The escaping tendency, or vapor pressure, of water molecules in an aqueous solution compared with that of pure water, typically abbreviated a_w. Activity is expressed mathematically as the ratio of two vapor pressures: a_w = p/p_o, where p is vapor pressure of the solution and p_o is vapor pressure of pure water. The ratio ranges from near 0 to 1. 0 and corresponds to percent relative humidity (% RH) of air in equilibrium with the aqueous solution. For pure water, a_w = p_o/p_o = 1. 00 and RH = 100%. By increasing the concentration of salt (or other solutes) in the solution, a_w decreases, because vapor pressure of the solution decreases. However, a_w never reaches zero. Known-activity, saturated-salt solutions are used to calibrate RH meters. Measuring RH of air above an oil mud is a simple way to measure the activity (salinity) of its water phase. Adjusting the salinity of the water phase is a way to control movement of water into or out of shales that are being drilled with an oil mud. Chenevert related a_w in oil mud to RH above the mud sample and devised a practical test using an electrohygrometer to measure RH, called the "Chenevert Method. "