When breaking blocks underground whilst raining, if I was looking up towards the ceiling and breaking blocks, it was possible for me to get flashes of rain coming through the surrounding blocks and onto my screen. It was as if the blocks around me vanished for a split second allowing the rain particles to come through.

I believe the easiest way to recreate this would be to go into a world, go underground, turn on rain and start breaking blocks whilst looking at approx. 45 degree angle towards the roof. I was using an iron pickaxe to mine the blocks if that makes any difference.