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Water vapor (H₂O) is the gaseous phase of water. In the atmosphere it is a variable trace constituent that makes up about 0.25 percent of the atmosphere by mass on average, with a global mean of about 25 kg·m⁻² of precipitable water. Its volume fraction ranges from a few parts per million in the dry lower stratosphere to about 3 to 4 percent in the warm, humid tropical boundary layer. It is the most important natural greenhouse gas, and it accounts for roughly half of the clear-sky greenhouse effect (with clouds, about three quarters). It is also the working fluid of the water cycle, and the latent heat that is released when it condenses is a major source of energy for weather systems. Water vapor is not the most abundant atmospheric gas, since nitrogen and oxygen make up about 99 percent of dry air, and the statement that it is the "most abundant greenhouse gas" is correct only in the sense that it exceeds carbon dioxide in concentration. The article on atmospheric water vapor covers its behavior in the atmosphere in more detail.

Unlike nitrogen and oxygen, water vapor is highly variable in space and time. It is concentrated in the lower troposphere, with a scale height of about 2 km, and it is most abundant over the warm tropical oceans. Its concentration is controlled mainly by temperature, because it condenses and precipitates, and not by emissions.

The water molecule is bent, with an H–O–H angle of about 104.5°, and it has a permanent dipole moment. It absorbs infrared radiation through its vibration-rotation bands, and not because it is "asymmetrical" in a general sense. Some key properties are:

• The molar mass is 18.02 g·mol⁻¹, which is lower than that of dry air (28.97 g·mol⁻¹), so that moist air is slightly less dense than dry air at the same temperature and pressure.
• The latent heat of vaporization is about 2.5 × 10⁶ J·kg⁻¹ at 0 °C, and the specific gas constant is R_v = 461.5 J·kg⁻¹·K⁻¹.
• The saturation vapor pressure at 0 °C is about 6.11 hPa, and it increases steeply with temperature.

Humidity is described by the vapor pressure e, the relative humidity RH = e / e_s (where e_s is the saturation vapor pressure), the specific humidity q ≈ 0.622 e / p, the dew point, and the precipitable water, which is the depth of liquid water that would result from condensing the vapor in a column (about 25 mm globally on average).

The saturation vapor pressure of a condensable gas rises steeply with temperature according to the Clausius-Clapeyron relation:

The fractional rate of increase is therefore L_v / (R_v T²). At T = 288 K, this is 2.5 × 10⁶ / (461.5 × 288²) ≈ 0.065 K⁻¹, or about 6.5 percent per kelvin. The value is about 7 percent per kelvin at 280 K and about 6 percent at 300 K, so "approximately 7 percent per degree Celsius" is a good approximation for typical near-surface conditions. For example, air at 30 °C can hold about 1.8 times as much vapor as air at 20 °C (23.4 hPa at 20 °C and 42.5 hPa at 30 °C). The change per degree Fahrenheit is a factor of 5/9 smaller.

The relation gives the maximum vapor pressure in equilibrium with liquid water, and it is not an "absolute limit" on the amount of vapor in an air parcel in all conditions, since air can be briefly supersaturated, especially with respect to ice. The atmosphere is also rarely saturated. The mean relative humidity of the troposphere is about 50 percent. In models and observations, relative humidity changes little as the climate warms, so that the specific humidity increases at roughly the Clausius-Clapeyron rate. The observed total column water vapor over the oceans has risen by about 1 to 2 percent per decade in recent decades, consistent with this rate.

The global mean precipitation does not increase at the same rate. It is constrained by the energy balance of the atmosphere, in which the latent heat released by precipitation must be balanced by radiative cooling, and it increases by about 1 to 3 percent per kelvin in models. The intensity of heavy precipitation, which depends on the moisture content of the air, increases at a rate that is closer to the Clausius-Clapeyron rate in many regions, so that the hydrological cycle changes with warming, and the changes include more intense rainfall events, and, in some regions, longer dry spells. The statement that the cycle becomes "increasingly volatile" is a simplification, and the changes differ among regions and seasons. The statement that the relation is "exponential" is approximately correct over a limited range of temperatures.

Absorption of radiation

Water vapor absorbs infrared radiation in the vibration-rotation band near 6.3 μm, in the pure rotation band at wavelengths longer than about 15 μm, and weakly in the continuum of the 8 to 12 μm window. It also absorbs near-infrared sunlight, near 0.94, 1.1, 1.4, 1.9, and 2.7 μm, which accounts for most of the solar absorption in the troposphere. It emits infrared radiation in all directions according to its temperature, and the downward emission contributes to the radiation that reaches the surface (about 340 W·m⁻² in total from the atmosphere and clouds). The claim that it "traps more heat than all other greenhouse gases combined" is correct in the sense of its contribution to the total natural greenhouse effect, but this is the effect of the existing water vapor, and not a measure of its role in the response to changes.

Feedback and not forcing

In climate dynamics, a radiative forcing is an imposed change in the energy balance, and a feedback is a response to a change in temperature. Long-lived, non-condensing gases, such as carbon dioxide, methane, and nitrous oxide, accumulate in the atmosphere, because they are not removed by precipitation. Water vapor is condensable and has an atmospheric residence time of about 9 days:

where B is the global burden of vapor and P is the global precipitation rate. Water vapor therefore adjusts rapidly to the temperature, and it acts as a feedback, which is the reason that it does not drive long-term warming. Direct human emissions of water vapor, for example from irrigation, have only small and local effects on climate. Stratospheric injections, such as that of the 2022 Hunga Tonga-Hunga Ha'apai eruption, are an exception, because the residence time in the stratosphere is years.

