"It is very hot today."
Translation:आज बहुत गर्मी है।
Yes, that's true. In Hindi, when using words to describe the temperature of something, you use the adjective form (गर्म) when the corresponding noun is present just before or after the word. Otherwise, you use the noun form (गर्मी) when you want to focus on the presence of temperature. Eg: "The room is hot" can be translated as "कमरा गर्म है" or "कमरे मैं गर्मी है". As far as weather is considered, you would either say "Today the weather is hot" ("आज मौसम गर्म है", the noun being weather) or "Today, it is hot"("आज गर्मी है", the noun being the temperature itself )