In Portal games, do you always exit a portal at a right angle to its surface?

Please help settle a debate between a group of friends. In Portal games, is exit velocity always normal to the portal surface being exited?

More importantly: please offer some kind of undeniable proof that will allow the debate among friends to end. I am nowhere near a machine with Portal installed for several days, but my memory of the series is that yes, exit velocity always is normal to the portal surface. Put more simply: if the exit portal is on the ground, no matter how you enter the entrance portal, you will come out of the exit portal moving vertically up. (Granted, you may be able to move out of that vertical column a split second later because of video game physics that allow forward motion to be initiated in midair, but if you do not press any movement controls after exit, you will rise/fall in a perfectly vertical column and end up re-entering the floor portal you just exited).