Water vapour is a naturally occurring gas formed by the evaporation of water. It can also be referred to as steam, water vapor, or moisture vapor. In scientific terms, it is called H2O, which is the chemical formula of water. Depending on its temperature and pressure level, water vapour can be visible or invisible. In terms of weather, it is a key component of the water cycle, and is also a greenhouse gas. In industrial processes, it can be used to generate energy, such as in steam turbines. Synonyms for water vapour might include humidity, atmospheric water, vapour, and aqueous vapour.