how to say it's cold\hot in hangul ?
Both formal and informal ways .

Question

Answer

For drinks, we can use 시원하다 for cold drinks,따듯하다for hot drinks.   
시원한 커피, 따듯한 커피

Answer

I would say it´s cold (the weather) is 추워,추워요, 춥습니다 (less formal to formal of the verb 춥다) and it is hot (also the weather) is 더워,더워요, 덥습니다 (less formal to formal of the verb 덥다).