With GPS navigation, airplanes can fly an exact route with very little margin of error. This results in plane flying through the same piece of sky....and the same air.
More than a few times I've had my plane rocked by flying in trail or a heavy jet. Even with FAA required entrail spacing, wake turbulence can occur.
On a trip back into base recently I snapped a photo of contrails produced by aircraft ahead of us that shows how we all fly through the same air. Kinda neat.
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