Water vapor
Water vapor is water that is in the form of a vapor, or gas. It is a part of the water cycle. When liquid water is heated to boiling point, 100 °C (212 °F), it turns into vapor. Water vapor can also be produced directly from ice; this is called sublimation. Steam is water vapor, but clouds are liquid water. The amount of water vapor in air is called humidity and it affects weather conditions. In the cold, breathing out causes the water vapor in the breath to freeze.
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Water vapor weighs less than air, thus slightly reduces the lift produced by an aircraft wing. It is also transparent, which means the small droplets that look like mist are actually liquid. Water vapor also is a big cause of the greenhouse effect. It is invisible, but makes visible water when it condenses.
