I initially put this one on the physics-side, but this might be a more appropriate place: https://physics.stackexchange.com/questions/666116/mollier-diagram-and-reading-the-relative-humidity-when-knowing-the-dry-and-wet

I had a thought-error when reading Mollier diagrams and thankfully we have the internet to assist, but even after finding the correct sources showing the answers, I can't get out of my thought-loop to understand why my thinking is wrong. I hope someone could in simple and very definite terms explain why my thinking is wrong.

So let's say that I have the dry bulb temperature of 20 Celsius and wet bulb temperature of 15 Celsius and I'd like to know the relative humidity.

enter image description here

My thought error was that I thought that I'd read this from the 100% humidity point at 15 Celsius, straight up along the absolute humidity lines. My thought here was that the absolute humidity is the same and the air's water content doesn't change no matter how much I spin my wet-bulb thermometer. So I'd have assumed from the image for the relative humidity to be about ~72% at 20 C. (red arrow)

However this source showed how it should be read and I was also able to test it online. So I should actually read it along the enthalpy lines, which gives me about 58% relative humidity at 20 C. (yellow arrow)



Now I'm a bit stuck: I can from one perspective understand that the enthalpy does not change and is constant, but I'm having a hard time to definitely understand why my initial assumption about the absolute humidity is the "wrong constant" in this case.

Could someone take it down to my level, and also possibly clarify if there is something I still seem to have misunderstood, or what should be good to recognize?

  • 1
    $\begingroup$ Do you mind if I explain this with the ASHRAE psychometric chart? $\endgroup$
    – el-cheapo
    Sep 17, 2021 at 12:00
  • $\begingroup$ Hi, not at all. Now I admit that it is some time since I last looked at these, and clearly I didn't understand it correctly then, but as I suppose it will be the same thing just from a "different angle". So I'm perfectly happy with it as long as I learn this. $\endgroup$ Sep 17, 2021 at 12:15
  • $\begingroup$ Just given my final answer down below $\endgroup$
    – el-cheapo
    Sep 17, 2021 at 12:30
  • $\begingroup$ hard to use a chart when I can't read the units or descriptions $\endgroup$
    – Tiger Guy
    Sep 17, 2021 at 19:12

1 Answer 1


Absolute humidity isn't the same if the dry bulb temperature changes. The definition of absolute humidity is the actual amount of water vapor in the air, while relative humidity is the ratio between the water vapor and the amount of water that the air can hold (saturation) at that given dry bulb temperature.

If the air gets hotter (dry bulb increases) the amount of water that the air can hold increases. While the amount of water in air remains the same (absolute humidity stays), the amount of water vapor that the air can hold changes, and thus changing the relative humidity. However, at some point if the air becomes saturated due to the change of the amount of water that air can hold, the absolute humidity will also change as some of the water vapor will condense.

If you go straight up with the red arrow you are implying that the hotter the air gets, the absolute humidity of the air remains the same. While in reality, at some point the absolute humidity changes when the air is saturated and some of the water vapor condenses.


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