In astronomy, a cold trap is a close to the surface layer of the atmosphere that is substantially colder than both the deeper and higher layers. The temperature of the air drops with increasing height above the surface of the earth reaching a low point (which for the earth resides at about 20 kilometers height). It is called a trap because it keeps ascending gases with high melting points in by freezing them to a solid which then drops back to the planet surface. The most important gas to be kept in that way, on the Earth, is water vapor, which without the presence of a cold trap in the atmosphere would gradually escape or dissociate into space, making life impossible. However, because of the cold trap water vapor condenses into droplets of liquid water or tiny ice crystals, forming clouds. The cold trap retains one-tenth of one percentof the water in the atmosphere in the form of a vapor at high altitudes. The cold trap is also a layer which above ultraviolet intensity is strong, since higher up the amount of water vapor is negligible. Oxygen screens out ultraviolet intensity. Some astronomers believe that the lack of a cold trap is why the planets Venus and Mars both lost most of their liquid water early in their histories.