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When you start your vehicle the hot exhaust gases contact the cold exhaust pipe(s) causing condensation. This is quite common and will quit after a mile or so.
On Cool mornings when the dew is falling or has just fallen we experience high humidity which is moisture if we payed attention during our school years. Now, your engine is a conductor to moisture aswe know from the appearence of water on the body. It also collects inside where it isn't visible to us. Here's another fact. Gasoline and Diesel both are comprised with a level of water/moisture. I can explain it best by telling you to stop by any Truck Co and watch as a truck that starts up after setting, you'll see the Blue smoke, then it changes to White, then dissappears until it's under a load where it changes to Black. Same with the farmers Diesel tractors. It's just that Gas burns HOTTER than Diesel and most people aren't aware of it when it happens in summer.