Water vapour

Explanation

Water vapour is water in its gaseous phase. It is abundant on Earth, playing a vital part in the water cycle, where the liquid water from the surface evaporates into vapour, reaches the atmosphere and condenses back to liquid water in the clouds, and precipitates on the surface as rain, hail or snow. It is also found in trace amounts in other planets as well, including their natural satellites. Water vapour is also observed on several asteroids and comets. Water vapour is an effective greenhouse gas that absorbs and traps the infrared radiation in the atmosphere.

Frequently Asked Questions

What is the difference between water vapour and steam?

Steam and water vapour are technically interchangeable terms, and they don't really have any differences.

✘ Misconception

Water vapour is visible, and steam is not visible. Clouds are made up of water vapour.

✔ Fact

Both steam and water vapour are two different terms that are used to represent the gaseous phases of water. The gaseous phase is invisible. When the water molecules in that invisible gaseous phase condense back to liquid droplets, it forms clouds. So clouds are made up of liquid droplets that are condensed from water vapour.

How water vapour forms snow?

When water vapour cool and expand in the atmosphere, the suspended dust and other particles in the atmosphere serve as nucleation sites for the vapour to condense back into liquid droplets. The temperature at which the condensation of vapour occurs is known as the dew point. When the temperature is below the freezing point of water the water vapour that evaporates in the atmosphere will be cooled further below the freezing point. If the dew point is well below 0 °C, instead of condensing back to liquid water, the supercooled vapour desublimate into ice crystals, which then precipitates as snow.

How water vapour works as a greenhouse gas?

A greenhouse gas is a gas that has the properties to absorb and emit the energy from the electromagnetic radiation from the Sun and the re-radiated infrared energy from the surface of the planet. On Earth, the greenhouse effect happens due to the presence of the greenhouse gases like water vapour, carbon dioxide, nitrous oxide, methane, and chlorofluorocarbons. These gas molecules are loosely bound to each other, and they vibrate on absorbing the infrared radiation. They then emit the same radiation to reach their original bound state. As water molecules have two hydrogen atoms bonded with an oxygen atom with a hydrogen bond, the molecule is polar with two lone pair electrons. These lone pair electrons repel each other and form the popular 'bent' structure of water molecule, loosening the bonds. Thus the water molecule can vibrate and then emit infrared radiation. This makes water vapour a greenhouse gas.