The Mixture of Water
Most of the water on Earth is found either on the surface, under the surface, or in the lowest layers of the atmosphere. As it is heated, water becomes a gas. This gas can and does freely mix with the atmosphere. However, water has a limitation that keeps it from traveling too high into the atmosphere. This limitation is caused by temperature. As water travels higher into the atmosphere, it is cooled down. This cooling down of water molecules causes them to once again turn into liquid form. As this happens, water becomes too heavy to remain in the air, and it falls back down towards the surface.