How does a jet engine work in the rain?
The engine is designed for it. Also large fan engines bypass most of the water around the engine core (where the fuel burns). The igniters are usually turned on so the engine doesn’t flame-out (they are normally off since the flame is self sustaining). However rain DOES NOT increase the performance of the engine like water injection does. When rain goes into the engine it turns into water vapor after the first few compressor stages, but it displaces the air. Vapor cannot sustain combustion so the engine loses power and water vapor is less dense than air so mass airflow goes down and thrust is lost. For water injection, the water is injected under pressure into the air that has already entered the engine, so it adds the to mass airflow. To compensate for the temperature loss, alcohol is added too.
