One of mine won't pee or do the other in the rain - or even if the ground is wet! :( When she was younger I would stay out for an hour trying to get her to go. Record was three hours, I was determined she would go but she didn't :( she went just inside the door as I walked in

From that day I threw all the toilet training books out and just kicked her out and didn't let her back in until she'd 'gone' I had to stand in the bath to make sure (the only way of looking out of the bathroom window!) but she now knows that if she doesn't go she stays there. Her daughter, I just put her out in the rain and didn't let her in until she was done, she now races out and does the necessary in double quick time ;)
Strangely, though both of them HATE the rain because they're made of sugar and won't walk even through a puddle, they LOVE playing in the water from the hosepipe

I can only think that the drenching they get from hosepipe water has strange magical powers!