The daily mean pressures at two locations on Mars, observed over 57% of a martian year, reveal a semiannual oscillation with a peak-to-peak difference that is 26% of the mean pressure. This intrinsically martian phenomenon is caused by exchange of CO2 between the atmosphere and the winter polar caps. The results are quantitatively in agreement with the simple model of Leighton and Murray (1966), indicating that radiaion and conduction are the dominant processes, while atmospheric heat transport is a minor effect. There is no evidence to support a significant role of adsorption on the regolith, on an annual time scale. Evidence is presented that the difference in pressure at the two landers varies with season and that the seasonal variation is not completely removed by hydrostatic correction for the difference in elevation. The mass of CO2 sublimed from the south polar cap is estimated to be ?7.9¿1012 metric tons, corresponding to a mean thickness of solid CO2 over the maximum extent of that cap of ?23 cm. Estimates are formed of the meridonal wind speed conveying gas out of the dissipating cap and the associated zonal geostrophic wind, both averaged over longitude. The results are ??2.3 ms-1 and Ug? 14 ms-1, respectively.