Water vapour is simply water that is present in its gaseous form instead of liquid form. It can be formed either through a process of evaporation or sublimation. Unlike clouds, fog, or mist which are simply suspended particles of liquid water in the air, water vapour itself cannot be seen because it is in gaseous form.
It is important to distinguish water vapour from steam, as the two tend to be confused with each other as the difference between the two is not always clear. Both steam and water vapour are water in gaseous forms, but the temperature difference between the two is what sets them apart. Water vapour is gaseous water that is below its boiling point, meaning that vapour can be condensed by increasing pressure without actually changing the temperature. Essentially, water vapour is cooler than steam, which is water vapour in gaseous form above its boiling point.
Water vapour is important for a number of different reasons, but its presence in the atmosphere is one of the most important. Water vapour is present within the atmosphere in varying amounts but is a vital component of the hydrologic cycle. In the atmosphere, water vapour can exist in trace amounts or even make up as much as 4% of the atmosphere. This concentration depends largely on where the water vapour levels are measured. On average, the value of water vapour in the atmosphere is 2-3%. In arid or very cold locations - such as polar regions - the amount of water vapour in the air is much lower.
Even on a clear day, water vapour exists in the atmosphere as an invisible gas - unlike clouds which are droplets of liquid water that can be seen. If the conditions are right, water vapour in the air can collect on small particles of dust, salt, or smoke in the air to form small droplets. These droplets gradually increase in size and over time become various forms of precipitation. Since water vapour is so prominent in the atmosphere and forms precipitation, water vapour is a major component of the hydrologic cycle. When water holding areas are heated by the Sun, some of the water being held evaporates and becomes vapour, powering the cycle.
In addition to being created by evaporating water, plants are capable of producing water vapour through a process of transpiration.
When hydrocarbons undergo combustion, water is created by the chemical reactions. However, the high temperatures mean that the water is in its gaseous form: water vapour. An example of a hydrocarbon combustion reaction is shown below to illustrate how water is created during this process.
Since water vapour is released when hydrocarbons are burned, water vapour is a fairly large component of the flue gases released from coal fired power plants or natural gas power plants. A surprisingly large amount of water is released when coal is burned. For example, when burning bituminous coal roughly 0.4 kilograms of water are produced for every kilogram of coal burned. This means that tonnes of water are be produced per hour at a large-scale coal fired power plant.
Water vapour moves across the Earth's surface, cycling through a variety of storage areas. It is this cycling that shapes the climate of different regions across the globe, providing precipitation and supporting life. As the climate changes, the distribution and cycling of water vapour changes as well.
Water vapour is actually the most abundant greenhouse gas in the atmosphere, and the most potent of all the greenhouse gases as a result of its chemical structure. Its presence accounts for around two thirds of the natural greenhouse effect. In the past there has been debate over how severe an impact water vapour has on global warming, but recently studies have confirmed that the amplification that water vapour has on heat is strong enough to double climate warming caused by increased CO2 levels.
As a greenhouse gas, water vapour serves as a positive feedback mechanism for global warming. This means that the warmer the world gets, the more water vapour will exist in the air as evaporation rates from oceans, lakes, and streams increase. Since there is then more water vapour in the air, the water vapour itself contributes to more warming. Human activities do not increase the overall water vapour content in the atmosphere, but human activities can cause more water to evaporate as a result of increased temperature of the atmosphere. Thus anthropogenic warming is enhanced by a greater water vapour content in the atmosphere.