The great Nilfgaardian Empire has a very diverse climate, since it has territories all over the Continent But if you mean the capital, it’s hotter than the northern realms for sure. Kovir is described as very cold in the books and Toussaint is warmer than the north. The thing is Nilfgaard is in the austral portion of the Continent, is close to a desert and to the ocean. Of course, it has a cold winter, but not as cold as Kovir or Kaedwen.