Earlier this semester we noted that air pressure is simply the pressure exerted by the weight of air above. Average air pressure at sea level is about 1 kilogram per square centimeter, or 14.7 pounds per square inch—also called 1 atmosphere. Specifically, a column of air  square inch in cross section, measured from sea level to the top of the atmosphere, would weigh about 14.7 pounds (▼). This is roughly the same pressure that is produced by a 1-square-inch column of water 10 meters (33 feet) in height. With some simple arithmetic, you can calculate that the air pressure exerted on the top of a small (50 centimeter-by-100 centimeter [20 inch-by-40 inch]) school desk exceeds 5000 kilograms (11,000 pounds), or about the weight of a 50-passenger school bus. Why doesn’t the desk collapse under the weight of the ocean of air above? Simply, air pressure is exerted in all directions—down, up, and sideways. Thus, the air pressure pushing down on the desk exactly balances the air pressure pushing up on the desk.

Visualizing Air Pressure

Imagine a tall aquarium that has the same dimensions as the small desk mentioned in the preceding paragraph. When this aquarium is filled to a height of 10 meters (33 feet), the water pressure at the bottom equals 1 atmosphere (1 kilogram per square centimeter [14.7 pounds per square inch]). Now, imagine what will happen if this aquarium is placed on top of our student desk so that all the force is directed downward. Compare this to what results when the desk is placed inside the aquarium and allowed to sink to the bottom. In the latter example, the desk survives because the water pressure is exerted in all directions, not just downward, as in our earlier example. The desk, like your body, is “built” to withstand the pressure of 1 atmosphere. It is important to note that although we do not generally notice the pressure exerted by the air around us, except when ascending or descending in an airplane or a tall elevator, it is nonetheless substantial. The pressurized suits that astronauts use on space walks are designed to duplicate the atmospheric pressure experienced at Earth’s surface. Without these protective suits to keep body fluids from boiling away, astronauts would perish in minutes.

The concept of air pressure can also be understood if we examine the behavior of gas molecules. Gas molecules, unlike molecules in a liquid or solid, are not bound to one another but move freely throughout the space available to them. When two gas molecules collide, which happens frequently under normal conditions, they bounce off each other like elastic balls. If a gas is confined to a container, this motion is restricted by its sides, much as the walls of a handball court redirect the motion of the handball. The continuous bombardment of gas molecules against the sides of the container exerts an outward push that we call air pressure. Although the atmosphere is without walls, it is confined from below by Earth’s surface and effectively from above because the force of gravity prevents its escape. Here, we can define air pressure as the force exerted against a surface by the continuous collision of gas molecules.

Measuring Air Pressure

When meteorologists measure atmospheric pressure, it is expressed in units called millibars. Standard sea-level pressure is 1013.2 millibars (mb). Although the millibar has been the unit of measure on all U.S. weather maps since January 1940, the media often use “inches of mercury” (inches of Hg) to describe atmospheric pressure. In the United States, the National Weather Service converts millibar values to inches of mercury for public and aviation use (▼).

Inches of mercury is easy to understand. The use of mercury for measuring air pressure dates from 1643, when Torricelli, a student of the famous Italian scientist Galileo, invented the mercury barometer. Torricelli correctly described the atmosphere as a vast ocean of air that exerts pressure on us and all objects around us. To measure this force, he filled a glass tube, which was closed at one end, with mercury. He then inverted the tube into a dish of mercury (▼). Torricelli found that the mercury flowed out of the tube until the weight of the column was balanced by the pressure that the atmosphere exerted on the surface of the mercury in the dish. In other words, the weight of mercury in the column equaled the weight of the same diameter column of air that extended from the ground to the top of the atmosphere.

When air pressure increases, the mercury in the tube rises. Conversely, when air pressure decreases, so does the height of the mercury column. With some refinements, the mercury barometer invented by Torricelli is still the standard pressure-measuring instrument used today. Standard atmospheric pressure at sea level equals 29.92 inches of mercury.

The need for a smaller and more portable instrument for measuring air pressure led to the development of the aneroid barometer (aneroid means “without liquid”). Instead of having a mercury column held up by air pressure, an aneroid barometer uses a partially evacuated metal chamber (▼). The chamber is extremely sensitive to variations in air pressure and changes shape, compressing as the pressure increases and expanding as the pressure decreases. A series of levers transmit the movements of the chamber to a pointer on a dial that is calibrated to indicate inches of mercury and/or millibars.

As shown in ▲ the face of an aneroid barometer intended for home use is inscribed with weather descriptions, such as fair, change, rain, and stormy. Notice that “fair” corresponds with high-pressure readings, whereas “rain” is associated with low pressures. Barometric readings, however, may not always indicate the weather. The dial may point to “fair” on a rainy day or to “rain” on a fair day. To “predict” the local weather, the change in air pressure over the past few hours is more important than the current pressure reading. Falling pressure is often associated with increasing cloudiness and the possibility of precipitation, whereas rising air pressure generally indicates clearing conditions. It is useful to remember, however, that particular barometer readings or trends do not always correspond to specific types of weather.

Historically, aneroid barometers were connected to a recording mechanism, a barograph, to provide a continuous record of pressure changes with the passage of time. Today, atmospheric pressure is more often recorded using electronic instruments that contain small sensors to detect air pressure changes. In the United States, pressure and other weather observations are collected by radiosondes, automated surface observation systems (▼), aircraft, buoy data, and volunteer observers. Beyond their utility in forecasting weather, barometers are also used in aircraft to measure increases in air pressure with altitude, decreases in air pressure with elevation by mountain climbers, and increases in air pressure with depth by scuba divers.

• Air has weight: At sea level, it exerts a pressure of 1 kilogram per square centimeter (14.7 pounds per square inch), or 1 atmosphere.

• Air pressure is the force exerted by the weight of air above. With increasing altitude, there is less air above to exert a force, and thus air pressure decreases with altitude—rapidly at first and then much more slowly.

• The unit meteorologists use to measure atmospheric pressure is the millibar. Standard sea-level pressure is expressed as 1013.2 millibars. Isobars are lines on a weather map that connect places of equal air pressures.

• A mercury barometer measures air pressure using a column of mercury in a glass tube sealed at one end and inverted in a dish of mercury. As air pressure increases, the mercury in the tube rises, and when air pressure decreases, so does the height of the column of mercury. Atmospheric pressure is measured based on the height of the column of mercury in the barometer. Standard atmospheric pressure at sea level equals 29.92 inches of mercury.

• Aneroid (“without liquid”) barometers consist of partially evacuated metal chambers that compress as air pressure increases and expand as pressure decreases.

• air pressure: The force exerted by the weight of a column of air above a given point.

• aneroid barometer: An instrument for measuring air pressure without the use of fluids. Consists of a partially evacuated metal chamber sensitive to pressure changes with an elastic top and a scale with a pointer to indicate change.

• mercury barometer: A mercury-filled glass tube inverted into a dish of mercury in which the height of the mercury column is a measure of air pressure.