<p>There is more to phase change than just what you see on that graph.  At temperatures and pressures that support multiple states (e.g. solid and gas) you have to look at the saturation vapor pressure of the gas and the actual vapor pressure (partial pressure) of the gas -- the line on the graph represents equilibrium between the states.  The differences between the two will tell you if you have net sublimation or net deposition and determine whether you end up with a solid or a gas. For solid CO<sub>2</sub> to form you need to have more deposition (transition from gas to solid) than sublimation.  The transition to solid may also need help nucleating, though I'm not familiar enough with CO<sub>2</sub> to say for sure.</p>

<p>To summarize, in your example Vostok at -90 C and 1000 hPa you have a rate of CO<sub>2</sub> molecules depositing and a rate of CO<sub>2</sub> molecules sublimating.  The partial pressure of CO<sub>2</sub> in the atmosphere is low enough that there is more sublimation than deposition, e.g. net sublimation.  To deposit CO<sub>2</sub> ice on the ground and have it stay there you would need atmospheric CO<sub>2</sub> concentrations at least at saturation, which we do not have.  </p>

<p>@Tristan provided some numbers in the comments that atmospheric CO<sub>2</sub> vapor pressures (partial pressure) are &lt;1 mm Hg (or less than about 133 Pa).  To get net sublimation you need to either reach saturation via increasing the vapor pressure (add more CO<sub>2</sub>) or reducing saturation vapor pressure (lower the temperature).  He provided an estimate that given CO<sub>2</sub> partial pressures temperatures approaching -140 C would be needed for net deposition and CO<sub>2</sub> snow.</p>

CC BY-SA 3.0
2015-05-29T13:38:04.790Z
Then, what are the CO2 concentrations necessary for net deposition to occur?
arkaia
CC BY-SA 3.0
2015-05-29T14:09:36.600Z
If we reach those CO₂ concentrations we're going to be in trouble!
gerrit
CC BY-SA 3.0
2015-05-29T14:19:55.497Z
I agree. I just wanted to see if we could provide that information in the answer for completeness.
arkaia
CC BY-SA 3.0
2015-05-29T14:21:03.830Z
@aretxabaleta I don't have the numbers handy but would be happy to accept edits / info from anyone that does.  I agree they'd be a valuable addition to the answer.
casey
CC BY-SA 3.0
2015-05-29T16:00:10.580Z
You won't get sublimation or condensation until you reach the saturation temperature for the *partial pressure* of the gas.  The partial pressure of CO2 in the atmosphere is less than 1 mmHg.  The saturation temperature at 1 mmHg is -134 C.  You would probably need to be colder than -140 C to get CO2 snow.
Tristan
CC BY-SA 3.0
2015-05-29T16:20:39.077Z
@Tristan I've added your info to the post, feel free to review what I wrote and edit if you want.  Thanks for some numbers!
casey
CC BY-SA 3.0
2015-05-29T21:18:03.503Z
@aretxabaleta to deposit CO2 at -78.5C one needs 1 atm pressure of CO2, that is having all air replaced with CO2. To depose CO2 at its current atmospheric concentration one needs to cool to some -140C.
Anixx
CC BY-SA 4.0
2015-05-29T13:35:59.840Z
2019-03-22T11:48:44.987Z
2019-03-22T11:48:44.987Z
casey
<p>The CO<sub>2</sub> phase diagram shows that at atmospheric pressure and about -78 °C temperature CO<sub>2</sub> becomes solid:</p>

<p><img src="https://i.sstatic.net/zO0fU.gif" alt="CO2 phase diagram">
<img src="https://i.sstatic.net/HPtp0.png" alt="Another CO2 phase diagram"></p>

<p>Wikipedia <a href="http://en.wikipedia.org/wiki/Carbon_dioxide" rel="noreferrer">confirms this</a>:</p>

<blockquote>
  <p>At 1 atmosphere (near mean sea level pressure), the gas deposits directly to a solid at temperatures below $−78.5\ \mathrm{^\circ C}$ ($−109.3\ \mathrm{^\circ F}; 194.7 ~\mathrm{K}$) and the solid sublimes directly to a gas above $−78.5\ \mathrm{^\circ C}$. </p>
</blockquote>

<p>The lowest temperature measured on Earth is about -90 °C (Vostok station). Does this not mean that CO<sub>2</sub> should fall as snow at the poles?</p>

CC BY-SA 3.0
2015-05-29T12:41:39.940Z
please avoid [duplicate](http://chemistry.stackexchange.com/q/32139/10262) questions on SE sites
bummi
CC BY-SA 3.0
2015-05-30T11:36:49.937Z
There is simply not enough CO2 in the atmosphere for that
Gimelist
<atmosphere><precipitation><poles>
Why are there no CO2 snowfalls on Earth?
Anixx
# Why are there no CO2 snowfalls on Earth?

