The air is ____hotter today than yesterday.

Two days (yesterday and today) are being compared in this case. This indicates, that we have to use a comparative degree in the given blank. The adjective is "hotter". To indicate the intensity of the hotness 'much' has been used in this case. Since hot is a word with a single syllable, the comparative degree would be "hot" + "ter" = Hotter and not more hot.