The influence of the season on the patch-to-background density ratio of polar cap patches in the nightside ionosphere was observed above northern Scandinavia around solar maximum (1999–2001). This is the first study of the seasonal effect in the nightside polar ionosphere. The observations were conducted by the European Incoherent Scatter Svalbard Radar under conditions favorable for patches based on the high-latitude plasma convection pattern, the interplanetary magnetic field, and an absence of in situ precipitation. Patch-to-background ratios of up to 9.4 ± 2.9 were observed between midwinter and equinox, with values of up to 1.9 ± 0.2 in summer. As the patch-to-background ratios in summer were <2, the enhancements could not formally be called patches; however, these were significant density enhancements within the antisunward cross-polar flow. Aberystwyth University's PLASLIFE (PLASma LIFEtime) computer simulation was used to model the observed seasonal trend in the patch-to-background ratio and to establish reasons for the difference between winter and summer values. This difference was primarily attributed to variation in the chemical composition of the atmosphere, which, in summer, both reduced the electron densities of the plasma drawn into the polar cap on the dayside and enhanced plasma loss by recombination. A secondary factor was the maintenance of the background polar ionosphere by photoionization in summer.