Last year I went to a weather conference and one of the talks was on hole punch clouds and how they are made.
The latest research shows they are not created by engine combustion, which was previously thought. The first requirement is the clouds have to be vertically thin. Researchers then discovered that the clouds usually form beneath C-130 planes (shown below). Beneath the wings of these planes temperatures are around 14 degrees warmer than the rest of the plane and surrounding environment. This temperature difference creates a dry punch of air falling from the sky evaporating the clouds beneath.
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