The weather add-on enables random weather changes (only for races) for which you can set up a custom probability when starting a new race. If you have set up a probability higher than 0%, it may eventually start raining during a race. This will be announced with a flash message and a voice announcement. After the first announcement, the rain will start in the next laps. As soon as the rain actually starts, there will be another flash message and a voice announcement.
When the rain starts, the speed of all drivers will be reduced to a configurable value. If the driver then enters the pits and stays there for at least 10 seconds (configurable), the tyres will be automatically changed to wet tyres. As soon as the tyres have been changed, the speed will be increased again (configurable). The duration of a rain phase is random and it’s end will again be announced.
Every driver can decide to change tyres anytime, independent from the current weather. If soon-to-start rain has been announced, a driver can even change their tyres before the rain actually started, which may give them a tactical advantage over other drivers who change tyres after the rain started.