For several races and events, you can select your weather to be dry, rainy, or mixed. The rainstorm is a torrential downpour with a difficult to see track and slippery handling when braking and turning. This can be a little too hard at times since you must literally have a course memorized to successfully navigate it. The mixed weather is interesting because you start in either the rain or under the sun, and the conditions will slowly change to the other during the race. This greatly affects your tire choice during pit stops, and is fantastic to watch too.
Clouds slowly roll in lap after lap when a storm approaches and the rain begins as a drizzle.
Sometimes it will build into a deluge while other times it will taper off. As a heavy storm clears, the sun pokes through the overhead clouds with some rain still falling. These conditions are selectable except in Championship mode where it is completely random from race to race. Another neat feature is the day to night to day transition in the actual Le Mans event. The sun slowly sets, allowing night to settle in.