water vapor
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Water vapor is the gaseous state of water, which is formed when liquid water evaporates or when ice sublimates. It is a major constituent of Earth's atmosphere, and plays a vital role in the Earth's weather and climate. Water vapor is also an important greenhouse gas, which helps to keep the Earth warm by trapping heat in the atmosphere. However, excessive amounts of water vapor can also lead to extreme weather events such as thunderstorms, hurricanes, and floods.