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 had completely froze the fjord.
After being aggravated by Anna, 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.
Elsa created her ice palace in the mountains above the snow line. Despite this, the first night of the winter saw snow laid down thick across the rest of the kingdom.
This made Anna 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, which Kristoff remarked as being "completely frozen", showing how bad the winter truly had become. They continued into the forest where they came across a forest of frozen willow trees. This gave the trio a chance to see the bueaty of winter through the sparkiling 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 Anna's horse 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 huge blizzard, making navigation on the fjord nearly impossible.
The storm eventually ceased and was brought to a complete end through the Great Thaw.