Shearing of a contrail occurs when the contrail is deposited in a region of windshear. This windshear can be either directional or velocity-wise. It causes the contrail, which has some extend in height, to deform or twist. This shearing is only visible if the contrail remains long enough in the atmosphere such that the windshear will have visible effect on it (typically longer than 5 to 10 minutes).
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