Convince the girl that your penis size is a result of the weather.
Girls often blame things on the weather. They pull that shit all the time. Why not try it? Explain to the girl that since it is so cold or so hot, your penis is smaller than usual. You could say, “Yeah, it’s because of all the rain we’ve been having. My penis is usually monstrous but for some reason whenever it rains my penis only gets yay big.”