By Jan Lievens
In my previous article, I promised to delve a bit deeper into what humidity really is… so, here goes – for once a rather scientific article.
What is the definition of water? Pure water is tasteless, odourless, and almost colourless. Water can occur in three states: solid (ice), liquid, or gas (vapour).
- Solid water—ice is frozen water. When water freezes, its molecules move further apart, making ice less dense than water. This means that ice will be lighter than the same volume of water, and so ice will float in water. Water freezes at 0° Celsius, 32° Fahrenheit.
- Liquid water is wet and fluid. This is the form of water with which we are most familiar. We use liquid water in many ways, including washing and drinking.
- Water as a gas—vapour is always present in the air around us. You cannot see it. When you boil water for example the water changes from a liquid to a gas or water vapour. As some of the water vapour cools, we see it as a small cloud called steam. This cloud of steam is a mini version of the clouds we see in the sky. At sea level, steam is formed at 100° Celsius, 212° Fahrenheit.
The water vapour attaches to small bits of dust in the air. It forms raindrops in warm temperatures. In cold temperatures, it freezes and forms snow or hail.
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How can water co-exist at three phases (solid, liquid and gas)?
Water exists in three distinct phases at what is called the triple point. Zero degrees C is defined by the triple point of water which is 273.16K at 611.2 Pa.
At this temperature water is in the process of changing from a solid state into the liquid phase or visa-versa. Molecules in the liquid phase can lose a bit of energy and solidify whilst solid water (ice) can gain some energy and melt.
This can be seen in melting ice where the solid ice exists for some time while the exposed surface melts.
Molecules in a liquid do not all have the same energy. The energies of the molecules can vary from a finite minimum, which would mark the transition back to a solid phase, up to an infinite energy (although the probability of this occurring is infinitely small).
The average energy of the molecules gives us the temperature of the liquid. Statistical thermodynamics can map out the energy distribution of the water molecules. At a certain energy molecule will have enough energy to evaporate, even if the water temperature is 0° C.
Because of these two effects it is possible for the water to exist as solid, liquid and gas at the same time.
What is humidity?
Humidity is the presence of water vapour in air (or any other gas). High humidity means there is a lot of water vapour in the air, low humidity means there is a little. Air is said to be saturated when it contains the maximum amount of vapour possible at a particular temperature.
In a cold room, you get various other issues that come to life…
The colder the air, the less it can contain moisture. So, if you have cold product and open doors, have bad insulation, hotter product, hot air movement, i.e., increasing the humidity, you are basically creating oversaturation and thus condensation.
At any given time, the amount of water vapour in the air is usually less than that required to saturate the air. Unless you influence it – as described above.
Humidity is an interesting and challenging measurement area because:
- water vapour gets everywhere!
- water vapour influences a vast range of physical, chemical, and biological processes.
- humidity measurement techniques
- are diverse.
- level of air humidity is expressed in many terms, quantities, and units.
What is water vapour?
Water vapour is one of the three states of water that occurs in human ambient temperatures. It is normally invisible and behaves like a gas. It is the condensation of water vapour that gives rise to clouds, rain, snow, dew, frost, and fog.
Even without condensing, water vapour can react with surfaces and penetrate materials.
More about water
- Water is a chemical.
- when water changes phase, its physical properties change – melting, freezing, condensation and evaporation.
- water is quite unique in that liquid; solid and vapour forms all occur in human ambient temperature ranges.
- water is polar – electron sharing leads to an uneven distribution of charge across the molecule. This makes water molecules electrically attracted to the surfaces, and sensitive to electric fields.
- water’s action on materials is compounded by temperature – therefore relative humidity is a useful measure.
- the phase state of water depends on the temperature and pressure.
Humidity and saturation
When a gas (or a space) holds the maximum water vapour possible at a given temperature, it is said to be saturated. If extra water is added to a saturated gas, or if its temperature is reduced, some of the water will condense.
Humidity and temperature
The capacity of a gas (or a space) to hold water depends on its temperature. The higher the temperature, the more water vapour it can contain.
- normal room temperature – air typically holds about 1 % of water vapour.
- hot-atmosphere has greater capacity to hold water vapour.
- cold- atmosphere has less capacity to hold water vapour.
