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The term relative humidity is very much misunderstood by those who do not need to deal with it as part of their work or studies.

Most people associate it with the weather, but it also plays a crucial role in thermal comfort and air conditioning design.

The general perception is that high humidity means conditions feel stuffy and uncomfortable whereas low humidity can lead to dry nose and throat.  This understanding, however, is not accurate and only applies to certain temperature ranges.

humidity

In reality, relative humidity is the percentage of water vapour in the air at a specific temperature compared to the maximum water vapour that the air can hold at that same temperature.

More simply put, air has the ability to absorb water vapour, or moisture, and its ability to absorb moisture varies depending on the temperature and air pressure.  Because air pressure does not play a big role in thermal comfort situations, let's focus on air temperature.

Think of it like a simple sponge.  A dry sponge is able to absorb a large amount of water, but as it gets wetter, its ability to absorb water is reduced. Eventually, it will be unable to absorb more and will release water if slightly squeezed. At this point, the sponge is considered to be saturated.

Like a sponge, air will readily absorb water vapour up to a point where it becomes saturated and can no longer absorb more. When the air it saturated, that's 100 per cent relative humidity. Before it reaches that point, relative humidity will be calculated based on the amount of water vapour (by weight) present in the air sample divided by the theoretical maximum water vapour the air would be able to absorb at the same temperature and pressure conditions.

Taking the analogy further, depending on how much you squeeze the sponge, it will retain more or less water. This applies to air for both temperature and pressure. The higher that air temperature, the more water vapour the air can absorb, whereas the cooler the air, the less water vapour it can absorb. This is where the understanding of relative humidity versus thermal comfort can cause confusion.

The cold bottle effect

As an example, air which is at 35 degrees Celsius at 40 per cent relative humidity contains approximately 14 grams of water vapour per kilogram of dry air. The layperson may assume air at 10 degrees Celsius at 90 per cent relative humidity contains a lot more water vapour, but in fact it only contains about seven grams of water vapour per kilogram of dry air. In fact, at 10 degrees Celsius, air would be fully saturated at 7.5 grams of water vapour per kilogram of dry air. This is because warm air has a much greater capacity to hold water vapour than colder air.

When air reaches 100 per cent relative humidity, it is unable to absorb any more water temperature and if the air temperature starts to drop, excess water vapour will condense out in to liquid water. This is how rain forms, or why a cold bottle "sweats" when it is taken out of the refrigerator. The air surrounding the bottle drops to a temperature near that of the bottle, and therefore its ability to hold its water vapour reduced to the point where the excess water is lost, creating condensation.

Human thermal comfort

Humans are warm-blooded, so our bodies produce heat to assist with our physiological functions. Our bodies use a number of processes to reject excess heat and keep our bodies at their optimum working temperature. These processes are:

Conduction: the process of direct heat transfer from our skin to any surrounding elements, whether our clothing, the chair we're sitting on or the air in contact with our skin. Provided our skin temperature is higher than the surrounding elements, we will transfer heat to the cool air around us. This can best be felt when we come in contact with cold water or when a cold wind is blowing.

Radiation: similar to conduction in that it relies on our skin temperature being at a higher temperature than our surrounds, but this process relies on infrared radiation to transfer the heat and does not require direct contact with a cooler surface. This is the same sensation you feel when standing in front of a fire or lying in the sun.

Evaporation: relies on water vapour and therefore is very much influenced by relative humidity in the air. This is our supplementary cooling system when the normal processes are not creating sufficient heat loss, and it is produced by sweating. This release of warm water from our skin to the surrounding air creates an evaporative cooling effect which is very effective in transferring large amounts of heat from our skin surface to the surrounding air. This is the function which is most affected by relative humidity of the air.

Given that air at low relative humidity has a much better capacity to absorb water vapour, it makes sense a person exposed to 35 degree Celsius temperatures and 25 per cent relative humidity (as in a Melbourne summer) will feel more comfortable than someone exposed to 35 degree temperatures and 60 per cent relative humidity (a Darwin summer). This is because at elevated temperatures, especially with the sun out, our bodies rely on sweating to provide the additional cooling capacity we need to keep our body temperature within its maximum allowable range.

Meanwhile, our thermal comfort levels would not differ much whether we were in 10 degree weather with 30 per cent relative humdity or 10 degree weather with 90 per cent relative humidity as we don't need to sweat to get rid of excess heat.

Effects on air conditioning

Relative humidity of air has a major effect on air conditioning processes as air conditioning systems use cooling coils to absorb heat from the air and deliver this to the conditioned space. The cooling of air changes its ability to absorb and retain water vapour, so typically when air is cooled via an air conditioning unit, its condition goes (for example) from 24 degrees and 50 per cent relative humidity to 14 degrees and 90 to 95 per cent relative humidity. The air conditioning coil is often at a temperature which causes water vapour to condense and drip from the coil surface.

The warmer and more humid the air is, the more energy is required to cool the air as the condensation of water vapour actually draws a lot more energy from the coil than the air component. So, for example, in Melbourne, the air conditioning systems condense a relatively small amount of water vapour from the air during summer, but in tropical regions, the moisture condensed is significant, and as such, energy consumption is much greater even when the actual air temperature may be lower.

Limiting the water vapour content within buildings in tropical regions is therefore very important in reducing energy consumption and improving internal thermal comfort levels. Vapour sealing a building is a key to achieving this.

Effects on evaporative cooling

Evaporative cooling systems are also greatly affected by relative humidity as these systems actually rely on the evaporation of water into the air stream to achieve a cooling effect. The higher the relative humidity, the less effective the evaporation process will be, which is why these systems work much better in dryer climates such as Melbourne and Adelaide, as opposed to Brisbane or Darwin, where they are not of much benefit. This is also why evaporative cooling has less and less effect at lower air temperatures, as the air’s ability to absorb moisture is much reduced.

In summary, relative humidity is more complex than most people realize and it must not be considered as an overall condition in isolation, but rather in conjunction with the ambient temperature it relates to. It plays an important role in our thermal comfort at higher ambient temperatures but much less of a role at lower temperatures.

 
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