating and cooling of air above a location causes the air pressure to change in that location, causing lateral variation in air pressure across a region. Air will flow from highpressure areas to low-pressure areas, forming winds. The daily heating and cooling of air masses by the Sun can in some situations cause the opposite effect, if not overwhelmed by effects of the heating and cooling of the upper atmosphere. Over large continental areas, such as the southwestern United States, the daily heating and cooling cycle is associated with air pressure fall and rise, as expected from the gas law. As the temperature rises in these locations the pressure decreases, then increases again in the night when the temperature falls. Air must flow in and out of a given vertical
column on a diurnal basis for these pressure changes to occur, as opposed to having the column rise and fall in response to the temperature changes.