premise: The jet stream is a two-mile-high column of wind and in this part of the world it deter- mines the boundary between the Arctic and Atlantic air. When it moves south, much of northern Europe and North America is subjected to cold air and when it moves north, northern Europe and North America enjoy warmer air. In the spring and autumn the jet stream moves much more than at other times of the year and this largely explains why spring and autumn weather in the northern hemisphere can be so changeable. In a matter of a few days the position of the jet stream can fluctuate and warm and possibly wet spring or autumn weather can suddenly be replaced by cold air and dry, clear skies.
hypothesis: If it were in fact untrue that the jet stream was a two-mile-high column of wind then the explanation of why spring and autumn weather in the northern hemisphere is so changeable would be compromised.