Watervapor

Watervapor, commonly referred to as water vapor, is the gaseous state of water. It consists of individual H2O molecules in the gas phase and is invisible under normal conditions, although it becomes visible as fog or clouds when it condenses into tiny droplets. Water vapor forms through evaporation from bodies of liquid water and through sublimation from ice and snow, and it can persist at a wide range of temperatures and pressures in the atmosphere.

In Earth's atmosphere, water vapor is a key component of the climate system. Its concentration varies with temperature and weather patterns, rising as air warms and decreasing as it cools and condenses. Water vapor is also a potent greenhouse gas because it absorbs infrared radiation, contributing to the natural greenhouse effect. Unlike some other greenhouse gases, its atmospheric amount is not directly controlled by a single emissions source; instead, it closely follows temperature and the hydrological cycle, creating important feedbacks in climate change.

Measurement and related concepts: relative humidity is the ratio of the current amount of water vapor to the maximum possible at a given temperature, while specific humidity and mixing ratio express water vapor as a mass-per-mass or mass-per-air value, respectively. The dew point is the temperature at which air becomes saturated and condensation begins.

Human observations and applications: water vapor drives weather formations such as clouds and precipitation, and is tracked in meteorology using instruments and satellites. In industry, steam (condensed water vapor) is used for heating and power generation, while humidity control is important in buildings and manufacturing processes.