The one they are really watching out for is just prior to a major earthquake, the air far above the area becomes very ionized.
http://www.livescience.com/14221-earthquake-prediction-atmosphere.html
“The theory, which in science circles is called “Lithosphere-Atmosphere-Ionosphere Coupling mechanism,” goes like this: Right before an earthquake, the stressed fault releases more gases, especially the colorless, odorless radon gas.
“Once in the upper-atmosphere ionosphere, the radon gas strips air molecules of their electrons, splitting them into negatively charged particles (the free electrons) and positively charged particles. These charged particles, called ions, attract condensed water in a process that releases heat. And scientists can detect this heat in the form of infrared radiation.”
Interesting!