1)  Water vapour in air:

All air contains water in gaseous form. The density of dry air is approximately 1.2kg/m3 at standard pressure and 20 degrees.  Moist air is lighter than dry air (because an H2O  molecule is smaller than the free N2 and O2 molecules in air).  The warmer the air the more water vapour it can contain.

Around Australia, depending on local climate and time of year, the water content of the air ranges from 6 to 24 g/m3 of air or 0.5-2% (g/kg dry air).
(Where 1.0% = 10g water vapour/ kg dry air, so the amount of water vapour/ m3 air at 1.0% ~ 12g/m3)

At any given location in Australia the water content in the air is stable day to day but varies through an annual cycle with its minimum in August and maximum in February.  For example:

Source:  “A High-quality Historical Humidity Database for Australia” by Chris Lucas, CAWCR Technical Report No. 024, July 2010

2)  Saturation & Dew point

As air cools, its relative humidity increases to 100% – or “saturation” – and vapour condenses as dew (or frost if below zero degrees). The saturation temperature at which condensation occurs is called the “dew point”.
For any given water vapour content there is a dew point temperature at which the air is saturated and condensation commences; the more water content the higher the dew point.  For example:

(With minima in August and maxima in February).

The following curve directly relates dew point to water vapour content (at standard pressure)…

ABCB dew point 1600pxw
(Source:  The graphics and some of the text on this page come from the ABCB Handbook: Condensation in Buildings 2014, Second Edition.)

As air cools below its dewpoint, water vapour condenses out reducing the water content to the % corresponding to its lower temperature…

ABCB cooling 1600pxw

As air is warmed above its dew point the water content remains constant but represents a decreasing percentage of the higher saturation limit of the new raised temperature.  This percentage is known as the “relative humidity” (RH)

ABCB RH 1600pxw

3)  Relative Humidity

RH is a measure of how close the air is to saturation.  A number of environmental issues relate more to the relative humidity of air than its actual water vapour content, for example…

ABCB Mould RH 1600pxw

This is why Relative Humidity is used more often than moisture content.

4) Moisture Vapour Transfer

All the above has particular significance in modern housing construction; as homes become better insulated and less leaky the building fabric must be designed to manage condensation and moisture vapour transfer – to let the building envelope ‘breathe’ as the moisture content in the air changes with the seasons.