This varies greatly by region, land use, and weather patterns. In some places and years, humans dominate; others, lightning. Because humans tend to start more fires near cities, campgrounds, and other accessible areas, those fires tend to be contained or extinguished quickly. Lightning starts tend to be on ridgelines, near peaks, and in other remote areas that are difficult to reach. Many of the big, long-burning fires are lightning starts, but they also tend to threaten fewer structures. Obviously there are exceptions all around.