The B-29 bomber, used in WWII, was designed as a high altitude bomber, capable of operating from altitudes in excess of 30,000 feet. However, starting with their first missions to Tokyo in Nov 1944, the B-29s encountered a 200+ mph jet stream which made accurate bombing impossible.
After General LeMay arrived in Jan 1945, he reduced their altitude to around 20,000 feet - sometimes as low as 8,000 feet. Because of improved bombing accuracy, higher bomb loads and acceptable losses, the B-29s never returned to 30,000 feet (except for recon and atom bomb missions).
The public perception appears to be that the B-29 could not be used as a high altitude bomber because of the jet stream. (There were other problems, of course, but the jet stream always seems to be singled out as the reason.)
The picture above has been described as an illustration of the effect of the jet stream on bombing accuracy. See XXI Bomber Command
My understanding of the jet stream (including the jet stream over the US) is that it is stronger in the winter, fairly localized and often changes speed, altitude and location.
Was it just bad luck that the B-29s encountered the jet stream over Tokyo in the winter of 1944? Could the jet stream have moved or weakened by Mar 1945 - when the low level raids were first conducted? Or is there a persistent high speed jet stream over Tokyo or mainland Japan every winter?
Vern Haugland of the AP wrote a lengthy article "World's Most Violent Winds Are Problem in Bombing Tokyo" [Evening Star, Washington, DC (Jan 8, 1945), Page A-2] which described, in great detail, problems caused by the winds encountered over Tokyo and the extreme cold at 30,000 feet.
The article did not use the term "jet stream". That term did not appear until 1949 when scientists published studies describing the discovery of "the jet streams of wind blowing from east to west around the world at different latitudes". [Evening Star, Washington, DC (Apr 20, 1949), p. A-6]
Thus, it is possible that, given their limited knowledge of what was happening, the military did not realize that what they were dealing with could have been a temporary phenomenon.
p.s. The picture above is probably not an illustration of problems caused by the jet stream because both the aircraft and the bombs should be traveling at the same speed within the jet stream. Instead, it could be illustrating the effects of wind shear, where winds at different altitudes are moving at different speeds.