Antarctic Temp Reaches -99.2C: Could CO2 Snow Have Fallen?

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anorlunda
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News reports in the past day tell us tha the temperature in Antartica reached -99.2C (-135.8F).

Wikipedia says that the melting point of CO2 is -78C (-109F)

Does that imply that there might have been natural CO2 snow falling there? If so, that is a very interesting fact.
 
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that's not the melting point, but where deposition/sublimation occur at ~1 atm. I do not know what CO2 "snow" might be like, as regular snow is due to the crystal structure / bonding between water molecules. I don't know if CO2 would do the same thing, but i suppose there should be some solid CO2 under those conditions.
 
anorlunda said:
Wikipedia says that the melting point of CO2 is -78C (-109F)
That's the temperature at which dry ice sublimates at one atmosphere of pressure. That does not mean that atmospheric CO2 will deposit as CO2 snow at that temperature. It would have to get *much* colder than that to have CO2 fall down as snow in our atmosphere.

The issue is the difference between partial pressure and atmospheric pressure. You would get CO2 snow at that temperature if the partial pressure of the CO2 was one atmosphere. It's not. The partial pressure of gaseous CO2 in the atmosphere is much lower than that because CO2 is a trace gas.
 
D H said:
That's the temperature at which dry ice sublimates at one atmosphere of pressure. That does not mean that atmospheric CO2 will deposit as CO2 snow at that temperature. It would have to get *much* colder than that to have CO2 fall down as snow in our atmosphere.

The issue is the difference between partial pressure and atmospheric pressure. You would get CO2 snow at that temperature if the partial pressure of the CO2 was one atmosphere. It's not. The partial pressure of gaseous CO2 in the atmosphere is much lower than that because CO2 is a trace gas.

Can you clear this up for me? It must be a similar situation to that of water vapour below 100C, at which the vapour pressure is 1. At -78C, the vapour pressure of CO2 is 1At. Are you just saying that you only get condensation when the partial pressure is higher than the vapour pressure at a given temperature? That would make sense and the partial pressure of CO2 must be about 0.03% of 1At. so the temperature would need to be lower than -150C (extrapolating from the info I have from Wiki)
 
sophiecentaur said:
Can you clear this up for me? It must be a similar situation to that of water vapour below 100C, at which the vapour pressure is 1. At -78C, the vapour pressure of CO2 is 1At. Are you just saying that you only get condensation when the partial pressure is higher than the vapour pressure at a given temperature? That would make sense and the partial pressure of CO2 must be about 0.03% of 1At. so the temperature would need to be lower than -150C (extrapolating from the info I have from Wiki)

Yes, that's correct. Think about dew formation. It only happens if the temperature goes below the dew-point which is much lower than the boiling temperature at 1 atmospheric pressure. The dew point is the temperature at which the atmosphere becomes saturated which is the temperature water would boil if the atmospheric pressure was as low as the water vapor partial pressure. I don't know what the frost point of CO2 on Earth is, but it is much lower that the sublimation temperature of -78C.