(Asked by: username_0 on 2015-05-29)

The CO2 phase diagram shows that at atmospheric pressure and about -78 °C temperature CO2 becomes solid:

![CO2 phase diagram](https://i.sstatic.net/zO0fU.gif)
![Another CO2 phase diagram](https://i.sstatic.net/HPtp0.png)

Wikipedia [confirms this](http://en.wikipedia.org/wiki/Carbon_dioxide):

> At 1 atmosphere (near mean sea level pressure), the gas deposits directly to a solid at temperatures below $−78.5\ \mathrm{^\circ C}$ ($−109.3\ \mathrm{^\circ F}; 194.7 ~\mathrm{K}$) and the solid sublimes directly to a gas above $−78.5\ \mathrm{^\circ C}$.

The lowest temperature measured on Earth is about -90 °C (Vostok station). Does this not mean that CO2 should fall as snow at the poles?

**Tags:** atmosphere, precipitation, poles

> **Comments:**
> **username_1**: please avoid [duplicate](http://chemistry.stackexchange.com/q/32139/10262) questions on SE sites  
> **username_2**: There is simply not enough CO2 in the atmosphere for that  

---

## 1 answers

### Answer by username_3 on 2015-05-29. Score: 19

There is more to phase change than just what you see on that graph. At temperatures and pressures that support multiple states (e.g. solid and gas) you have to look at the saturation vapor pressure of the gas and the actual vapor pressure (partial pressure) of the gas -- the line on the graph represents equilibrium between the states. The differences between the two will tell you if you have net sublimation or net deposition and determine whether you end up with a solid or a gas. For solid CO2 to form you need to have more deposition (transition from gas to solid) than sublimation. The transition to solid may also need help nucleating, though I'm not familiar enough with CO2 to say for sure.

To summarize, in your example Vostok at -90 C and 1000 hPa you have a rate of CO2 molecules depositing and a rate of CO2 molecules sublimating. The partial pressure of CO2 in the atmosphere is low enough that there is more sublimation than deposition, e.g. net sublimation. To deposit CO2 ice on the ground and have it stay there you would need atmospheric CO2 concentrations at least at saturation, which we do not have.

@username_6 provided some numbers in the comments that atmospheric CO2 vapor pressures (partial pressure) are <1 mm Hg (or less than about 133 Pa). To get net sublimation you need to either reach saturation via increasing the vapor pressure (add more CO2) or reducing saturation vapor pressure (lower the temperature). He provided an estimate that given CO2 partial pressures temperatures approaching -140 C would be needed for net deposition and CO2 snow.

> **Comments:**
> **username_4**: Then, what are the CO2 concentrations necessary for net deposition to occur?  
> **username_5**: If we reach those CO₂ concentrations we're going to be in trouble!  
> **username_4**: I agree. I just wanted to see if we could provide that information in the answer for completeness.  
> **username_3**: @aretxabaleta I don't have the numbers handy but would be happy to accept edits / info from anyone that does.  I agree they'd be a valuable addition to the answer.  
> **username_6**: You won't get sublimation or condensation until you reach the saturation temperature for the *partial pressure* of the gas.  The partial pressure of CO2 in the atmosphere is less than 1 mmHg.  The saturation temperature at 1 mmHg is -134 C.  You would probably need to be colder than -140 C to get CO2 snow.  
> **username_3**: @username_6 I've added your info to the post, feel free to review what I wrote and edit if you want.  Thanks for some numbers!  
> **username_0**: @aretxabaleta to deposit CO2 at -78.5C one needs 1 atm pressure of CO2, that is having all air replaced with CO2. To depose CO2 at its current atmospheric concentration one needs to cool to some -140C.  

---

