Dropsondes are launched from research aircraft to measure vertical profiles of temperature, humidity, pressure and wind in the atmosphere while falling to the ground. Onboard the aircraft Polar 5 of the Alfred Wegener Institute for Polar and Marine Research (AWI), they are deployed on arctic and antarctic campaigns. Here we compare temperature and humidity data from Vaisala RD94 dropsondes with data from the permanently installed sensors of the aircraft. Dropsonde profiles are combined with profiles measured during ascent or descent by the aircraft nearby. The aerosol lidar AMALi is used to identify the presence and altitude of clouds. Furthermore the time constants of the dropsonde temperature and humidity sensors are deduced from atmospheric data. The overall agreement between aircraft and dropsonde data is good, but for high humidities as in clouds the dropsonde shows a systematic dry bias of almost 10 %. Data suggest a temperature dependency of this bias. The combination of dry bias and time lag of the sensors can make cloud detection from dropsondes alone difficult in cold environments.