vapor

Vapor is the gaseous phase of a substance that is typically liquid or solid at room temperature. A substance becomes vapor through evaporation or sublimation, and the resulting gaseous form may exist at temperatures below its critical point. Water vapor is the most abundant greenhouse gas in the lower atmosphere and plays a key role in humidity, cloud formation, and the Earth's energy balance; its concentration is temperature dependent and subject to weather dynamics.

The amount of vapor in the air is described by vapor pressure, which is the equilibrium pressure of the vapor above its liquid or solid, and by relative humidity, the ratio of actual vapor pressure to its saturation pressure at a given temperature. When air reaches saturation, condensation can occur, producing clouds, fog, or dew.

In materials science and manufacturing, vapors are generated for deposition techniques such as chemical vapor deposition (CVD) and physical vapor deposition (PVD), used to create thin films and coatings. Vaporization methods include boiling, evaporation, and sublimation, depending on the substance and conditions. The concept is also used in environmental and atmospheric studies to model moisture transport, energy exchange, and climate processes.