Rainbow storm, or Prisma storm, as it was also known, was a weather phenomena that occurred on Yavin 4. This brilliant display occurred when the moon traveled behind Yavin Prime and spent time in its shadow. As a result, nights would get very cold and water crystals would form in the upper atmosphere. As the moon came out behind Yavin Prime, the sunlight from Yavin (star) would be shattered by millions of prisms. The light would dance and crackle through sky to give the effect of varicolored lightning and swirling rainbows. It would only last for a few minutes, before the sun rose high enough to light up all of Yavin 4.
Rainbow storm was first recorded by the Sullustan naturalist, Dr'uun Unnh, who saw it when he came to Yavin 4 with the Rebel Alliance in 0 ABY. He told the other Rebels about it and some would make a point of climbing up high every day, so as to watch it. General Jan Dodonna noted the names of some of these people and assigned them to the newly-constructed watch towers.
Corran Horn would later watch the phenomena, when he came to the Jedi Praxeum on Yavin 4 in 11 ABY.