Marine Glossary

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standard atmosphere

Also: International Starndard Atmosphere (ISA)

1. As a unit of pressure: a unit accepted for use with the International System of Units; 1 standard atmosphere is equal to 101,325 pascals (= 1013.25 hectopascals = 1013.25 mb). Although this is used as a standard to represent a typical surface pressure, it is not the actual average pressure at any particular location. See atmospheric pressure for record values and averages of real pressures and ART-5 for further discussion. This unit of pressure is the one used to related inches of mercury to mb, in that the ISA is also defined as exactly 760mm of Hg, which is 29.921 inches of mercury.

2. As a description of the atmosphere itself: a hypothetical vertical distribution of atmospheric temperature, pressure, and density which is taken to be representative of the atmosphere for various purposes. It starts at the surface at 1013.25 mb at 59° F and density 0.0744 pounds/cubic ft. The lapse rate up to 36,000 feet is 3.55° F/1,000 feet. See Appendix of Modern Marine Weather and the Barometer Handbook for more details.