Researchers have discovered the existence of high-altitude “radiation clouds” that can expose airplane passengers to high levels of radiation. These clouds were discovered as part of the NASA-funded Automated Radiation Measurements for Aerospace Safety (ARMAS) program.

By Avery Thompson Popular Mechanics 2-7-17

It’s widely known that radiation levels are generally higher in the upper atmosphere versus on the ground, simply due to the higher levels of cosmic rays. However, when studying these levels of radiation, researchers detected small pockets where radiation levels suddenly spiked, up to double the normal level. These spikes could not be explained by normal sources of radiation like cosmic rays.