Atmospheric Pressure

The most basic atmospheric element is air pressure. Pressure itself is a measure of the force per unit area. The concept of force is usually best understood by considering Newton's second law, which states that the acceleration of a body is directly proportional to the force exerted on that body and inversely proportional to its mass. In other words, force (F) is equal to the product of mass (M) and acceleration (a).

Using International System (SI) units of measure, M is given in kilograms (kg) and acceleration in units of meters per second per second (m/s2, also expressed as m s-2). Given F = Ma, then force is described in units of kg m s-2, which is a newton. A newton (N) is equal to the amount of force required to accelerate a mass of 1 kg at the rate of 1 m/s2, or 1 m s-2. So if force is given in newtons, then pressure must equal to N per unit area, or N/A or N A-1. This unit of measure for pressure is known as a pascal (Pa). Given that a unit area would be provided in square meters (m2), a pascal would be equal to kg m s-2 m-2, or kg m-1 s-2.

In general, the mass of the atmosphere varies slightly depending on location. When dealing with air, however, we need to consider Pascal's law, which states that fluids (gases and liquids) transmit pressure in all three dimensions. For this reason, mass of the atmosphere varies depending on the volume of space considered, so to compare pressure across space, one must standardize the unit of measure to a constant volume, a measure known as “density.” Typically, the mass is given in kg and the volume in cubic meters (m3); therefore, we consider atmospheric mass, or density, in units of kg/m3 (or kg m-3).

Gravitational acceleration, the other term in force, also varies depending on distance away from the center point of Earth. If Earth were a perfect sphere, then gravitational acceleration would be constant at the same height. However, even though Earth is best described as a geoid, the variation in gravitational acceleration due to a nonspherical body within all heights of the troposphere are negligible. For this reason, and to simplify the situation, we assume gravitational acceleration within the troposphere to be a constant 9.8 m/s2. To ensure that this assumption is recognized, all heights above Earth's surface are referred to as “geopotential”; that is, with a vertical datum of mean sea level on a spheroid-shaped Earth.

Atmospheric pressure, then, is the summation of all mass within a 1-m2 column of air that extends to outer space, where pressure is 0 Pa. On average, if we consider all sea level point locations over Earth 24 hrs. (hours) per day for 30 yrs. (years) or more, the pressure would be 101,325 Pa. Since geographers and atmospheric scientists frequently work with pressure, the SI units are simply too large to deal with on an operational basis. As a result, a new unit of measure called the millibar (mb) was developed in 1909 by Sir Napier Shaw. An mb equals 100 Pa, so average atmospheric pressure at sea level is 1,013.25 mb, or simply 1,013 mb.

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