Ground-based measurements of the O I 8446 ¿ airglow brightness are made from the Millstone Hill and Arecibo Observatories. These measurements are made at various times throughout the year, and this paper reports those made between the years of 1993 and 1996. The maximum brightness of the 8446 ¿ airglow emission is found to be less than about 35 Rayleighs during the nighttime at both these midlatitude sites. During the summer this emission is present for only a short time in the evening and morning. During the winter the emission can persist throughout the night. This observed behavior is in agreement with the calculations of current models that predict similar intensities from excitation by photoelectron impact on atomic oxygen. A close examination of these data, however, reveal that differences exist between the model predictions and the measured brightness. One difference is a persistence of the nighttime 8446 ¿ brightness to times when the model predicts little or no emission from photoelectron production. A second difference is an occasional enhancement of the morning 8446 ¿ brightness that is not predicted by the model. The former of these differences implies a shortcoming in current understanding of the 8446 ¿ production mechanisms and must be resolved before atomic oxygen can be monitored from the ground using this emission. The latter may have important consequences for our understanding of upper atmosphere dynamics. ¿ 2000 American Geophysical Union