Due to hotter weather and more days without rain, the length of forest fire season has grown by 19 percent over the past 35 years, says a team of researchers. South America had the largest increase, at 33 days. In addition, the area affected by the longer fire season has doubled in size since 1979. “For things to burn, they have to have the proper conditions,” said Mark Cochrane of South Dakota State University, one of the researchers. Four factors increase the chance of fire: high temperatures, low humidity, high wind speeds and rain-free days. In their project, researchers combined fire-danger indexes with global climate databases to see where conditions favored fires and for how many days. All four factors trended toward fire-supporting conditions over the time period that was studied.