 In January 1945, LeMay was transferred from China to relieve Brigadier General Haywood S. Hansell as commander of the XXI Bomber Command in the Marianas.He became convinced that high-altitude precision bombing would be ineffective, given the usually cloudy weather over Japan.Furthermore, bombs dropped from the B-29s at high altitude (above ) were often blown off of their trajectories by a consistently powerful jet stream over the Japanese home islands, which dramatically reduced the effectiveness of the high-altitude raids.Because Japanese air defenses made daytime bombing below jet stream-affected altitudes too perilous, LeMay finally switched to low-altitude nighttime incendiary attacks on Japanese targets, a tactic senior commanders had been advocating for some time.