At any place water vapour in the air varies greatly. In the lower air it tends to be some 2% of the volume, but varies from very little to about 5%. The amount tends to decrease upward, so that the mean vapour content at 1200 m is only one tenth that at sea-level, though the actual amount is ever varying.
At any given temperature there is a limit to the amount of water that can be held as vapour. When the air is saturated any fall in temperature should cause condensation. Relative humidity (RH) is the proportion of the actual mass of water vapour in a given volume of air to the maximum amount that could be contained at that temperature. Thus "dry" air over a hot desert may contain as much water vapour as saturated air over Artic waters, or indeed more. If the humidity exceeds 100 per cent, moisture will begin to condense from the air. If the air contains only half the water it can hold at that temperature, the RH is 50 per cent
All air contains water vapour of varying quantities. Warm air can hold more moisture than cool air. The dew point indicates the amount of moisture in the air. The dew point temperature is the temperature at which the air can now longer hold all of its water vapour, and some of the water vapour must condense into liquid water. The condensed water is called dew. The higher the dew point, the higher the moisture content of the air at a given temperature. Conversely, the dew point of humid air will be higher than the dew point of dry air.
When the dew point approaches 75 degrees F, most people can "feel" the thickness of the air as they breathe, since the water vapour content is so high (about 20 grams of water vapour per kilogram of dry air, or 2% of the air's mass).
Humidity is measured by condensing the moisture in the air. Absolute humidity is the quantity of water in a particular volume of air. If all the water in one cubic meter of air were condensed into a container, the container could be weighed to determine absolute humidity. The amount of vapor in that cube of air is the absolute humidity of that cubic meter of air. The absolute humidity changes as air pressure changes.
Air will normally contain a certain amount of water vapour. As I understand it, a liquid water molecule has been heated and expands into a water vapour molecule. The water that condenses on someone's glasses as they walk into a humid room, would previously have occupied a space in the air that was perhaps hundreds of times greater. The condensation comes from bursting bubbles of water vapour in the room which collect on the cooler glasses. The water vapour starts to look like some kind of invisible foam that fills a room.
I think that when we talk about water vapour making up only 2% of the air it is something of a misnomer, because when we speak of water vapour I think that it IS the air. We think there is barely any hydrogen in air. This theory says we are surrounded by it. Somehow, oxygen fixes the hydrogen so it does not float off into space. Oxygen acts like a bag of ballast to the hydrogen gas "bubble".
A container pressurized with steam can be condensed into a pool of water which now occupies a space 99.93% less. This space that has been left contains quite literally nothing. It is a vacuum. The container would look like it had been sucked-in. The water vapour creates air pressure inside the container. Can I suggest then that it is also water vapour which creates air pressure in the atmosphere?
Weather and climate By David Money