water vapor

American  

noun

1. a dispersion, in air, of molecules of water, especially as produced by evaporation at ambient temperatures rather than by boiling.

---

water vapor Scientific  

1. Water in its gaseous state, especially in the atmosphere and at a temperature below the boiling point. Water vapor in the atmosphere serves as the raw material for cloud and rain formation. It also helps regulate the Earth's temperature by reflecting and scattering radiation from the Sun and by absorbing the Earth's infrared radiation.

2. See also vapor

---

Etymology

Origin of water vapor

First recorded in 1875–80

Example Sentences

The UV radiation generated by this process can break apart water vapor molecules, leading to the formation of hydroxyl.

From Science Daily • Apr. 21, 2026

Catalytic converters turn toxic gases into water vapor and carbon dioxide, making car exhaust fumes less environmentally damaging.

From Los Angeles Times • Apr. 3, 2026

That storm turned water vapor into ice at natural gas wellheads, blocking fuel production.

From Barron's • Jan. 21, 2026

A familiar physical example appears at the critical point of water, defined by a precise temperature and pressure at which liquid water and water vapor become indistinguishable.

From Science Daily • Dec. 16, 2025

There are small traces of nitrogen, water vapor, argon, carbon monoxide and other gases, but the only hydrocarbons or carbohydrates present are there in less than 0.1 parts per million.

From "Cosmos" by Carl Sagan