http://en.wikipedia.org/wiki/Condensation
discussion from another board: You need to run the vehicle long enough to let the pipe get hot, which stops the production of condensation. When the exhaust is hotter then the inside surface of your exhaust system, you produce condensation. That causes condensation everytime you start up the vehicle, in cold weather, you produce more. If you run the engine long enough, it will empty the water from the low lying areas, which is why you should not just jump into your vehicle in the winter, drive 1 mile and stop it. That will rust out your exhaust quickly. In the summer, you produce less condensation because the pipe and the exhaust stay at a closer temperature which produces less condensation.