We were driving down the highway and heard terrible sounds of glass breaking. We weren't sure what it was at first but heard it at least twice more. My wife opened the sunroof cover and saw that it was shattered. It was a very hot day and a cold front and thunderstorms were coming through (though it hadn't started raining yet). We could not identify a rock or other object that did the damage and I observed that the glass was blown outward (away from the interior of the car).