Water vapor, a major greenhouse gas, is represented by the chemical formula H2O.
Each water molecule contains two hydrogen atoms bonded to one oxygen atom. Water can exist as ice, liquid water, or vapor, but its chemical formula remains H2O in all three states. “Water vapor” specifically means the invisible gas phase—not mist, fog, or clouds, which contain tiny liquid droplets or ice crystals.
Water vapor is Earth’s most abundant greenhouse gas and strongly absorbs infrared radiation emitted by the surface. Unlike carbon dioxide, however, it usually acts mainly as a feedback: warmer air can hold more water vapor, which amplifies warming caused by other factors. The atmosphere’s water-vapor amount therefore changes rapidly through evaporation, condensation, clouds, and rainfall.
A common mix-up is confusing H2O with greenhouse gases such as carbon dioxide (CO2), methane (CH4), or nitrous oxide (N2O). Those compounds are chemically distinct, even though all can absorb infrared energy.