Ion Storms were incredibly destructive storms caused by Tiberium. When a significant amount of tiberium gases were present in the atmosphere, these storms tended to develop.
There were many telltale signs as to the start of an Ion Storm. First off, tiberium clouds began forming high above at an alarming rate, quickly blotting out the sun in the daytime. Not long after, the Storm would begin generating electromagnetic energy, which then developed into intensely powerful lightning bolts. The electromagnetic interference shut down radar systems in the area, grounded hovercraft, and outright destroyed most air vehicles. While bigger aircraft, such as the GDI Kodiak, were safe from the EMP effect, they were not safe from the lightning bolts. The heat and energy in the bolts was enough to vapourize a human being on contact, completely destroy vehicles, and cause exception structural damage to buildings.
Though these storms only occured very often in red zones and deep yellow zones, there was a number of incidents in which an Ion Storm developed in a populated Yellow Zone, and sometimes travelled as far as into a Blue Zone. In light of these incidents, GDI set up Ion Storm Shelters designed to protect civilians from the hellish lightning.
During the First Tiberium War, the Scrin were observed utilising localised Ion Storms on Planetary Assault Carriers and Storm Columns, though how the Scrin managed this is not known, although Scrin air force activities, from weapons fire to movement, were reported at operating 5x faster than usual during an ion storm, meaning that while the bolts were indeed harmful to their infantry and possibly ground units, their air force was not harmed, and in fact operated better, giving the reason for the naming of the Stormrider. This was observed again in the Fourth Tiberium War, where Scrin warships utilised a similar Ion Storm technology.
The last reported Ion Storm was in Southeast Australia on November 18th, 2014, moving slowly across the area until the tiberium recession grid eliminated it.