It will most likely be weeks before the County Fire west of Sacramento is completely extinguished. By Friday it had consumed nearly 140 square miles -- an area larger than Las Vegas.

Firefighters say it was a vicious cycle of weather conditions, terrain and vegetation that made it one of the fastest-growing fires in recent memory. But there was something else at work: a relatively new challenge confronting fire crews.

Scientists have noted that nighttime temperatures -- overnight lows, in particular -- are rising at a faster rate than daytime highs.