“Rainbows are formed when light shines through water. The light is bent and reflected, often leading to the formation of the colors of the rainbow.” – How Stuff Work
It appears mostly during rainy conditions and can show up on other occasions too. Similar to a science experiment explosion in a chemistry lab.
What is it?
“Lightning is an electrical discharge caused by imbalances between storm clouds and the ground, or within the clouds themselves. Most lightning occurs within the clouds.” – National Geographic