Accurately determining and reducing the climate impact of aviation and its uncertainties is one of the pressing challenges of our times. Contrail cirrus are estimated to contribute more than half of the total effective radiative forcing from aviation, yet the uncertainties in their optical and radiative properties are large. In contrast to midlatitude cirrus, high-latitude cirrus are less anthropogenically influenced; thus, they are more pristine. However, little is known about Arctic cirrus properties and their role in the amplified warming of this region. The Cirrus in High Latitudes (CIRRUS-HL) mission using the High Altitude and Long Range Research Aircraft (HALO) provides measurements in mid-and high latitudes during summer (June/July) 2021, exploiting HALO’s capabilities and a comprehensive cloud–aerosol–trace gas and radiation instrumentation. The results of 24 HALO flights provide new insights into both natural cirrus and contrail cirrus properties in high (60°–76°N) and midlatitudes (38°–60°N). In particular, we find lower ice water content (−42%) and lower number concentrations (−88%) of cirrus particles with larger mean diameters (+22%) in high latitudes. Ice supersaturated regions were frequently observed in midand high latitudes, with median in-cloud relative humidity over ice between 105% and 122%. Mean aerosol number concentrations in the midlatitudes were reduced by up to 80% compared to pre-COVID-19 times. Less air traffic during the COVID-19 lockdowns, reduced contrail cirrus coverage, and lower ice nucleating particle concentrations in high latitudes help to explain the observed differences in cirrus properties. The extensive dataset will be used to improve weather and climate models.