The first pun is when the original poster says, "Fuck the weather," meaning that he is expressing an extreme disdain for the current weather, but he twisted its meaning to "have sexual intercourse with the weather." The second one is stating that it is under 18. Literally, this means that temperature is lower than 13 degrees, but another meaning can come from the age one becomes an adult. Altogether, OP is saying that you cannot perform sexual intercourse with the weather because it is under-aged. There were probably a shitload of grammatical and spelling errors, but whatever. I need some sleep.