I know that a parcel of air can rise, cool and create clouds and rain but I'm struggling to understand how:

  1. absolute humidity
  2. relative humidity
  3. actual vapour pressure
  4. saturation vapour pressure

change as a parcel of air with an unchanging amount of water vapour rises, expands and cools.

I think that absolute humidity and relative humidity increase, but is there a difference how absolute and relative humidity behave and how actual vapour pressure and saturation vapour pressure behave?


1 Answer 1


The imortant point is that absolute humidity is defined as

mass of water per volume of air

So if the parcel of air expands, and still has the same water content, there is less mass of water per volume, hence the absolute humidity decreases.

Maybe most if not all of the question depends on this and is much more clear now?

  • 2
    $\begingroup$ For more clarification, I'd like to understand where the misunderstanding is - could you tell why you thought that absolute humidity would increase? $\endgroup$ Apr 27, 2015 at 10:53
  • $\begingroup$ +1. Note that some define absolute humidity as the mass of water per unit mass of air. With this definition, the absolute humidity of an isolated parcel of air remains constant (at least until water starts condensing). With either definition, absolute humidity doesn't increase, so asking why the OP thought absolute humidity would think that is a good question. This is the core of the OP's misunderstanding. $\endgroup$ Apr 27, 2015 at 18:22
  • $\begingroup$ @DavidHammen Who uses that definition? I noticed that Wikipedia seemed to just leave it undefined in the place where I expected one of these definitions. (the german version of the page explicit on it) Hmm... would be worth a question here, if there is any kind of source for the other variant. $\endgroup$ Apr 27, 2015 at 19:18
  • $\begingroup$ I rechecked en.Wikipedia - the definition of abs. humidity is unambiguously to be mass of water per volume of gas. It's just the first sentence, that should - and tries to be - a definition makes no sense: "Absolute humidity is the total amount of water vapor present in a given volume of air": "amount of water present" explicitly ignores the unit. $\endgroup$ Apr 27, 2015 at 19:24
  • $\begingroup$ @DavidHammen Regarding en.wikipedia definition - first sentence - in case you wonder where it is: fixed now. $\endgroup$ Apr 27, 2015 at 19:36

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