Water vapour

Figure 1. Although invisible, water vapour is one component of the wet steam released by a boiling kettle, the visible portion is microscopic droplets of water suspended in the air.

Water vapour is water in a gaseous state instead of a liquid state. It can be formed either through a process of evaporation or sublimation. Unlike clouds, fog, or mist, which consist of tiny, suspended particles of liquid water in the air, water vapour itself cannot be seen because it is in gaseous form.

Water vapour in the atmosphere is often below its boiling point. When water is boiled, it evaporates much more rapidly and produces steam. Steam often contains tiny droplets of liquid water, which are what is seen when water is boiling. Since both water droplets and water vapour are present, this is called wet steam (also called wet vapour). As the mixture gets hotter, the liquid water droplets evaporate, producing dry steam (also called dry vapour). Many power plants use water vapour in the form of steam as a working fluid to drive turbines that generate electricity. Dry steam performs better, although wet steam is also used.

Atmospheric Water Vapour

Water vapour is important for many reasons, but its presence in Earth's atmosphere is one of the most important roles. Water vapour is present within the atmosphere in varying amounts but is a vital component of the hydrologic cycle. Atmospheric water vapour ranges from trace amounts to approximately 4% of the atmosphere by volume, depending on location and atmospheric conditions. This concentration depends largely on where the water vapour levels are measured. On average, water vapour makes up about 2-3% in the atmosphere. 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 consist of visible droplets of liquid water. If conditions are right, water vapour can condense onto tiny particles of dust, salt, or smoke, forming small water droplets. As these droplets grow larger, they can eventually form precipitations such as rain, snow, sleet, or hail. Since water vapour plays a central role in forming precipitation, it is a major component of the hydrologic cycle. When the sun heats water holding areas, some of the water evaporates into the atmosphere as water vapour, helping to drive the cycle.

In addition to evaporation, plants release water vapour into the atmosphere through a process known as transpiration.

Product of Combustion

When hydrocarbons undergo combustions, one of the products of the chemical reaction is water. However, because combustion occurs at high temperatures, the water is produced in its gaseous form, known as water vapour. An example of a hydrocarbon combustion reaction is shown below to illustrate how water is produced during combustion.

Figure 2. Methane combining with 2 oxygen to form carbon dioxide, two water molecules (in the form of water vapour) and heat.

Since water vapour is released when hydrocarbons are burned, it is a significant component of the flue gases released by coal-fired power plants or natural gas power plants. A substantial amount of water vapour is produced when coal is burned. For example, when bituminous coal is burned, approximately 0.4 kilograms of water are produced for every kilogram of coal burned. This means that large coal-fired power plants can produce many tonnes of water vapour every hours.

Climate Impacts

Water vapour moves continuously throughout the Earth's surface as part of the water cycle, cycling between the atmosphere, oceans, land, and living organisms. This continuous cycling influences regional climates, produces precipitation, and helps sustain life on Earth. 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. Its chemical structure allows it to absorb heat radiated from Earth's surface, helping to maintain the planet's natural greenhouse effect. Water vapour is responsible for approximately one-half to two-thirds of Earth's natural greenhouse effect, depending on the method of estimation. Climate models and observations show that increasing atmospheric water vapour amplifies the global warming caused by rising concentrations of carbon dioxide and other greenhouse gases. This water vapour feedback is estimated to approximately double the global warming that would occur from carbon dioxide alone.

As a greenhouse gas, water vapour creates a positive feedback cycle for global warming. As Earth's temperature increases, evaporation from oceans, lakes, and streams also increases, adding more water vapour to the atmosphere. The additional water vapour traps more heat, which leads to further warming. Human activities do not directly increase atmospheric water vapour in the same way they increase carbon dioxide. Instead emissions of greenhouse gases warm the atmosphere, which increases evaporation and allows the atmosphere to hold more water vapour. Thus anthropogenic warming is enhanced by the greater water vapour content in the atmosphere.

For Further Reading

• Hydrologic cycle
• Greenhouse effect
• Flue gas
• Working fluid
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