Questions & Answers about Veðrið er kalt svo ég fer ekki út.
Because kalt here is a predicative adjective—coming after the linking verb er (“is”). Predicative adjectives always appear in the strong (uninflected) form. Only attributive adjectives (those placed directly before a noun) take weak endings if the noun is definite. E.g.:
• Predicative: Veðrið er kalt (“The weather is cold”)
• Attributive: kalda veðrið (“the cold weather”)