1. n. [Drilling Fluids]
The pressure exerted by a vapor escaping from a liquid. It quantifies the tendency of molecules to enter the gaseous phase. The vapor pressure of water increases as temperature increases and reaches one atmosphere pressure (760 mm Hg or 14.7 psia) at the boiling point (100°C or 212°F). The activity of an aqueous solution is the ratio of vapor pressures: aw = p/po, where p = vapor pressure of a solution and po is vapor pressure of pure water. Since this is a ratio of vapor pressures, activity is not a strong function of temperature.