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

Although vapor exchanged across hyper‐arid surfaces without free liquid affects the water budget of sand seas, its mechanism is poorly documented for want of accurate instruments with fine spatial resolution. To rectify this, we report bulk density profiles and spatiotemporal variations of vapor mass fraction just below the surface of a mobile dune, acquired with a multi‐sensor capacitance probe sensitive to tiny water films adsorbed on sand grains. We also record wind speed and direction, ambient temperature and relative humidity, net radiation flux, and subsurface temperature profiles over 2 days. The data validate a non‐linear model of vapor mass fraction. Unlike heat, which conducts through grains, vapor percolates across the interstitial pore space by advection and diffusion. On time scales longer than evaporation, adsorbed films equilibrate with their surroundings and hinder molecular diffusion. Their non‐linear coupling with subsurface temperature generates inflections in vapor profiles without counterpart in simpler diffusive systems. Pore advection arises as wind induces subtle pressure variations over the topography. During periods of aeolian transport, flowing sand dehydrates the surface intermittently, triggering evanescent vapor waves of amplitude decaying exponentially downward on a characteristic length implying an adsorption rate governed by a kinetic‐limited activated process. Finally, the probe yields diffusive and advective exchanges with the atmospheric boundary layer. During the day, their combined flux is smaller than expected, yet nearly proportional to the difference between vapor mass fraction at the surface and aloft. Under stabler stratification at night, or during aeolian sand transport, this relation no longer holds. Plain Language Summary Deserts inhale and exhale water vapor through their surface. Although this process affects the water balance over vast sand seas, it is poorly understood for want of sensitive instruments. We discover how it operates using a new probe that detects tiny amounts of moisture on sand grains. Our analysis reveals 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 dow