A transient decrease in FUV dayglow emission from a ~103 km2 region of Earth's upper atmosphere is one expected outcome of the collision of a stony meteoroid as small as 50 cm in diameter. Computational models of the entry of projectiles between 210 and 21,000 kg predict the ejection of low-density plumes of air from the stratosphere and mesosphere at velocities of up to 5 km/s. The plumes reach an altitude of over 1000 km, displacing the atomic O-rich thermosphere, and replacing it with photoelectron- and O-poor air from beneath 80 km. The FUV dayglow is normally dominated by O emissions so the plume appears as a dark spot when viewed from above. FUV dayglow holes are reported at rates ~104--106 larger than the impact flux of objects in the size range modeled here, suggesting either 1) the observed rate is greatly overestimated, 2) there is an additional mechanism, or 3) much smaller impactors (which we have not modeled) can also generate FUV darkening plumes. ¿ 1997 American Geophysical Union