Water vapor is the gaseous state of water that occurs when it evaporates from surfaces like oceans, lakes, and rivers. It is an essential part of the water cycle, where water moves between the earth and the atmosphere. When water vapor rises, it cools and can condense into clouds. In the atmosphere, water vapor plays a crucial role in weather and climate. It helps regulate temperature by trapping heat, which is known as the greenhouse effect. Additionally, water vapor can lead to precipitation, such as rain or snow, when it condenses and falls back to the earth.