Our olds had a cracked head and the smoke was ungodly amounts of white. Check your fluids. It cracked cause of a manifold gasket failed and if I had been on top of things (resivoir) I would of caught it.

Like Franc said moisture can play tricks too. My sentra seemed to be blasting out white smoke on the highway as I started my commute. As I looked around I saw a few other cars doing it too.

If your not sure you could put a code reader on it as well