Relative humidity and moisture
Relative humidity
The amount of moisture - or water vapour - suspended in the air is called relative humidity, often abbreviated to RH.
It is expressed as a percentage and is 'relative' to the maximum amount of water vapour that the air can absorb at that temperature.
For example, if the RH is 50%, it means that the air is holding half of the total amount possible at that temperature.
As the RH approaches 100%, the closer it gets to being fully saturated.
Once it reaches saturation point, any additional moisture will result in condensation, or 'dew'.
This graph shows the amount of water that the air can carry between -20° C and 50° C.
You can see that at 50% RH (shown by the green line), every kilogram of air will contain nearly 50 grams of water when the temperature is at 50°, but once the temperature drops below zero, there is hardly any moisture at all.
Because Australia is such a big continent, it too has big variations in average levels.
This map shows the average annual RH readings around the country at various times of the day. It's based on a map from the Building Code of Australia.
You can see a more detailed version of this map on the Australian Bureau of Meteorology (BOM) website.
Learning activity
Let's take a closer look at the climatic zones around Australia. Click on the map at right to see it in a larger window.
The map uses mathematical signals to express 'more than' and 'less than'. For example:
'< 60%' means less than 60%
'> 60%' means more than 60 %.
See if you can find where your own town or city is located on the map. What climatic zone does it fall into? What is the average RH at 9 am?
If you live in a regional area and you're having trouble deciding which band it falls into, have a look at the BOM map. It shows many more towns and has more detailed zone bands.