The water vapor feedback is positive. When the surface warms, the atmosphere holds more vapor, which increases the absorption of infrared radiation, so that there is more warming. Its strength is about +1.8 W·m⁻²·K⁻¹ (with an uncertainty of about ±0.2), and the lapse rate feedback (about −0.5 W·m⁻²·K⁻¹) partly offsets it, so that the combined water vapor and lapse rate feedback is about +1.1 W·m⁻²·K⁻¹. The warming for a forcing ΔF is:

where λ_P is the Planck response (about −3.2 W·m⁻²·K⁻¹) and λ_f is the sum of the other feedbacks. For ΔF = 3.7 W·m⁻², which is a doubling of CO₂, the warming without feedbacks is 3.7 / 3.2 ≈ 1.2 K. With the water vapor feedback alone, the net parameter is −3.2 + 1.8 = −1.4 W·m⁻²·K⁻¹, and the warming is about 2.6 K, more than twice the value without feedbacks. Together with the other feedbacks, the equilibrium climate sensitivity is assessed as about 3 K, with a likely range of 2.5 to 4 K. The description that the feedback "roughly doubles" the warming is therefore reasonable for the water vapor feedback taken alone, and the total amplification from all feedbacks is larger. The strength of the feedback depends on the changes of water vapor in the upper troposphere, where the effect on the radiation is largest, and observations from satellites are consistent with the models.

Evaporation and condensation

The evaporation of water requires energy. The energy is needed to overcome the intermolecular attraction (hydrogen bonds) between the molecules in the liquid, and not to break the bonds inside the molecules, which remain intact. It is taken from the surface, which cools it, and it is released in the atmosphere when the vapor condenses. The global mean latent heat flux is about 80 to 90 W·m⁻², which is the largest non-radiative term in the surface energy balance. An evaporation rate of 3 mm per day (3 kg·m⁻² per day) corresponds to a flux of 3 × 2.5 × 10⁶ / 86,400 ≈ 87 W·m⁻². The energy that is stored is mostly that of the sun, but it also includes heat from the ocean. Evaporation is a cooling of the surface, and not an "absorption of solar energy" that is removed from the budget.

The vapor is transported by the winds from regions of net evaporation, mainly the subtropical oceans, to regions of net precipitation, the tropics and the mid-latitudes, and the latent heat is transported with it. At mid-latitudes, much of the moisture transport occurs in narrow filaments, the atmospheric rivers. The Hadley cell carries moisture to the tropical convergence zone, where it rises, and the descending branch in the subtropics is dry. The Walker circulation is a zonal circulation over the tropical Pacific, and it redistributes moisture and heat from east to west, and it does not transport vapor poleward. The poleward transport of energy in the atmosphere is carried mainly by the Hadley cell in the tropics and by eddies in the mid-latitudes. The moist static energy, which includes the contribution of latent heat, is:

Convection and storms

When moist air rises, it cools, and when it reaches saturation, the condensation releases latent heat. This makes the air more buoyant than a dry parcel, so that the moist adiabatic lapse rate (about 4 to 7 K per km in the lower tropical troposphere) is less than the dry rate (9.8 K per km). It is the source of the energy of deep convection and thunderstorms. In tropical cyclones, the evaporation from warm ocean water, over about 26.5 °C, supplies the energy, and the release of latent heat in the eyewall drives the circulation. Mid-latitude cyclones are driven mainly by horizontal temperature contrasts (baroclinic instability), and the release of latent heat contributes to their intensity. The release of latent heat warms the air, and the buoyancy of the warmed air produces motion, so that it is not a "violent" release of energy by itself, and the energy of the winds comes from the conversion of potential energy.

The concentration of water vapor falls rapidly with height, because the temperature falls. The tropical tropopause, at about 16 to 17 km, has a temperature of about −75 to −80 °C, so that the saturation mixing ratio is very low. Air entering the stratosphere is dried by freezing out of the vapor (the "cold trap"), and the entry mixing ratio is about 3 to 4 parts per million by volume. In the stratosphere, the oxidation of methane produces additional water, about two molecules per methane molecule, so that the mixing ratio is about 5 to 6 parts per million at higher levels. Water vapor in the stratosphere is also a source of the OH radicals that take part in the chemistry of ozone, and it is a component of polar stratospheric clouds.

Water vapor is measured by radiosondes, by ground-based GPS networks (which measure the delay of the signal by the total column), by microwave and infrared sounders on satellites (such as AIRS, IASI, and SSM/I), and by limb sounders. Reanalyses combine the data with models. Trends are estimated with care, because of changes in instruments, and the trends in the upper troposphere and over land are more uncertain than the trend of the total column over the oceans. Observed relative humidity over land has declined since about 2000, in part because land warms faster than the ocean, and this contributes to an increase in the vapor pressure deficit.

• Atmospheric water vapor
• Greenhouse gas
• Clausius-Clapeyron relation
• Latent heat
• Humidity
• Climate sensitivity
• Water cycle