Excitation energy can be deposited in air by a number of different mechanisms:
* Ionizing radiation is the cause of blue glow surrounding sufficient quantities of strongly radioactive materials in air, e.g. some radioisotope specimens (e.g. radium or polonium), particle beams (e.g. from particle accelerators) in air, the blue flashes during criticality accidents, and the eerie/low brightness "purple" to "blue" glow enveloping the mushroom cloud during the first several dozen seconds after nuclear explosions near sea level. This post-explosion effect has been observed only at night from atmospheric nuclear tests owing to its low brightness, with observers noticing it following the pre-dawn Trinity nuclear test, as well as Upshot-Knothole Annie, Operation Fishbowl, and the Cherokee shot of Operation Redwing.
* Within minutes after the steam explosion that caused the Chernobyl accident at 01:23 local time, a number of employees at the power station went outside to get a clearer view of the extent of the damage. One such survivor, Alexander Yuvchenko, recounts that once he stopped outside and looked up towards the reactor hall he saw a "very beautiful" laser-like beam of light bluish light, caused by the ionization of air, that appeared to be "flooding up into infinity".
* Cathode rays in air produce this blue glow.
* Electrical discharge in air is the cause of blue light emitted by electric sparks, lightning, and corona discharges (e.g. St. Elmo's fire).
* Auroras, the sometimes observable blue-violet hues emitted by nitrogen at lower altitudes