Two kanji can be used to write あたたかい: 暖かい and 温かい. The former is used especially in regards to air temperature...
Is the second kanji used for liquids or something?
I read the difference is 暖 you feel with your whole body (weather) and 温 is only part of the body (hot food).
Because I sometimes struggle with subtle differences (if any) between present and future tense, how would you say "Will it be warm today?"
Same thing. There is no distinction between present and future tense in simple sentences like this
Why is atsui not applicable here?
"あつい" means "hot". あたたかい means "warm"
In Kanji, "hot (weather)" is 暑い and "warm (weather)" is 暖かい
I wonder, why here is no Kanji accepted as an answer while, beforehand it is accepted with the same word "あたたかい"? It is the same meaning and the same word, but it is irregular accepted.