When the air holds the maximum amount of water vapour at a particular temperature, it is said to be saturated. In air this can happen if:
- Capacity is reduced by a lowered temperature and amount of water vapour stays consistent.
- More water vapour is added.
How do we quantify humidity?
- Humidity quantities can be expressed in several different ways:
- The quantities include:
- relative humidity – strictly dimensionless but generally expressed as a percentage with use unit symbol ‘%RH’.
- dew point – expressed in units of temperature.
- fraction or ratio – strictly dimensionless, expressed as a percentage, or smaller fractions 3/103 (3/1000)
There is no official specialised SI unit for humidity measurement.
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Relative humidity
- Relative humidity (RH) is:
- a measure of degree of saturation with water vapour
- a ratio that compares the amount of water vapour in the air with the amount of water vapour that would be present in the air at saturation at a particular temperature.
- most used measure of humidity, for example in weather forecasts.
- it is usually expressed as a percentage with the symbol ‘%RH’ e.g., ‘the humidity is 51 %RH’.
- the term ‘relative humidity’ is commonly abbreviated to RH (not this is different from the unit symbol %RH)
Relevance of relative humidity
- if relative humidity and temperature are high the air feels damp
- a condition of 100 %RH means the air is totally saturated with water vapour and will feel much hotter than the actual temperature.
- relative humidity is strongly governed by temperature.
- interaction of water vapour with materials is often in proportion to relative humidity.
- lowering relative humidity increases evaporation and drying.
Imagine a parcel of air at known temperature and relative humidity, at 20°C and 50 %RH. If we vary only temperature without adding or removing water (or anything else), the relative humidity changes. The degree of saturation is increased or decreased simply by changing temperature. Relative humidity falls when temperature rises (and rises when temperature falls).
Examples, approximate, values:
| Temperature °C | 15 | 20 | 25 | 30 | 35 | 40 |
| Relative humidity (%RH) of the same air at different temperatures | 100* | 71 | 50 | 36 | 27 | 19 |
*Above saturation
Saturation
When a gas (or a space) holds the maximum water vapour possible at a given temperature, it is said to be saturated. If extra water is added to a saturated gas, or if its temperature is reduced, some of the water vapour will condense.
Partial pressure of water vapour
A gas mixture such as air is made up of several pure gas components (such as oxygen, nitrogen, and others). The total pressure of the gas is said to be the sum of partial pressures of the component gases.
In room air, the partial pressure of water vapour might be around 1000 pascals (Pa). (Compare this to typical atmospheric pressure, which is around 100 000 Pa).
Saturation vapour pressure (SVP)
SVP is the maximum pressure of water vapour that can exist at a given temperature.
Expressed in units of pressure such as pascals (Pa), (or in non-SI units such as millibars (mbar)).
At any given temperature, there is an upper limit to how much water vapour a gas (or space) can contain. For example, at 21°C the SVP of water is around 2500 Pa.
Dew point (or dew-point temperature)
So, what is dew point?
- This is the temperature at which condensation (dew) would occur if a gas (air) were cooled.
- in effect the temperature at which air becomes saturated in equilibrium with water (100 %RH).
- dew point is expressed in temperature units e.g., ‘today the dew point in my office is 10°C.
- dew-point temperature is always lower or equal to the air temperature.
- if there is further cooling of air the dew-point temperature falls with the air temperature and more dew/fog/cloud forms.
- if dew-point temperature equals air temperature – dew, fog, and clouds form – at this point relative humidity is 100%.
Dew point is a useful measure for two reasons:
- the dew point tells us what temperature to keep a gas, to prevent condensation.
- dew point is an absolute measure of the gas humidity (at any temperature) and relates directly to the amount of water vapour present (partial pressure of water vapour).
Partial pressure (of water vapour) is the part of the overall pressure exerted by the water vapour component in a gas. Expressed in units of pressure such as pascals (Pa) (or millibars (mbar)).
If the condensation is ice, then the term frost point is used.
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For clouds to form and rains to start the air must reach 100 %RH, but only where clouds are forming or where the rain is coming from. This normally happens when the air rises and cools.
Often rain will be falling from clouds where relative humidity is 100 percent into air with a lower relative humidity. Some water from the rain evaporates into the air as it is falling through but not enough to bring the humidity up to 100 percent.
