You frequently hear said Mars is much drier than any place on Earth as
an indication of the difficulty of having liquid water on Mars, as in
this news release:

Deep down, Mars harbors a lot of ice
Frozen water may even be drinkable, scientists say.
"Mars is extremely dry, drier than any (place) we have on Earth," said
Gerhard Neukum, a German scientist who has analyzed stereo images of
the Martian surface recorded by the European Space Agency's Mars
Express satellite that began orbiting the fourth planet from the Sun in
late 2003."
http://www.sfgate.com/cgi-bin/articl...&feed=rss.news

However, during southern Winter, Antarctica can actually have less
atmospheric water vapor than the highest concentrations of water vapor
on Mars:

South Pole Transmissivity Plots.
"Because of its high altitude, low water vapor column, and low
temperatures, Antarctica may contain some of the driest and thus best
sites for infrared, submillimeter, and millimeter astronomy [Bally,
1989; Harper, 1989; Chamberlin and Bally, 1996]. Potential sites on the
Antarctic plateau vary in elevation from nearly 3000~m to over 4000~m.
The center of the plateau is in a permanent high-pressure zone where
air is descending from high altitudes. Temperatures at the south pole
range from 200~K to 260~K [Chamberlin and Bally, 1994]. Measurements of
the precipitable water vapor column [e.g., Smythe and Jackson, 1977;
Burova et al., 1986] show that the water column can be as low as 50
microns in the austral winter, and is rarely above 1~mm."
http://casa.colorado.edu/~bally/AT/cara.html

The term "precipitable microns" means the thickness of liquid water you
would have if the entire water vapor content in a column were condensed
to liquid.

Mars can have water vapor content above 100 precipitable microns over
the northern pole in Summer, though the measurements close to the
equator can be as low as 10 precipitable microns.
However, I believe there are some locations even near the equator on
Mars that are far above the 10 micron level. For instance Noctis
Labyrinthus frequently shows dense low lying clouds/fogs that give the
appearance of precipitation carrying clouds on Earth:

Noctis clouds.
http://photojournal.jpl.nasa.gov/jpe...213_modest.jpg

An even better image from Mars Express appears on the first page of this
report:

Adsorption water driven processes on Mars.
D. Möhlmann
http://sci.esa.int/science-e/www/obj...objectid=36779 [pdf
file]

Mars Express image.
http://uplink.space.com/attachments/...ctisclouds.jpg


Bob Clark