Atmospheric pressure

Atmospheric pressure is the pressure at any given point in the Earth's atmosphere. In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. Low pressure areas have less atmospheric mass above their location, whereas high pressure areas have more atmospheric mass above their location. Similarly, as elevation increases there is less overlying atmospheric mass, so that pressure decreases with increasing elevation. A column of air 1 square inch in cross section, measured from sea level to the top of the atmosphere, would weigh approximately 14.7 lbf. A 1 m² (11 sq ft) column of air would weigh about 100 kilonewtons (equivalent to a mass of 10.2 tonnes at the surface).

The standard atmosphere (symbol: atm) is a unit of pressure and is defined as being equal to 101.325 kPa. The following non-standard units are equivalent: 760 mmHg (torr), 29.92 inHg, 14.696 PSI, 1013.25 millibars. One standard atmosphere is standard pressure used for pneumatic fluid power (ISO R554), and in the aerospace (ISO 2533) and petroleum (ISO 5024) industries.

Average sea-level pressure is 101.325 kPa (1013.25 mbar) or 29.921 inches of mercury (inHg) or 760 millimeters (mmHg).

The highest sea-level pressure on Earth occurs in Siberia, where the Siberian High often attains a sea-level pressure above 1087.0 mbar. The lowest measurable sea-level pressure is found at the centers of tropical cyclones.

Lows and highs; Barometric pressure variations from day to day

At first thought, you might expect the air of the atmosphere to distribute itself uniformly over the surface of the earth so that the barometric pressure at the same elevation would be the same everywhere. After all, the water in a pond or lake "seeks its own level," and, except for the tides, the water in the oceans does as well.

But this is not the case with the atmosphere. Several factors, among them the rotation of the earth and the uneven heating of the earth's surface by the sun, come together to cause the air to seemingly slosh about so that at some places it is piled up deeper than at other places. The entire surface of the earth is covered with these mountains and valleys of air, in an ever-changing pattern.

Where the air is heaped up into a pile, the barometric pressure on the ground will be greater than the average value. This forms what meteorologists call a High, meaning a high pressure area. And where the air is less deep, referring to a valley, the barometric pressure will be lower than the average value and will represent a Low, or low pressure region. You have heard the weather people talk about a "high pressure ridge" or a "low pressure trough."

The lowest pressures are probably associated with hurricanes. Hurrican Mitch in 1998 produced a low of 27.08 in/Hg, and Floyd in 1999 gave us a low of 27.32. The lowest sea level pressure ever recorded in the western hemisphere was associated with Hurricane Gilbert in 1988. An NOAA aircraft recorded a low pressure of 26.22 in/Hg on September 13, 1988. These are the extremes, however, Normally, lows are not nearly that pronounced, seldom reaching a pressure as low as 29.00.

Pressure Equivalents:

1 Atmosphere (as a unit of pressure) = 14.7 psi = 34 ft water column = 76 cm/Hg

     = 29.92 in/Hg = 1.013 bar = 1,013 mbar

1 bar = 1,000 mbar = 14.5 psi = 29.53 in/Hg = 0.9869 atm

1 psi = 27.68 inches water column = 2.036 in/Hg = 6.859 kPa

1 kPa = 1,000 Pascals = 0.1458 psi

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Dr. Amar Nath Giri
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