Water vapor in the winter arctic tropopause region is important because, after the tropical tropopause region, the winter arctic tropopause has the coldest temperatures in the tropospheric northern hemisphere. This suggests the potential for cloud formation that can remove water vapor from a part of the atmosphere where radiatively active gases (such as water) exert a disproportionate influence on the earth's radiation budget. Previous work by the current authors has shown that this potential for cloud formation extends into the lowermost stratosphere, with 20% of air parcels having ozone values between 300 and 350 ppbv experiencing ice saturation in any given 10 day period during the late winter. In fact, temperatures are cold enough that 5-10% of the parcels experience ice saturation even if the water content is below the prevailing stratospheric value of 5 ppmv.

This work describes a case study of clouds observed by aircraft near the winter arctic tropopause during the SAGE Ozone Loss and Validation Experiment (SOLVE). This provided a unique opportunity to examine dehydration processes in this region since in situ water, tracer, cloud particle, and meteorological data were all available simultaneously from NASA's DC-8 aircraft. During this period, temperatures were cold enough at the tropopause to produce saturation mixing ratios of 3-4 ppmv. Thus, cirrus clouds were actually observed within the stratosphere. Back trajectories indicate that the air in these clouds came from lower latitudes and altitudes. The study describes the nature of the clouds, the history of the air, and the implications for the upper tropospheric water budget.