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

CO2 phase diagram Another CO2 phase diagram

Wikipedia confirms this:

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?

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    $\begingroup$ please avoid duplicate questions on SE sites $\endgroup$ – bummi May 29 '15 at 12:41
  • $\begingroup$ There is simply not enough CO2 in the atmosphere for that $\endgroup$ – Gimelist May 30 '15 at 11:36
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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.

@Tristan 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.

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  • $\begingroup$ Then, what are the CO2 concentrations necessary for net deposition to occur? $\endgroup$ – arkaia May 29 '15 at 13:38
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    $\begingroup$ If we reach those CO₂ concentrations we're going to be in trouble! $\endgroup$ – gerrit May 29 '15 at 14:09
  • $\begingroup$ I agree. I just wanted to see if we could provide that information in the answer for completeness. $\endgroup$ – arkaia May 29 '15 at 14:19
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    $\begingroup$ 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. $\endgroup$ – Tristan May 29 '15 at 16:00
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    $\begingroup$ @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. $\endgroup$ – Anixx May 29 '15 at 21:18

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