Humidity (absolute humidity) is simply the actual water content of air (expressed in gram/cubic meter, or as partial pressure of the water vapor, or in any such unit). But absolute humidity it is not particularly interesting to us, not in everyday life. What is much more interesting, rather, is how close the air is to becoming completely saturated with water. It is then that your shirt clings to your back, your ass to the chair and condensing water-drops start forming on various objects. Outdoors this state of affairs corresponds to raining or snowing.

Temperature dependence

Problem is, air can dissolve more water (in absolute terms) the warmer it is. Well, this is not altogether surprising. It’s a bit like dissolving sugar or salt in water – the warmer it is, the more sugar or salt can you dissolve.

So the question “how much water does the air contain, when condensing water-drops start forming and my shirt clings to my back” can not be answered once and for all. Sadly, it has a different answer for every temperature. Here is a table that illustrates the situation:

          Air temp,   Water content of
          deg C       saturated air,
                      gram/cubic meter
          --------    ----------------
          -10               2.1
            0               4.8
           10               9.4
           18              15.4
           20              17.3
           30              30.4
           40              51.2
           50              83.0
                          .....
           90             423.5

As you can see, there is a huge difference between how little water cold air can hold and how much water warm air can hold – the maximum water content almost doubles for every 10 C increase in temperature.

Relative to saturation

We are fortunately not all that often plagued by completely water-saturated air, with shirts clinging and drops forming all around us. But we would very much like to know how close we are to such an uncomfortable state of affairs. So this is where the concept relative humidity comes in. It tells us how many per cent saturated the air is, at a certain temperature. So a relative humidity of 50% means that the air holds 50% of what it can maximally hold at that temperature (say 30 C).

If you consult the table above, then you can calculate for yourself how much water every cubic meter of 30-degree air contains at 50% relative humidity. Yes, 50% of 30.4 gram is 15.2 gram.

Cooling can increase relative humidity

But let us assume that you would like to cool the air in your room by 10 degrees, to 20 C. It still contains the same amount of water, 15.2 gram/cubic meter, so it still has the same absolute humidity, absolutely. But what about the relative humidity? Well, at 20 C air can maximally hold 17.3 gram per cubic meter, and your air contains 15.2 gram. So the relative humidity is now 100 x 15.2/17.3 = 88%. This is tolerable, but maybe a bit less comfortable than a relative humidity of 50%. However, if you would cool the air in your room even further, to 18 C, then things start getting bad: 100 x 15.2/15.4 = 98.7% relative humidity, which feels most uncomfortable. Of course, if you cool the air in your room with the help of an air conditioner, then you will hardly get this terrible relative humidity. The reason is that the air conditioner removes some of the water, so that the relative humidity stays nearly the same even at the lower temperature.

Warming makes air dry

In wintertime, in cold climates, a different problem appears – uncomfortably low relative humidity. You can see this easily from the table above. Let us say that it is –10 C outside and it is snowing, signifying that the outside air is saturated with water vapor. According to the table, every cubic meter of saturated –10 C air contains 2.1 gram of water vapor. This air from the outside now enters your room and is warmed to 20 C, a temperature at which a cubic meter can hold as much as 17.3 gram. The relative humidity indoors is consequently 100 x 2.1/17.3 = 12%. This is far too dry for comfort – your skin feels dried out and static electricity starts building up, giving you nasty sparks and jolts and putting the electronic circuits of your computer in jeopardy. The remedy is an air humidifier, a device that sprays a water mist into the indoor air.

NOTE: The above writeup correctly shows the use and practical meaning of the concept of relative humidity. I have used analogies to provide some explanation as well, but analogies are never perfect. The full explanation of how water vapor and air interact requires an understanding of the physical behavior of gases, which is beyond the scope of this writeup. For example, water vapor above liquid water at a certain temperature always has a certain pressure, even if there is no air present, so analogical terms like “water content” and “air holding/dissolving water” may be helpful in a practical sense, but they are strictly speaking irrelevant. Also, pressure affects the physical processes considerably, but since atmospheric pressure varies very little on the surface of the earth, this may be disregarded from a practical everyday point of view.

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