Learn Icelandic: Það er ansi kalt úti í dag. - It is pretty cold outside today.

When describing the weather or using the dummy subject það (it), Icelandic always uses the neuter singular form of the adjective. Even though weather has no physical gender, the grammatical structure requires the neuter form kalt rather than the masculine kaldur or feminine köld.

Is the word ansi considered formal or informal?

Ansi is mostly used in conversational Icelandic and informal writing, much like saying something is 'pretty' or 'rather' cold in English. If you were writing a formal weather report, you would be more likely to use words like frekar or talsvert.

I sometimes confuse úti and út. Why is úti the only correct choice for this sentence?

Icelandic strictly distinguishes between location (where something is) and motion (where something is going). Because this sentence describes a stationary state, which is the temperature right now, you must use úti, which means 'outside' indicating location. You would only use út if there were motion, such as walking 'out' into the cold.