"Sky holes", also known more formally as "Hole Punch Clouds" or "Punch Hole Clouds", are not some secret Government experiment, nor Alien cloaked spacecraft. They are instead, holes that form in thin, puffy cloud layers.
There are many hypothesis for what causes them: jet planes passing through the cloud layer, wind, and others. But they remain an enigmatic example of mother nature's incredibly complex weather processes.
The prevailing theory is that Cirrocumulus clouds contain both ice crystals, and supercooled water droplets. The introduction of the extra water from the jet exhaust causes a rapid increase in the rate of crystallization, and all the water turns to ice as the ice crystals get larger. This rapidly desaturates the air, which then pulls in moisture from the air around it, thus spreading the hole. The larger ice crystals then drift downwards (as virga), since they are heavier, resulting in the characteristic “V” or “wing” cross-section of the central cirrus uncinus cloud, which looks a bit like a bird or an angel, if you are imaginative enough.
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