Learn Icelandic: Það er rosalega kalt úti. - It is extremely cold outside.

It has to match its subject. The sentence uses the dummy subject það (it) to talk about the weather. Since það is a neuter pronoun, the adjective must take its neuter form, changing from kaldur to kalt.

Where does the word rosalega come from?

It is an adverb formed from the adjective rosalegur, which means "terrible" or "huge". By changing the ending to -a, it becomes an adverb. In everyday Icelandic, it's a very common way to emphasize things, much like saying "terribly cold" or "crazy cold" in English.

I've seen the word út used for "out" before. Why does this sentence use úti?

Icelandic uses different words depending on whether there is movement or not. Úti describes a stationary location (being outside), which fits this sentence perfectly since the weather isn't traveling anywhere. Út is used for direction (moving to the outside), like when you say ég fer út (I am going out).

If I want to emphasize the location and start the sentence with Úti, how does the sentence change?