<p>The saturation mixing ratio is not how much water vapor the air has in it... but how much it <em>could</em> have, based upon the temperature.  Since <strong>adiabatic ascent/descent</strong> means cooling or heating, <strong>the saturation mixing ratio will change</strong>.  The actual mixing ratio on the other hand won't change unless you lift the parcel so much that it cools to the dewpoint and beyond and starts condensation.</p>
<p>When you're working with a skew-T or such, anytime you see the word <em>saturation</em>, it signals to refer to the <strong>temperature</strong> part of the profile (the right/red line) and <strong>not the dew point</strong> part (the left/green line).</p>
<p>Anytime you see that word saturation, it's a different entity... (unless <span class="math-container">$T=T_d$</span>) <em>saturation</em> mixing ratio is different than mixing ratio, <em>saturation</em> vapor pressure is different than vapor pressure, etc.  And each are so tied to the temperature/possibility of what can exist at that point and not the dew point/current existing moisture.</p>
<p>On a skew T, the saturation mixing ratio is the same shape, up and to the right, along/parallel to the example mixing ratio lines usually given (in dashed light green on this image).  But they aren't the same line (unless <span class="math-container">$T = T_d$</span>):</p>
<p><a href="https://i.sstatic.net/iDR8X.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/iDR8X.png" alt="enter image description here" /></a></p>
<p>The saturation mixing ratio line starts from the temperature, the regular mixing ratio starts from the dew point.</p>
<blockquote>
<p>I think the mixing ratio (rv=ρv/ρd) is constant as long as no water
vapor is condensed out, but saturation mixing ratio always confuses me
a little.</p>
</blockquote>
<p>Indeed the (regular) mixing ratio is constant as long as there's no condensation and no outside change to the parcel... for the moisture you stay with the same mixing ratio in ascent (until saturation), such as when calculating the LCL.  But you <a href="https://en.wikipedia.org/wiki/Lifted_condensation_level" rel="nofollow noreferrer">follow the dry adiabat</a> (lines going up to the left on a skewt like this one) for the temperature for condensation-less ascent (so never use the saturation mixing ratio line, the one coming from just the temperature).</p>

CC BY-SA 4.0
2022-01-23T22:02:12.603Z
2022-01-24T02:13:22.223Z
2022-01-24T02:13:22.223Z
JeopardyTempest
<p>Does saturation mixing ratio change during adiabatic ascent/descent? I think the mixing ratio (<span class="math-container">$r_v = \rho_v/\rho_d$</span>) is constant as long as no water vapor is condensed out, but saturation mixing ratio always confuses me a little.</p>

<meteorology><thermodynamics><water-vapour>
Does saturation mixing ratio change during adiabatic ascent/descent?
theWrongAlice
# Does saturation mixing ratio change during adiabatic ascent/descent?

(Asked by: username_0 on 2022-01-23)

Does saturation mixing ratio change during adiabatic ascent/descent? I think the mixing ratio ($r\_v = \rho\_v/\rho\_d$) is constant as long as no water vapor is condensed out, but saturation mixing ratio always confuses me a little.

**Tags:** meteorology, thermodynamics, water-vapour


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## 1 answers

### Answer by username_1 on 2022-01-23. Score: 2 | Accepted answer

The saturation mixing ratio is not how much water vapor the air has in it... but how much it *could* have, based upon the temperature. Since **adiabatic ascent/descent** means cooling or heating, **the saturation mixing ratio will change**. The actual mixing ratio on the other hand won't change unless you lift the parcel so much that it cools to the dewpoint and beyond and starts condensation.

When you're working with a skew-T or such, anytime you see the word *saturation*, it signals to refer to the **temperature** part of the profile (the right/red line) and **not the dew point** part (the left/green line).

Anytime you see that word saturation, it's a different entity... (unless $T=T\_d$) *saturation* mixing ratio is different than mixing ratio, *saturation* vapor pressure is different than vapor pressure, etc. And each are so tied to the temperature/possibility of what can exist at that point and not the dew point/current existing moisture.

On a skew T, the saturation mixing ratio is the same shape, up and to the right, along/parallel to the example mixing ratio lines usually given (in dashed light green on this image). But they aren't the same line (unless $T = T\_d$):

[![enter image description here](https://i.sstatic.net/iDR8X.png)](https://i.sstatic.net/iDR8X.png)

The saturation mixing ratio line starts from the temperature, the regular mixing ratio starts from the dew point.

> I think the mixing ratio (rv=ρv/ρd) is constant as long as no water
> vapor is condensed out, but saturation mixing ratio always confuses me
> a little.

Indeed the (regular) mixing ratio is constant as long as there's no condensation and no outside change to the parcel... for the moisture you stay with the same mixing ratio in ascent (until saturation), such as when calculating the LCL. But you [follow the dry adiabat](https://en.wikipedia.org/wiki/Lifted_condensation_level) (lines going up to the left on a skewt like this one) for the temperature for condensation-less ascent (so never use the saturation mixing ratio line, the one coming from just the temperature).


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