Earth's atmosphere includes billions and billions of gallons of evaporated water: in fact, water vapor is Earth's most abundant greenhouse gas. Like the stuffing in an insulated sleeping bag, water vapor absorbs heat radiating from the sun-warmed surface of the Earth. When water evaporates from oceans, rivers, or the leaves of plants, it cools the Earth’s surface. When water vapor condenses into clouds or rain, it releases heat back into the atmosphere. As a result, the volume of water vapor—the specific humidity—plays a key role in setting Earth’s surface temperature.