Vertical air movement at 600 mb, computed daily by the Joint Numerical Weather Prediction unit of the National Meteorological Center, and averaged by months as part of a project sponsored by the Office of Civil and Defense Mobilization, is about 0.4 cm/sec over much of North America; the standard deviation is roughly the same magnitude. Averages computed separately for the 00 UT and 12 UT observations show good agreement in the various months, and change slowly from month to month during the six months, December 1958 through May 1959, analyzed.