It has a mean volume ratio of 94.9%.[1] In winter polar regions, the surface temperature can be lower than the frost point of CO2. CO2 gas in the atmosphere can condense on the surface to form 1–2 m thick solid dry ice.[2] In summer, the polar dry ice cap can undergo sublimation and release the CO2 back to the atmosphere. As a result, significant annual variability in atmospheric pressure (≈25%) and atmospheric composition can be observed on Mars.[32] The condensation process can be approximated by the Clausius–Clapeyron relation for CO2.[33][3]