Please tell me why in the above sentence we write: cold, not coldly, because this sentences is the answer to the question: How is it outside today (-for example)? and the reply to the :,how’’ should be adverb: coldly and not adjective: cold.
You cannot say “It is coldly outside.” “To be” is a linking verb and therefore requires adjectives. “Cold” is a complement, something that gives information about the subject. And, of course, describing nouns is the traditional role of adjectives. “Cold” is not performing the traditional adverbial role of describing how an act was performed. There is no act here at all.
“How” does not mean “in what manner” in the sentence you used. It means “What is the nature of?”