Moisture existing as a gas in air. Warm air can hold more water vapour than cold air. Water vapour in the air creates a pressure much like any other gas. Cold air has a relatively low vapour pressure, but warm air with larger amounts of water vapour has a greater pressure. The difference in pressures cause the vapour to do strange things such as penetrating building materials in the direction from high to low vapour pressure. French (Vapeur d'eau)
The most abundant greenhouse gas; it is the water present in the atmosphere in gaseous form. Water vapour is an important part of the natural greenhouse effect. While humans are not significantly increasing its concentration, it contributes to the enhanced greenhouse effect because the warming influence of greenhouse gases leads to a positive water vapour feedback. In addition to its role as a natural greenhouse gas, water vapour plays an important role in regulating the temperature of the planet because clouds form when excess water vapour in the atmosphere condenses to form ice and water droplets and precipitation. () Up to 2% of the atmosphere is water vapour and, as a greenhouse gas, it causes about 60% of the total greenhouse effect. But because water vapour is rapidly eliminated from the atmosphere in the form of clouds, it is not considered to be part of the human-induced part of the greenhouse effect. Instead, as global warming brings about a long-term increase in the amount of water vapour in the atmosphere, the additional greenhouse effect caused by the extra water vapour is classed as a climate feedback. ()