Comprehensive car insurance covers damages from an "act of God," or events that are not caused by a car driving into something else. An "act of God" can include things like damage from a heavy tree branch falling on your car. Since you have no control over when or why a tree branch would fall on your car, this kind of accident would be covered under your comprehensive policy.
^ Berger, Allen N.; Cummins, J. David; Weiss, Mary A. (October 1997). "The Coexistence of Multiple Distribution Systems for Financial Services: The Case of Property-Liability Insurance" (PDF). Journal of Business. 70 (4): 515–46. doi:10.1086/209730. Archived from the original (PDF) on 2000-09-19. (online draft Archived 2010-06-22 at the Wayback Machine)