Using a new probe that detects tiny amounts of moisture on sand grains, a team of researchers from Cornell University, the Ecole Polytechnique de l’Université de Nantes, and the Université de Rennes determined that desert surfaces exchange less moisture with the atmosphere than expected, and that water evaporation from individual sand grains behaves like a slow chemical reaction.
Louge et al. show how water vapor penetrates powders and grains. Image credit: Pexels.
The capacitance probe developed by Cornell University’s Professor Michel Louge and colleagues uses multiple sensors to record everything from solid concentration to velocity to water content, all with unprecedented spatial resolution.
The scientists used their new instrument to study the moisture content in sand dunes to better understand the process by which agricultural lands turn to desert, an interest that has only become more urgent with the rise of global climate change.
“The wind flows over the dune and as a result creates imbalances in the local pressure, which literally forces air to go into the sand and out of the sand,” Professor Louge said.
“So, the sand is breathing, like an organism breathes.”
“That ‘breathing’ is what allows microbes to persist deep inside hyper-arid sand dunes, despite the high temperature.”
The authors found that vapor infiltration is considerably slower in dry sand, and that wind flowing over a dune creates weak internal air currents contributing to the transport of moisture. Their strength depends on dune location, wind speed and direction.
When wind is strong enough to let dry sand meander over a dune, the resulting rapid variation in surface moisture sends evanescent waves of humidity downward.
An analysis of these waves implies that water evaporation from individual sand grains behaves like a slow chemical reaction.
The exchange of moisture with the atmosphere is not always driven by the difference between humidity at the dune surface and in the ambient, as current models assume, and it is weaker than they predict.
In future, the team’s probe can be used as ‘ground truth’ to calibrate satellite observations over deserts, explore extraterrestrial environments holding scant water, and detect moisture contamination in pharmaceutical products.
“The probe will have a number of applications — from studying the way soils imbibe or drain water in agriculture, to calibrating satellite observations over deserts, to exploring extraterrestrial environments that may hold trace amounts of water,” the researchers said.
The team’s paper was published in the Journal of Geophysical Research: Earth Surface.
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M.Y. Louge et al. Water vapor transport across an arid sand surface — non-linear thermal coupling, wind-driven pore advection, subsurface waves, and exchange with the atmospheric boundary layer. JGR Earth Surface, published online March 21, 2022; doi: 10.1029/2021JF006490
