Marine Glossary

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constant-pressure surface

An imaginary surface along which the atmospheric pressure is everywhere equal at a given instant. Also called isobaric surface. If the air is completely uniform, then there is a flat isobaric surface on the ground of say 1013 mb, meaning measured from anywhere on this surface the pressure would be 1013 mb. If we were to go up in elevation to say 275 feet, then the pressure would drop to about 1000 mb. And in this uniform air example, there is another isobaric surface parallel to the ground at an elevation of 200 feet. This would be called the 1000 mb surface. (See G228 to compute pressure drop versus elevation.)

This concept is not much used directly in marine weather but is crucial to understanding the maps available over radiofacsimile. The most important for mariners is the 500-mb map, which is just a depiction of the elevation contours of the 500 mb isobaric surface. When the air is cold, the surfaces are closer together, which would pull down the 500 mb surface level over colder air. See G401.

Constant-pressure chart: The synoptic chart for any constant-pressure surface, usually containing plotted data and analyses of the distribution of, e.g., height of the surface, wind, temperature, and humidity. Constant-pressure charts are most commonly known by their pressure value; for example the 500-millibar chart. Also called isobaric chart.