Thunderstorm

A thunderstorm is a weather condition which takes place randomly during some rain- or snowfalls. All changes from these weathers apply, but since lightning occurs and the light level falls below 7, thunderstorms are distinct. This means that during a thunderstorm, aggressive mobs will be able to spawn, even in the brightest of day.

Trivia
When Lightning hits a Pig Spawner lots of Zombie Pigmen will appear, even on a server with mobs turned off.(SinglePlayer Needs Confirmed)
 * If you are near lightning when it strikes, you will hear the same sound as that of a TNT explosion.
 * There is a small chance that lightning will set trees on fire. This can happen when lightning strikes really close to a tree and the fire starts under the tree. The rain may put the fire out before the thunderstorm finishes.
 * If lightning strikes a portal it will activate.

Bugs

 * If lightning hits a sheep, it will not get sheared.