An analysis of the time and space variability of density in the middle atmosphere has been developed in this report, affording a means to determine density changes that occur over distances out to 200 nmi and time periods of 1 to 72 hr. Spatial variations - The rms differences between densities at locations 50, 100, and 200 nmi apart are provided in Tables 4, 8 and 11 for low- and middle- latitude stations at altitudes from 20 through 60 km. These rms values are estimates of the day-to-day variability around the mean density gradients given in Table 5 for low latitudes and Tables 9 and 12 for middle latitudes. Time variations - The rms variability of density for time periods from 1 to 12 hr in tropical regions is given in Table 13 for altitudes up to 60 km. Estimates were derived using autocorrelation theory and an analysis of the diurnal variation of density, which is the dominant periodic cycle at low latitudes. Analyses of available density observations at several midlatitude locations for altitudes between 30 and 60 km provide reasonable estimates of the magnitude (and changes with season and latitude) of the variability of density with time for periods up to 72 hr. Variations of density at these latitudes are relatively small during summer and do not increase significantly with time. Rms variations remain between 1-1/2 and 4 percent for all periods up to 72 hr.