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Competencies covered

MSFFL2004: Moisture test timber and concrete floors

MSFFL3003: Inspect sub-floors

Relative humidity and moisture

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Moisture is the presence of a liquid in small amounts. Generally the liquid is water, sometimes with trace elements dissolved in it.

Water is a very special substance, because it can exist as a solid, liquid or gas.

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In its solid form, it turns to either ice or snow, depending on how closely the ice crystals are packed together.

As a gas, it becomes vapour or steam.

In between these two states it is a liquid, and one of the essential ingredients of life - in fact, humans are made up of about 70% water.

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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'.

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Because warm air can hold more water vapour than cool air, the actual amount of moisture present at any given RH level will be higher on a hot day than on a cold day.

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.

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The average relative humidity in different countries also varies, depending on the size and location of the landmass and surrounding oceans.

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.

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Learning activity

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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.

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