How does hoarfrost form in the morning?

  • Context: Undergrad 
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Discussion Overview

The discussion centers around the formation of hoarfrost in the morning, exploring the physical processes involved, including sublimation, condensation, and the role of temperature and pressure. Participants are examining the conditions under which hoarfrost appears, particularly in relation to atmospheric pressure and humidity levels.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant notes the triple point of water and questions how hoarfrost can form at atmospheric pressure, suggesting a connection to partial pressure and the Law of Dalton.
  • Another participant shares their impression that supersaturated air plays a significant role in the formation of hoarfrost, although they express uncertainty about the technical details.
  • A participant questions the understanding of sublimation at atmospheric pressure, indicating a lack of clarity on the topic.
  • There is a proposal that hoarfrost may be formed through reverse sublimation or a specific type of condensation followed by freezing, raising questions about the dew point near the surfaces where hoarfrost forms.

Areas of Agreement / Disagreement

Participants do not appear to reach a consensus on the mechanisms behind hoarfrost formation, with multiple competing views and uncertainties remaining in the discussion.

Contextual Notes

Participants express uncertainty regarding the technical aspects of sublimation and condensation, as well as the specific conditions necessary for hoarfrost to form. There are unresolved questions about the dew point and the role of supersaturation.

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I was wondering how it comes that we have hoarfrost in the morning. Water has his tripplepoint at 0.001°C and 10³Pa, so subblimation occurs at lower pressure and temperature. But we have atmospherical pressure of 10^5Pa, so how comes we have hoarfrost?

http://www.lsbu.ac.uk/water/phase.html

I think it has something to do with partial pressure (Law of Dalton), but I'm not sure how...
 
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I could be wrong about the technicalities, but when I have seen hoarfrost(an otherwordly, beautiful site, eh?)on trees, shrubbery, fences, etc... I was under the impression that the primary mechanism involved(other than freezing temps)was supersaturated air.
Any insight?
 
So no one knows why water sublimates at this pressure?
 
Is hoarfrost formed by (reverse) sublimation? Maybe it's only a special kind of condensation, folowed by freezing? What is the dewpoint right next to the leaves or whatever where the hoarfrost forms?
 

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