Grammar help please!

[NorkyButNice]

In the sentence "It is hotter today" - what is the word "hotter" (ie verb, noun, adjective, adverb etc)?

adjective

It's an adjective, in the comparative form.

I think it's an adjective. A comparative one. Because you're sort of saying "It's a hotter day today"

I thought so - am having an argument with a guy at work who reckons it's an adverb as it's describing the verb "is"?

No, it's describing the "It" - the day.

To be an adverb it would be "hotly" - you might say something hotly, then it would be an adverb because it's descibing the way you are saying it.

It is actually a predicative adjective (a type of complement). It defines or modifies the subject (in this case It)