Rain falling on fire, whether caused by players or lightning, no longer puts those fires out. Fires burn until they naturally age out. Fires also spread as though the rain was not falling. This makes thunderstorms very destructive.

While I first noticed this in 15w43b, I went back to the older snapshots and tested this. In versions prior to 15w38, rain puts fires out very quickly. In 15w38b and up, fires behave as though there is no rain falling on them.

This results in widespread destruction during thunderstorms if lightning strikes a wooden structure or a forest.

To test this, wait for rain/thunderstorm (or start one with a command) and use a flint and steel, or find a flat area and use a /fill command to create a large area of fire. In versions prior to 15w38, the rain will almost immediately extinguish all fire it touches. In 15w38b and up, the fires age out naturally, with no help from the rain. Changing the difficulty setting does not change this behavior.