Rain Drip Shader/Material

Okay, I have been wondering for quite a while now and surfed the forums with no avail. I have been wanting to have animated 'Rain drip/streaks' on a window, and/or the main camera when you look up (We will cover that after)

I really need to find out how to create the rain drip effect on the window. I have tried using the (Pro) Glass texture option and using a Normal Map which gives that refracted 'water' look but I do not know how to animate the normal map, I have tried using a Spritesheet.js but that doesn't work on Normals.

If there is any other way to give this water drip/streak/droplet effect on a material/shader the information would be much appreciated.