Relative humidity measurements near saturation at temperatures well below 0°C

During our yearlong deployment at SHEBA, the experiment to study the Surface Heat Budget of the Arctic Ocean, we collected the most extensive set of surface-level humidity data ever obtained over sea ice. We typically had 13 humidity sensors operating simultaneously and recording hourly data in and around the main SHEBA ice camp. These sensors were three different types of Vaisala capacitance probes and experienced temperatures between -42° and 4°C. We also have four months of hourly data collected with a cooled-mirror dew-point hygrometer that operated over sea ice in the Weddell Sea (Antarctica). This sensor experienced temperatures between -38° and -2°C. All sensors agree that, in all seasons, the near-surface relative humidity over sea ice, when figured with respect to saturation over ice, is virtually 100%. We attribute this result to open leads that can easily supply enough water vapor to routinely saturate the thin polar boundary layer. We also identified and will discuss some ambiguities that all these sensors displayed at air temperatures below -25°C.