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If you look closely at these two pictures you can see how the aircraft contrail cuts the lower cumulus clouds at a slightly different place in the second picture. It is not dramatic, but one clue that the upper winds at the altitude of the aircraft, probably about 35,000 ft, may be moving in a different direction to the lower winds moving the clouds. The upper wind direction is likely to be the more consistent and can be gauged by lining contrails or cirrus clouds up against fixed features like high landmarks, or even by lying under a tree.
