Polar stratospheric clouds form at extremely low temperatures in the upper atmosphere. Should the temperature rise, clouds won't form. In this visualization, sequential temperature readings taken in the research area for SOLVE (Stratospheric Ozone Loss and Validation Experiment) are plotted against a threshold temperature for PSC formation. These are clouds essentially made of nitric acid. Note how the area covered by the clouds increases as winter progresses. The red point on the map indicates the location of Kiruna, Sweden, the SOLVE staging area.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 184.108.40.206.0