Marine Glossary

|| || || || ||

vapor pressure

1. In the most general terms, the pressure exerted when a solid or liquid is in equilibrium with its own vapor. Vapor pressure is a function of the substance and of the temperature. This definition would imply the saturation vapor pressure, meaning this pressure would not increase with time.

When any substance is placed in an evacuated container, it immediately begins to give off gases, either by evaporation (liquids) or by "outgassing" as is said of solids. This causes the pressure in the container to rise until it reaches the saturation vapor pressure of that substance at that temperature. Increase the temperature and more vapor will be given off. When you get into a brand new car you can usually smell the vapors outgassed from the various materials used. Water has a saturation vapor pressure of about 24 mb at room temperature. When this vapor is given off into air, it will contribute about 1.5% to the total mass of the air. See G179.

2. More specifically: the pressure of water vapor in the air; that part of the total atmospheric pressure which is due to water vapor.

See humidity and related discussion in ART-13 on humidity. G179 is a table of the vapor pressure of water at different temperatures.