1. When it's just THAT hot out
2. When trying to convey to someone just how hot it is, as vividly as possible
Note: Ballsack can be replaced with Camel Toe to be gender friendly
"You going to the dessert this weekend"
"Nah, Man...I checked the weather report and it's gonna be hotter than a sweaty ballsack in Dubai"
"Damn, Bro...THAT hot"
"Yeah, Man...THAT hot"