The troposphere is the lowest level of the atmosphere. Depending on the dryness of the air over the particular area it can be anywhere from less than 1% to as much as 5% of the atmosphere but only in the troposphere. As you get higher in the air the temperature drops and water vapor condenses to liquid and eventually ice crystals. Above the troposphere pretty much all water is going to be liquid or solid.
Since the source of atmospheric water is the surface of the Earth it only makes sense that the most amount of water is going to be in the troposphere.