Question

why does water appear on the outside surface of our car when ac is on?

Answer 1

STFW: http://en.wikipedia.org/wiki/Condensation
AC cools the window -> water condenses
If you ask why it is on the outside: The air outside the car must have more water vapour than the inside. (During winter (I have never experienced condensation during summer) it is usually on the inside.

Answer 2

the window pane cools resulting in the water vapour in the environment to cool down and form water droplets u can see the same thing happening if u put cold water in a tumbler