Humidity can be expressed as a fraction or ratio
- The proportion of water vapour in a gas can be specified as:
- ratio (dry basis)
- fraction (wet basis)
- They can be measured in terms of mass, volume, or amount of substance moles.
Mass fraction
Fraction expressing mass of component per total mass of substance present, may be written with no units (said to be ‘dimensionless’), for example ‘water vapour in nitrogen at a mass fraction of 0.002 (or 2 parts in 103)’. Alternatively, in mass units such as ‘kilograms of water per kilogram of (humid) gas’.
Mass ratio (mixing ratio) of air
Mass of water vapour per unit mass of dry air. May be written with no units (said to be ‘dimensionless’), but often expressed in grams of water per kilogram of dry gas.
Volume fraction
Fraction expressing volume of component per total substance present. May be written with no units (said to be ‘dimensionless’), for example ‘water vapour in nitrogen at a volume fraction of 5 parts per million (or 5 parts in 106)’.
Mole
Amount of substance which contains as many elementary entities as there are atoms in 12 grams of carbon 12. Expressed in moles (symbol: mol).
NOTE: When the mole is used, the elementary entities must be specified as atoms, molecules etc.
Amount fraction (amount-of-substance fraction) also known as mole fraction
Amount (number of moles) of a component as a fraction of the total amount of substance present, also known as ‘mole fraction’. May be written with no units (said to be ‘dimensionless’, or of ‘dimension 1’) for example ‘water vapour in nitrogen at a mole fraction of 0.005 (or 5 parts in 103)’. Alternatively, in ‘moles of water per mole of gas’.
Parts per million (ppm)
Informal way of expressing trace water content (amount fraction or ratio; mass fraction or ratio; or volume fraction or ratio). ‘1 ppm’ means ‘1 part per million’ or ‘1 part in 106′.
Concentration
The amount of mass of water vapour per unit volume.
For example, room air might typically contain about 10 grams of water vapour per cubic metre.
Air temperature measurement
Air temperature measurement is a key measurement alongside relative humidity. This is because the ‘relative’ aspect is effectively ‘relative to temperature’ (how saturated the gas is at its current temperature).
For any given air sample, a rise in temperature means a fall in relative humidity – at a humidity of 50 %RH a temperature rises from 20 °C to 21 °C will cause relative humidity to fall by about 3 %RH.
Thermometers are integral in most relative humidity probes. Alternatively, temperature is measured alongside using resistance thermometers, thermocouples, or thermistors.
Heat or humidity?
Thermal comfort levels are a combination of temperature and humidity and moisture content of the air, as well as air movement.
High temperatures make the body perspire. If relative humidity is high, less perspiration can evaporate from our bodies into the air and the body cannot cool down as effectively.
Why is humidity important?
- Humidity measurement is important because:
- It affects many properties of air, and of materials in contact with air.
- Water vapour is a key agent in both weather and climate, and it is an important atmospheric greenhouse gas. (Without water vapour we would be 31°C colder on Earth).
- A huge variety of manufacturing, storage and testing processes are humidity critical. Humidity measurements are used to prevent condensation, corrosion, mould, warping or other spoilage – highly relevant for foods, pharmaceuticals, chemicals, fuels, wood, paper, and many other products.
- Air-conditioning systems in buildings often control humidity, and significant energy may go into cooling the air to remove water vapour – Humidity measurements contribute both to achieving correct environmental conditions and to minimising the energy cost of this.
Et voila! Now you have a deeper insight on what RH is… this mysterious subject is now clearer… we hope,
Just never ever forget that water and air have been on earth for over 4.4 billion years, and they are extremely smart – they always take the shortest route and the path of the least resistance.
And please always remember that preserving quality after harvest does not come by chance…
In practical postharvest there are a lot of ‘wannabe experts’ but unfortunately there are very few real experts…
References:
- National Physical Laboratory
About Jan Lievens
Jan Lievens, born in Belgium, is a graduate civil engineering(B) and international senior consultant for engineered applied postharvest technology at UTE South Africa. With over 20 years of experience in this field, he is widely regarded as a specialist in the fruit-, vegetable- and flower industry with regards to humidity, airborne bacteria and ethylene removal, both locally and internationally. Furthermore, he also designed airflow-friendly packaging systems for the industry with proven results.