Zach is on point.
To simplify the entire setup, let me use a thing we see commonly to simplify what happens.
Moisture that builds up on the outside of a coke bottle is really just water in the air we breathe. As the air cools from being in contact with the cold coke bottle, water dropets deposit on the surface. Now, let's apply this to your exhaust. Water in gaseous form is a part of your exhaust. Therefore, in colder climates and colder start ups, you'll see much more water coming out of the tail pipe because it is condensed down into a liquid form. The same amount of water is still coming out of your tailpipe, but because of the temperature the exhaust is at once it exits your tailpipe, you may or may not see water. Make sense?
And now I fee like bill nye the science guy...
