Game concept
Winter Severity
From the game's encyclopedia
Winter steadily drains Supply over time, making armies lose Supply while standing in a barony affected by Winter. The Supply penalty increases with each severity. Winter has three different severities; Mild, Normal, and Harsh.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Winter Severity is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Winter steadily drains Supply over time, making armies lose Supply while standing in a barony affected by Winter." It covers Mild Winter, Normal Winter and Harsh Winter. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.