When an asteroid impacts a planetary surface, ejecta are excavated along ballistic trajectories whose loci define an inverted cone shape. The outward advancing motion of this curtain displaces atmosphere to generate a ring vortex whose winds can entrain, transport, and deposit ejecta and fine-grained surface materials. Curtain width and velocity, particle concentration, size distribution and motion parallel to the curtain, and the density, viscosity, and compressibility of the surrounding atmosphere all influence the vortex circulation strength. As analogs to an advancing ejecta curtain, we tested the effect of inclined solid and porous plates on vortex formation in a low-speed wind tunnel. We found that hydraulic resistance, a measure of energy losses for 1-D porous flow, governs the position along a porous plate where it becomes effectively permeable and flow separation occurs. The resulting flow field is similar to that over an inclined solid plate of the same effective length. Energy losses through the top, permeable portions of the plate reduce circulation strength by only 7% relative to flow over a solid plate. The two parameters needed to estimate circulation strength, curtain velocity and impermeable height, can thus be determined by coupling an impact model with published hydraulic resistance data. These tests also served to calibrate a numerical model, which we then applied to investigate the influence of atmospheric compressibility and particle motion parallel to the curtain (see Part 2 <Barnouin-Jha et al., this issue>). These two studies provide a method to predict the curtain-induced flow and consequent patterns of debris deposition associated with impacts on planets with atmospheres. ¿ 1999 American Geophysical Union