The word contrail is short for (jet) aircraft condensation trail. Contrails can cause a day to be cloudy or overcast that would otherwise have been clear-sky. The presence of contrails is an indication of high moisture in the upper region of the troposphere. Part of a contrail freezes, while the other part descends (like mammatus or virga) and evaporates. The frozen remnant of the contrail may have the appearance of a thin line, or vortex. Usually the contrail will transform into cirrus or cirrocumulus, or dissipate before a transition occurs.
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