Multiple <a href="https://en.wikipedia.org/wiki/contrail" class="extiw" title="en:contrail">contrails</a> in an area with high airline traffic.
NOAA
<a rel="nofollow" class="external free" href="http://www.wrh.noaa.gov/fgz/science/contrail.php">http://www.wrh.noaa.gov/fgz/science/contrail.php</a>