Formula D
How does the weather change during a race, and when does it become fixed?
The rulebook's Weather Conditions section gives the chart shifts and the fixed ends.
The black die sets the starting weather from the chart. In a changeable race, a 5th-gear roll of 20 or a 6th-gear roll of 30 triggers another black-die roll: good weather moves the conditions one line up, rain moves them one line down, and changeable leaves them as they are. Good and rain are fixed once reached, and a rain result makes the whole race wet.