This report suggests clouds may be harder to form on Mars than
thought:

RELEASE: 07_89AR
NASA Study Reveals Less Water in Mars' Clouds.
Dec. 6, 2007
http://www.nasa.gov/centers/ames/new...7/07_89AR.html

Nevertheless clouds at such low temperatures have been observed on
Mars.
Also even at such low temperatures it is still possible some
proportion of the condensed water in the clouds is in liquid form. For
instance actual measurements of Polar Stratospheric Clouds on Earth
show that liquid water aerosols with nitric and sulfuric acid can be
liquid down to -80 C:

Polar Stratospheric Clouds.
"Type I a (Nitric acid trihydrate particle - NAT)
crystalline particles forming at 195 K,
Type I b (Supercooled ternary solution - STS)
spherical liquid particles forming at 193 K,
Type II (Water ice) ice crystals forming below 188 K."
http://www.iap-kborn.de/optik/psc/psc_e.htm

Balloon-borne measurements of Polar Stratospheric
Clouds.
"First results of in-situ measurements of the chemical
composition of PSCs are presented, which have been
performed over Kiruna, Sweden, in the early morning of
January 25, 1998. PSCs at 21.5 and 23 km and
temperatures between 187 and 192K have been
encountered with a balloon-borne experiment, a
combination of a PSC mass spectrometer and a
backscatter sonde (operated by N. Larsen, Danish
Meteorological Institute, Copenhagen, Denmark) [1]. In
the clouds, a simultaneous increase of condensed
water, nitric acid, and backscatter ratios has been
observed. The measured molar ratios of water to nitric
acid indicate the presence of liquid mixtures of
water, nitric acid, and sulfuric acid, so-called
supercooled ternary solution particles (STS)."
http://www.mpi-hd.mpg.de/mauersberger/schreiner/#Ballon

Because these Earth clouds are stratospheric, they occur at pressures
near those on the surface of Mars. Then low lying fogs or clouds on
Mars would occur at similar pressures and temperatures to the liquid
water containing PSC's on Earth.

Also a recent report suggests some liquid water in clouds could remain
down to -140C(!)

New Observations On Properties Of Water
"ScienceDaily (Dec. 13, 2006) -- Recent research on the properties of
water reveals information relevant for cloud physics and even
cryopreservation science.
"Experimental studies conducted by Ph.D. Anatoli Bogdan at the
University of Helsinki, Finland, have received broad interest in the
scientific world, as the results might have applications even in the
cryopreservation of cells and tissues. Bogdan's results show that
mixture droplets consisting of sulphuric acid and water can be slowly
cooled down to-140 degrees Celsius and then heated again without ice
formation."
http://www.sciencedaily.com/releases...1213104104.htm

Reversible Formation of Glassy Water in Slowly Cooling Diluted Drops
J. Phys. Chem. B, 110 (25), 12205 -12206, 2006. 10.1021/jp062464a S1520-6106(06)02464-3
Web Release Date: June 6, 2006
http://pubs.acs.org/cgi-bin/abstract...jp062464a.html

This might make it possible for even Polar Mesopheric Clouds on Earth
to contain liquid water. The Aeronomy of Ice in the Mesosphere (AIM)
satellite was recently launched to study such clouds. If such clouds
are also found to contain some proportion of liquid water, that would
greatly increase the range of possibilities for liquid water clouds on
Mars.

The last statement in the NASA news release is also misleading:

"The amount of water in the martian atmosphere varies greatly in space
and time," Colaprete observed. Clouds in the atmosphere largely
control the amount of water that comes off of the north pole and
migrates to the south pole.
"If all the water in the atmosphere were to freeze out to the surface,
it would make a layer of ice about one-fifth the thickness of a human
hair, according to Colaprete.
"Cloud mass is typically only 10 to 20 percent of the total water
content. However, the thin martian atmosphere is much more sensitive/
reactive to the influence of these clouds," he said."

Since the water vapor content on Mars is known to be so low that
implies that the water content in any cloud must be even lower. But
actually it is because overall the cloud cover of the entire planet is
relatively low.
But the water content in precipitation clouds can be much higher than
the water vapor content in the surrounding area.
For instance during a storm you can have many inches of rainfall or
snowfall. But the water vapor content on Earth is at most 5 to 6
precipitable cm, about 2 to 2.5 inches (the amount of water vapor in
an atmospheric column if it were condensed.)

The NASA report focused on clouds at very cold temperatures -100C.
But it is known there are daytime clouds/fogs very close to the
surface on Mars where the temperatures will be much higher than this,
frequently above -40C for instance. This a temperature at which pure
water in clouds can undergo supercooling to remain in liquid form.
Indeed supercooling is a major part of cloud formation on Earth:

Cloud.
http://en.wikipedia.org/wiki/Cloud


Bob Clark