Fog on camera case with chest mount I've got a chest mount for my camera (sony hdr-as15, if it's important). I went on a race last Sunday - it was like +15-17 Celsius. After about an hour of riding I've noticed that the case is all foggy and the picture turned from this
 into this: 
So, what can be done to prevent this?
Normally I would not expect this at 15C-17C, its something that happens at 5C. 

Presuming the case is sealed and was dry before closing it up, the cause is because the inside of the case was warm with high humidity before being taken into colder air. Most likely the warm air inside your home is damp, or the case has been sealed up for a while. When taken outside, the air in the case cooled, and was no longer able to hold the water vapour so it  condensed on the coldest part - the lens that points away from you as that gets no body heat. 

To prevent this, leave the camera case open when in storage, and store in a dry location (stuffed in you gear bag with your damp cloths and shoes form the last ride is really not a good idea). If the temperature changes is high - e.g when I head out on 0C degree morning, I leave the camera and case open in cold outside air before sealing it up for the ride, ideally for at least an hour.  

An alternate, if conditions are suitable, is to use a non-waterproof case that has massive slots  to allow airflow.