Humidity can be expressed in different ways which can lead to confusion if the incorrect term is used. This guide explains the different terms and how they are used in practise.
Relative humidity (RH)
Expressed commonly as a percentage value ‘%rh’, this refers to how saturated a gas (or space) is with water vapour compared to the amount of water vapour that would be needed to completely saturate that gas at a given temperature. It is a measure that looks at both water content and temperature.
The ability of air to hold water changes significantly with temperature therefore RH can be very volatile as temperature conditions change.
The term ‘relative humidity’ is often abbreviated to ‘RH’ but the measurement would be expressed as its percentage value e.g. 70 %rh.
Absolute humidity (AH)
Often just referred to as ‘humidity’, this is a measure of the actual amount of water vapour in a given volume of air, expressed in grams per meter cubed of air e.g. 8 g/m3.
Unlike RH, AH does not take temperature into consideration, it is simply the total amount of water vapour present in a given volume of air. AH in the atmosphere ranges from zero to 30 g/m3. This latter value is when the air is saturated at 30 °C.
Note: Vapour pressure is a measurement term that closely follows AH. Vapour pressure is an indication of a liquid’s rate of evaporation and is expressed in pascal (Pa).
The dew point is another useful measure and is the saturation temperature for water in air. At temperatures below the dew point, water will leave the air and form as condensation or dew when it forms on a solid surface. The dew point is an absolute measurement and is considered a more useful way of measuring the humidity and comfort of the air.
Most people are comfortable with a dew-point temperature of 16 °C or lower. At a higher dew point (often around 21 °C), most people feel hot or ‘sticky’ because the amount of water vapour in the air slows evaporation of sweat and we therefore find it harder to cool down.
Density of humid air
The density or ‘heaviness’ of air is also an important consideration. When water vapour is added to air (i.e. the air becomes more humid) its density reduces – which seems counterintuitive.
How can adding water vapour make air LESS dense?!
For a particular volume of any gas the number of molecules present is constant at a given temperature and pressure. So if (for example) five molecules are added to the volume of gas, then the number of molecules must also reduce by five so that the total number remains constant.
With regards to air, the mass (molar) of a dry air molecule is more than the mass of a water molecule. Therefore, if water molecules (water vapour) are added then the number of dry air molecules must decrease by the same number. Our volume of gas now contains the same number of molecules but more of them now have less mass so its mass per unit volume (i.e. density) decreases.
Since humidity is impacted by water vaporisation and condensation, which, in turn, is highly influenced by temperature and pressure, all of these variables impact on the density of air.
Relative humidity versus absolute humidity – understanding some common misconceptions
RH and AH are not interchangeable and can lead to confusion. The two examples below can show how RH can be misleading:
- On a cold day, the dew point might be 1 °C and the air temperature 2 °C. The resulting relative humidity would be 93 %rh. This high value is misinterpreted as ‘high humidity’ but few people would refer to such conditions as ‘humid’.
- Conversely, on a hot summer’s day, with a dew point of 18 °C, and an air temperature of 30 °C, the relative humidity might be 49 %rh. A ‘low’ RH, but high in absolute terms.
Using relative humidity and absolute humidity as a means of condensation control
When looking at controlling the water content in air the %rh scale can cause confusion. For example, the idea that air with RH of 70 %rh could be suitable for use in extracting water vapour seems unlikely – until you consider absolute humidity.
If we look at 1 m3 of air at a temperature of 14 °C, the RH versus AH is as follows:
Air with an RH of 70 %rh would have an AH of 8g/m3
Air with an RH of 100 %rh would have an AH of 12g/m3
So, assuming that the less-saturated air will readily saturate to 100 %rh, this means every 1 m3 of replacement air can absorb a further 4 g/m3. A fan extracting at 54 m3/h will, in ideal conditions, remove an extra 216 ml of water/hour.
However in practise this rate is compromised by:
- Replacement evaporated air from wet surfaces
- Dilution. This has an impact at the point of extraction, although the fan will be moving air at the rate stated, the fan stretches the air – lowering the pressure as it extracts. The amount the fan expands the air is aligned to how easy it is for air replacement to occur.
A realistic extraction rate in this example therefore would be around 50 ml/hour (this is subjective and based on experience).
A practical example in the domestic environment
However if we look at a typical domestic environment where air temperature is often higher the volume of water that can be extracted is much more significant. Here the same mass of water in the air as our previous example (8 g/m3) will produce the following figures.
If we look at 1 m3 of air at a temperature of 20 °C, the AH versus RH is as follows:
Air with an AH of 8g/m3 will have an RH of 47 %rh
Air with an AH of 12g/m3 will have an RH of 70 %rh
Air with an AH of 17g/m3 will have an RH of 100 %rh
This means every 1 m3 of replacement air can absorb a further 9 g/m3. A fan extracting at 54 m3/h will, in ideal conditions, remove an extra 486 ml of water/hour.
Although in reality (subjective) this would be closer to 100ml/h.