<p>The imortant point is that absolute humidity is defined as</p>

<p><strong>mass of water</strong> per <strong>volume of air</strong></p>

<p>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.</p>

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

CC BY-SA 3.0
2015-04-27T10:53:51.090Z
For more clarification, I'd like to understand where the misunderstanding is - could you tell why you thought that absolute humidity would increase?
Volker Siegel
CC BY-SA 3.0
2015-04-27T18:22:46.257Z
+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.
David Hammen
CC BY-SA 3.0
2015-04-27T19:18:36.577Z
@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.
Volker Siegel
CC BY-SA 3.0
2015-04-27T19:24:36.847Z
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.
Volker Siegel
CC BY-SA 3.0
2015-04-27T19:36:12.660Z
@DavidHammen Regarding en.wikipedia definition - first sentence - in case you wonder where it is: fixed now.
Volker Siegel
CC BY-SA 3.0
2015-04-29T01:34:04.747Z
This answer does not address the other issues raised about vapour pressures & relative humidity
Fred
CC BY-SA 3.0
2015-04-29T05:36:34.010Z
@Fred Yes- that's intended, it's a chain of questions - and the OP got the first one wrong  in a basic way. So my expectation is that now - after clarifying that error - he iether can understand the remaining points, because this big issue at the start blocked him. Or - maybe more probable - needs to add details to the question to understand what the OP is understanding. Otherwise, this would end up as homwork question list - nobody cares to understand. Which may be the original issue to begin with, of course.
Volker Siegel
CC BY-SA 3.0
2015-04-27T10:50:37.407Z
2015-04-27T10:50:37.407Z
Volker Siegel
<p>I know that a parcel of air can rise, cool and create clouds and rain but I'm struggling to understand how:</p>

<ol>
<li>absolute humidity</li>
<li>relative humidity</li>
<li>actual vapour pressure</li>
<li>saturation vapour pressure</li>
</ol>

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

<p>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?</p>

CC BY-SA 3.0
2015-04-27T02:04:06.680Z
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<meteorology><atmosphere><homework><humidity>
How and why humidity and vapour pressure change as a parcel of air with an unchanging amount of water vapor rises, expands, and cools
Elsa
# How and why humidity and vapour pressure change as a parcel of air with an unchanging amount of water vapor rises, expands, and cools

(Asked by: username_0 on 2015-04-27)

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?

**Tags:** meteorology, atmosphere, homework, humidity

> **Comments:**
> **username_1**: Welcome to Earth Science S.E.! If you need any assistance with our site, please visit [The Help Center](http://earthscience.stackexchange.com/help). Also, please see [this Meta post on homework questions](http://meta.earthscience.stackexchange.com/questions/180/homework-questions).  

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## 1 answers

### Answer by username_2 on 2015-04-27. Score: 3

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?

> **Comments:**
> **username_2**: For more clarification, I'd like to understand where the misunderstanding is - could you tell why you thought that absolute humidity would increase?  
> **username_3**: +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.  
> **username_2**: @username_3 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.  
> **username_2**: 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.  
> **username_2**: @username_3 Regarding en.wikipedia definition - first sentence - in case you wonder where it is: fixed now.  
> **username_4**: This answer does not address the other issues raised about vapour pressures & relative humidity  
> **username_2**: @username_4 Yes- that's intended, it's a chain of questions - and the OP got the first one wrong  in a basic way. So my expectation is that now - after clarifying that error - he iether can understand the remaining points, because this big issue at the start blocked him. Or - maybe more probable - needs to add details to the question to understand what the OP is understanding. Otherwise, this would end up as homwork question list - nobody cares to understand. Which may be the original issue to begin with, of course.  

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