You don’t really have to go to Siberia to get minus 50 degress celsius. Mostly if you’re flying at >41,000 feet the temperature goes lower than that that you better be sure your anti ice working. :-)
Temperature drops 2deg.Celsius per thousand feet. This is just a rough estimate. Some books state 2.5 and some state 3.0
When we were flying down to Peru in November, the external temperature shown on the video screen varied between -50 and -60, and that was even as we crossed the equator.