Martian Clouds
http://planetscapes.com/solar/eng/marscld.htm

Click on the link for the Noctis Labyrinthis image. It has
downloadable links for large GIF and TIFF files.

This page has several different examples of the types of clouds seen
on Mars. I found interesting this passage:

"Fog
Fog often appears in low-lying areas. It typically occurs in the
southern hemisphere especially in the Argyre and Hellas basins. It
forms frequently in craters. Occasionally, it occurs in higher
regions such as Sinus Sabaeus and Solis Planum. (Courtesy Calvin J.
Hamilton)
"Clouds in Noctis Labyrinthis
This image shows early morning fog in the Noctis Labyrinthis, at the
westernmost end of Valles Marineris. This fog, which is probably
composed of water ice, is confined primarily to the low-lying
troughs, but occasionally extends over the adjacent plateau. The
region shown is about 300 kilometers (186 miles) across. (Courtesy
NASA/LPI)"

Another low latitude location more clearly gives the indication of
low lying fogs:

Cross multiple parallel bands on eastern Sinai Planum.
http://ida.wr.usgs.gov/html/m08078/m0807848.html

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.
Perhaps though sufficient water vapor to form the crystals only
occurs at low altitudes. For example there were some Pathfinder
observations that suggest most of the water vapor at the Pathfinder
site was restricted to the bottom 1 to 3 kms above the surface.
On the other hand this can't be the entire explanation because we
know Martian clouds sometimes formed of H2O can form at high
altitudes.
Another key consideration is the temperature at which these ice
crystal fogs can occur. On Earth fogs are most commonly seen as
liquid water droplets, but there are cases where they occur as ice
crystals. Interestingly, the air temperature for this to occur has to
be not only below freezing but actually below around -30 to -40
degrees centigrade.
Above this temperature fogs will form but usually be still in the
form of supercooled liquid droplets. The temperature at which the
transition from supercooled liquid water droplets to ice crystals
occurs is not exact because the supercooled water will nucleate to
ice at a higher temperature in dusty conditions.
See these sites on ice fogs and supercooled water:

Ice Fog a Product of Temperature, Topography, Dogs.
http://www.gi.alaska.edu/ScienceForum/ASF13/1319.html

Winter Rainbows And Supercooled Water.
http://www.gi.alaska.edu/ScienceForum/ASF7/748.html

Cloud, fog & precipitation.
http://www.auf.asn.au/meteorology/section3.html [link may be inactive,
see google cached link:
http://216.239.41.104/search?q=cache:t0EhxfJWP2QJ:www.auf.asn.au/meteorology/section3.html&hl=en&lr=&ie=UTF-8&strip=1
]

An image showing atmospheric temperature readings from MGS and the
Opportunity rover shows the -40 C cut-off temperature for supercooled
liquid water droplets may also apply to low lying clouds up to within
2km, 6000 ft:

Image from:

MARS EXPLORATION ROVERS UPDATE
Spirit Arrives at Bonneville Crater; Opportunity Moves to Berry Bowl;
and Both Rovers Turn 'Eyes' to the Skies
12 March 2004
http://www.planetary.org/news/2004/mer-update_04-03-12.html

Key questions here are how is this temperature of -40C for which
atmospheric water can still be supercooled effected by a CO2
atmosphere and how is it effected by low atmospheric pressure?
It is likely that atmospheric water can be supercooled even under
Martian conditions. Then even if the sites at which the observed fogs
occur are a few degrees below zero the water may still be in
supercooled liquid form.

Bob Clark