Temperature and its effect on humidity

A given quantity of air cannot go absorbing moisture indefinitely. There comes a point where the air cannot hold any more moisture. Ay this point we say the air is completely saturated or at 100% humidity. To appreciate what happens to air when it is heated or cooled, let us imagine an air sample as a set of marbles (as per above image)

The diagram shows that water vapour can be held between the molecules of air and as the air is heated the air becomes less dense so the molecules move further apart in proportion to the temperature rise. Thus air at, say 21C can absorb twice as much water vapour as the same volume of air at 10C.

We may establish as an example that air at 20C may contain 14.7g of water per kg or dry air (14.7g/kg). If a sample of air did in fact contain this amount of water we would say that the air had reached its saturation point or that the humidity was absolute and could not absorb any additional water.