Water vapour is defined as the amount of water in gas phase (in grams per cubic metre) of air. Often the term "humidity" is used in meteorology.
The specific humidity is the amount of water in gas phase (measured in grams in a total air volume with a mass of 1 kg). The commonly used parameter "relative humidity" is defined as the ratio of the actual water vapour pressure in the air to that of the saturation (or equilibrium) water vapour pressure. Above the water vapour saturation pressure, which is a function of temperature, at 100% relative humidity, any additional water vapour will condensate, resulting in the formation of clouds and precipitation.
Water vapour is one of the most significant constituents of the atmosphere since it is the means by which moisture and energy (as latent heat) are transported through especially the troposphere and lower stratosphere. Aside from the role of water vapour in balancing the atmospheric heat budget, water vapour is obviously the source of precipitation. In any vertical column of air, the amount of water vapour provides operational meteorologists with a value of the maximum potential precipitation which could be retrieved from that column of air in optimal conditions. Water vapour also plays a key role in the formation of clouds and fog.
Water vapour is highly variable, both temporally and spatially, and is therefore one of the most difficult quantities to predict with numerical weather prediction (NWP) models. Typically, NWP model fields are initialised using existing model data coupled with observational data. The availability of water vapour measurements, with high spatial and temporal resolution, is therefore crucial for providing reliable weather forecasts. Given that approximately half of the energy in the atmosphere is transported by water vapour, other parameters such as cloud cover and surface temperature are also better forecast with superior water vapour information.
Although the actual amount of water vapour in the atmosphere is relatively low (~1%), water vapour is one of the most crucial greenhouse gases, and numerous scientific studies have determined that around 60-70% of atmospheric warming can be attributed to the atmospheric water vapour greenhouse effect. Moreover, warmer air can contain also more water vapour than cold air (this is the Clausius-Clapeyron equation, stating that the water holding capacity increases at about 7% per degree Celsius increment in temperature). In addition, evaporation will increase for higher temperatures where water is available (from oceans, lakes, plants, soils, etc.). Combining these effects leads to a positive water vapour feedback mechanism, by which an initial surface warming due to increasing levels of greenhouse gases like CO2 and methane, is amplified by 2 to 3 times! It should be clear that water vapour plays a vital role in the global climate system and climate model predictions. And we have not yet mentioned the role of water vapour in the climate models for the amount of cloud formation, precipitation, reflection of sunlight from cloud tops, etc. Despite the large role of water vapour, the water vapour feedback mechanism and this cloud feedback mechanism remain the largest sources of uncertainty in climate sensitivity estimates.