For a fixed altitude, the known seasonal variations of pressure in the mesosphere are less than a factor of two. However, the components of the pressure, the total concentration and temperature, vary in opposition to each other. Thus, a rate for a process with an inverse temperature dependence can undergo a much larger seasonal variation than does the pressure. In particular, the loss rate associated with the three-body process Na+O2+M→NaO2+M, which has a negative temperature dependence, annually varies by a factor of four at 80 and 85 km for 60 ¿N and by less than a factor of two at 30 ¿N. Using a recent sodium model, sodium concentrations for 60 ¿N at 80 and 85 km are shown to change by factors of three and two, respectively. Other possible contributors to the seasonal variation of mesospheric sodium are argued as less important.