Water Vapor

As water evaporates from Earth’s surface, it enters the atmosphere as water vapor. As the water vapor rises in the atmosphere, it cools; the kinetic energy of the water molecules decreases, and the molecules condense to form clouds and eventually fall as precipitation.

Water vapor is a greenhouse gas because (similar to a greenhouse) it absorbs radiation emitted by Earth’s surface, trapping the heat and increasing the temperature of the atmosphere. When the temperature of the atmosphere increases, the water molecules in the atmosphere have more kinetic energy. Water molecules with higher kinetic energy are less likely to condense into a liquid. This means that the amount of water vapor in the atmosphere increases as the atmosphere warms. More water vapor traps more radiation, increasing atmospheric temperature, which leads to further warming.

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In contrast, water molecules in cooler air have less kinetic energy and are more likely to condense and eventually fall back to Earth as precipitation. If the air temperature decreases, the amount of water vapor in the atmosphere decreases, which leads to further cooling.