The atmospheric pressure on Mars is claimed by NASA to be only 0.6% that of Earth's atmospheric pressure. At this pressure, water exists only in the solid and gas phases, and not in the liquid phase. However, the atmosphere on Mars has clouds. On Earth, clouds contain water in the liquid phase. Mars also has hurricanes.

Despite the supposed Mars atmospheric surface pressure of only 0.6 kPa, Mars has wind pressures capable of starting and sustaining major dust storms. Surface gravity on Mars is 0.376g, or 37.6% that of on Earth. The highest wind speeds claimed for Mars are less than 600 kilometres per hour. A wind speed on Mars of 600 kilometres per hour would then give the same wind pressure as a wind speed on Earth of 10 kilometres per hour.

On Earth, how much dust is picked up with a wind speed of 10 kilometres per hour? On Earth, how many dust storms have a wind speed of no more than 10 kilometres per hour?

I recently discovered some new complicating factors that I did not previously consider so please disregard, for now, the Earth-equivalent wind speed number of of 10 kilometres per hour. (Sorry about that.) It is a complicated calculation.

People, who have spent more time researching dust storms on Mars than I have, have written:

http://marsprogram.jpl.nasa.gov/mgs/sci/fifthconf99/6117.pdf
“A major source of uncertainty, in the modelling of dust on Mars, concerns the mechanisms by which dust is injected into the atmosphere. Observations indicate that the sizes of aerosol particles are of order a few micrometres, yet theoretical arguments and wind tunnel data using simulated Mars conditions [4,5] appear to show that wind speeds are generally not high enough to surmount the large thresholds required for dust to be lifted from the surface simply by the near-surface wind stress.”

http://www.lpi.usra.edu/meetings/lpsc2009/pdf/1961.pdf
“It is not yet fully understood how particles of the order of a few µm in size are lifted from the surface into the atmosphere [3, 4]. The most common assumption is that wind stress picks up dust particles from the surface [5]. Wind speeds at mean elevation must be larger than ~30 m/s to lift particles and place them into atmospheric suspension [5, 6]. Such high winds occur locally on Mars, but the average wind speeds as measured at the Viking lander sites and values derived from global circulation models are much lower [7, 8]. At the elevations of the Arsia Mons Caldera the atmospheric pressure is around 1 mbar, which requires wind speeds 2-3 times higher than at the Mars mean elevation for particle entrainment.”

The first post above incorrectly states: “A wind speed on Mars of 600 kilometres per hour would then give the same wind pressure as a wind speed on Earth of 10 kilometres per hour.”

The correct statement is: “A wind speed on Mars of 100 kilometres per hour gives the same wind pressure as a wind speed on Earth of 10 kilometres per hour.”

Also, wind speeds in dust storms on Mars have been measured to be between 50 and 100 kilometres per hour.

Http://www.lpi.usra.edu/meetings/lpsc2009/pdf/1961.pdf
“... At the elevations of the Arsia Mons Caldera the atmospheric pressure is around 1 mbar, which requires wind speeds 2-3 times higher than at the Mars mean elevation for particle entrainment.”

According to the “Royal Netherlands Meteorological Institute”, “The wind pressure can be approximated by:
Pressure = ½ x (density of air) x (wind speed)² x (shape factor)”.

Tripling the wind speed, but keeping the air density and the shape factor constant, results in 9 times the wind pressure. Requiring wind speeds 3 times higher is like requiring wind pressures 9 times higher.

Changing the wind pressure to be 9 times, but keeping the wind speed and the shape factor constant, requires 9 times the air density. For an ideal gas, 9 times the air density requires 9 times the absolute pressure, keeping everything else constant.

The claimed atmospheric pressure at the bottom of Valles Marineris is 0.9 kPa. 9 times this claimed atmospheric pressure is about 8 kPa.

According to the pressure-temperature phase diagram for water, for an atmospheric pressure of about 8 kPa, or 0.08 atm, ice water melts at about 1°C, and liquid water boils at about 60-70°C.

Http://www.lpi.usra.edu/meetings/lpsc2009/pdf/1961.pdf
"The analysis of active high altitude dust devils indicate that they do not follow the season of maximum insolation. The initiation of dds only by heating of near surface air by insolation as well as the dust lifting process under such low pressure (~1 mbar) environments are difficult to explain."

Dust devils are common on Earth and Mars. There are great similarities or overlap in 11 traits: (1) Geographic distribution, (2) Seasonal Occurrences, (3) Electrical Properties, (4) Size, (5) Shape, (6) Direction of Rotation, (7) Diurnal Formation Rate, (8) Lifetime and Frequency of Occurrence, (9) Wind Speed, (10) Core Temperature Excursions, and (11) Dust particle size. The only significant differences lie in measured absolute and relative pressure excursions in the cores of Martian and terrestrial dust devils. These differences hinge on the accuracy of 1 type of instrument used (a Tavis magnetic reluctance diaphragm used for the two Vikings and one Pathfinder lander), and a 26-gram instrument developed by the Finish Meteorological Institute. The device is based on the Vaisal Barocap technology. If the Tavis device was flawed, then the Vaisala sensor is likely questionable. Why? Because it was built to record and/or send data on the wrong pressure range - only 5 to 12 mbar. In other words, it could only be properly used to record pressure ranges indicated by the earlier Tavis device. The implication of this is that Mars might have far higher pressure than previously believed, and it may be far friendlier to life than currently advertised. All this is discussed in detail on my site at http://davidaroffman.4t.com/photo5.html. This investigation is ongoing, I still need to hear from the folks who built the Tavis sensor.