I went out to my car to leave work for lunch and discovered that my rear windshield had shattered between my arrival and leaving for lunch. I found out by closing my drivers’ door and having a significant portion of the rear windshield fall into the back of the car, scattering glass everywhere. The entire rear glass was shattered and fell out of place with any movement of the car. There were no apparent cracks in the glass prior to shattering. The weather was unusually cold for this region, ranging between 16-22 degrees F when it shattered, and the car was parked in a parking deck, covered but exposed to the weather. Sky was clear and no rain/snow. I was on the 3rd level of the parking deck and backed into my space, facing the exterior of the deck, so no opportunity for a projectile to shatter the window.