In article ,
Rodney Blackall wrote:
In article ,
Robert Clark wrote:

I'm wondering if these low level fogs are ice crystals instead of
liquid water droplets why are they occurring in low level locations?
A low altitude site would make sense for *liquid* water on Mars since
that would provide a higher atmospheric pressure. But you would think
that ice crystals could form at any atmospheric pressure.

Fog / cloud will form when the atmosphere becomes saturated with respect
to water / ice.

H2O conc. will be greatest near the surface normally.

The coldest atmosphere will either be high up or near the night-time
surface.

If condensation occurs at temps. 0 deg C then you get water fog.
If condensation occurs at temps. - 40 deg C then you get ice fog.

G'day Rodney,
I'm curious about this -40C stuff. Is there actually some physical
phase change or whatever involved here, or is it just a convenient
rule of thumb not to be taken too literally?

If condensation occurs at temperatures between 0 & -40 then water fog will
form on 'ordinary' condensation nuclei and ice fog around 'ice'
condensation nuclei; I doubt we know which is commonest on Mars.

In a mixed fog ice will grow at the expense of water and the resulting
larger crystals gradually settle out (often with interesting optical
phenomena).


Cheers, Phred.

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LID