The theory that describes how vapor phase of a substance condenses into liquid and solid phases based on classical bulk thermodynamic principles. This leads to the conclusion that water vapor will start condensing into liquid water or ice when the environment marginally surpasses saturation (100% relative humidity with respect to plain water or ice surface, respectively). In reality, it is almost always necessary to have foreign particles serving as condensation nuclei or ice nuclei for the condensation to occur even when the environment is somewhat supersaturated.