The planetary boundary layer height (PBLH) is a diagnostic field related to the effective heat capacity of the lower atmosphere and it constrains motion in this layer as well as impacting surface warming. Here we used radiosonde data from five icebreaker cruises to the Chukchi and Beaufort Seas during both spring and fall to derive PBLH that were compared with results from the ERA5 reanalysis. The ERA5 PBLH was similar to but slightly lower than the observation. Clear and consistent seasonal changes were found in both the observation and the reanalysis: PBLH decreases from mid-May to mid-June and subsequently increases after August. The comparison with ERA5 shows that biases in PBLH are a function of wind direction that are largest for northerly flow conditions, suggesting that the availability of upwind observations is important in representing processes active in the planetary boundary layer over the Arctic Ocean.