Elsa's winter was a period of abnormal weather characterized by the freezing of summer in Arendelle. During this period, temperatures plummeted, and ice and snow covered the landscape. It was triggered by Elsa's flight from Arendelle after she completely froze the fjord.
After being aggravated by Anna during her coronation, Elsa exposed her powers to the kingdom and decided to flee. However, as she raced across the fjord, Elsa inadvertently caused the water to freeze; after the queen fled into the mountains, snow began to fall, worrying the citizens of Arendelle. Hoping to bring back summer, Anna set out in search of her sister, leaving Prince Hans in charge during her absence.
After she had fled from Arendelle, Elsa created her ice palace on the North Mountain above the snow line. Despite this, the first night of the winter saw snow laid down thick across the rest of the kingdom.
Journey to restore summer
The harsh weather conditions made Anna's journey into the forest particularly difficult; she quickly encountered trouble, but was able to recruit Kristoff to help her continue with the journey from Wandering Oaken's Trading Post.
Anna, Kristoff, and his reindeer Sven continued their journey into the mountains; from a high viewpoint, they were able to catch sight of Arendelle. The entire region was completely covered in snow, and its severity led Kristoff to question if it could be undone. They continued into the moutains where they came across a forest of frozen willow trees. This gave the trio a chance to see the beauty of winter through the sparkling frozen water dropelts hanging from the branches. A nearby waterfall had also frozen solid.
Meanwhile back in Arendelle, the citizens were trying to cope as best as they could with the unnatural weather. When Kjekk returned without her, Hans decided to venture into the mountains with a small task force to find her. His mission resulted in returning to Arendelle with Elsa, where she was locked up in the dungeon.
Despite this, the winter only became worse as Elsa's fear grew. Even though she was in the dungeon at the time, the wind was already picking up outside on the fjord. After she had escaped onto ice, the winds grew into a giant blizzard, making navigation on the fjord nearly impossible.
The storm was eventually ceased by Elsa's grief, and after her reconciliation with Anna, she was able to use love as a means to end the winter through the Great Thaw.