Water is a highly polar molecule and so interacts strongly with other polar molecules (other water molecules and/or ions in the air), ensuring that it never really gets high enough in the atmosphere to escape because interactions with other elements in the atmosphere keep it locked in essentially the same place. read more
Water vapor, e.g., the atmosphere, is "held against" the earth by the earth's gravity. There may be a trace amount of the atmosphere escaping over time, but it would not be a significant amount. read more
On the Earth, water can exist in all three forms namely as a solid, liquid, or gas. Evaporation transforms liquid water into water vapour which can then freely move in the atmosphere as a gas. Now, atmospheric molecules, including water vapour molecules, are in perpetual motion in all directions. read more