Triple state

From Glossary of Meteorology

triple state

(Or triple point.) The thermodynamic state at which three phases of a substance exist in equilibrium.

For water substance, the triple state occurs at es = 0.611 kPa, and T = 273.16 K, where es is the saturation vapor pressure of the water vapor and T the Kelvin temperature. At this state the specific volumes of the water vapor, water, and ice are, respectively,

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