It's the TE-form of the verb, in this case, the dictionary form is ATSUKU NARU. Though we know the sentence would be continued, for example, ATSUKU NATTE KIMASHITA (it's getting hot), I need the whole sentence if you have. Anyway, ATSUKU NATTE doesn't mean "TOO HOT", as you said, it's "ATSUSUGIRU / ATSUSUGIMASU". *about the TE-form, please take a look this page: http://japanese.about.com/od/grammarlessons/a/031101c.htm I hope this was helpful.