Translation:I am sweating because the veranda is so warm.
I wouldn't say it's because of "niin". I think the main reason here is that saying "veranta on kuuma" sounds like it's so hot you can't even step on it without burning your feet. But then again, I'd definitely use "kuuma" with other contexts anyway but usually with the inessive (-ssa/-ssä) or adessive (-llä/-llä) case - "terassilla on kuuma" = "it's hot at the terrace" or "teltassa on kuuma" = "it's hot in the tent". Basically you could also say "verannalla on kuuma" but for some reason it sounds weird for me, too, I'd probably use that if I was at that veranda myself and for saying "Tässä/Täällä verannalla on muuten tosi kuuma.", "It's, by the way, pretty hot at/on this veranda."
And I'm aware that using the adessive case sounds the same as saying "minulla on kuuma" aka "I'm feeling hot/really warm", which is probably why I'd prefer the latest sentence I wrote.
Thank you. It does show that even for Finns language is not carved in stone, i.e. some interpretation is needed. The problem for us foreigners is though that our intuition was not developed in Finnish but a different language. Still, thank you for the explanation.