You frequently hear said Mars is much drier than any place on Earth, as in that news release:

"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."

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."
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 a as low as 10 precipitable microns.
However, I believe there are some locations 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 Labyrinthus