If this were winter I would agree with the previous post. In the middle of summer, especially when it is hot, you don't normally see water vapor on a cold start. It could be a leak in the head. There is no gasket. Volvo uses some kind of sealant. Take a thin black marker pen and mark the side of the coolant reservoir when the engine is cold, and then watch carefully the next week or so, before you start, whether the coolant level stays exactly the same or whether you losing some. If the latter, then you might suspect a leak in the head. Dick