I did quite a bit of testing on Eiger Nordwand Short Track Weather Change... With rain NON-fixed.
When I say testing, not actual racing, but I don't think it matters... I left a car on the track while doing other things (not on the ps3). I did this several times.
My conclusion is that what you get is random.
HOWEVER
There do seem to be some typical patterns, as to how long the rain will last.
But though I found patterns with the VISUAL appearance of rain... The changes in the track grip can vary wildly from that. So there seems to be 2 factors going on there. The visual of the rain... and the slippery condition of the track during rain, or after the sun comes out and the track starts to dry.
Even during what looks like heavy rain, the grip of the track varies a great deal!! It will always be more slippery during rain than on the REGULAR track. It's never grippy when it's raining. But there seems to be a variation of just how bad it might be while raining... or how long it will take for the track to dry after the sun comes out.
For example, in the Spitfire races we had 3 divisions. So that was 3 separate
identical (as far as settings) races on Eiger Nordwand Short Track weather change 18 Laps... sports soft tires. Which in the Spit at that restriction, was roughly a 1/2 hour or less - even with the worst conditions. So from the time the track was loaded for qualifying, to the end of the race, 45 minutes or less would have elapsed.
Division X race @ Eiger
(photos here) - Rained the entire time. It was very heavy through qualifying & most of the race. The sun never appeared. However, though the track
was slippery... it really was not that bad at all, especially compared to how bad it is sometimes. Nobody was really fussing hard. Not like a dry track, surely. But the hairpins were fairly manageable, and the sweeping turn was rather okay as long you stayed on the driving line.
Division Y race @ Eiger
(photos here)- It was raining at the start of qualifying, but not very heavy. By the time the race started the sun appeared. However, the track remained INCREDIBLY slippery for the ENTIRE race. Right from the get-go, everyone was fussing at a spinning crawl around the hairpins going sideways counter-steering like mad. It took til about lap 7 before people started picking up the pace just a tad. But there was always mist behind the cars, even on the last laps, when the sun had been shining since lap 1. And just generally, I would say the track was more slippery at the start of the race, than during the entire Division X race where the sun never came out & it rained the entire time. And the track at the end of the 18 laps, was rather almost as slippery as the 1st race in the rain.
Division Z race @ Eiger
(photos here) - When the track was loaded, it was raining. But by the time everyone got on the track for qualifying within a minute... the sun came out. The track dried. It was a tad slippery the 1st few laps. But there was never any mist behind the cars by the time the race started. By halfway through the race, it was almost completely dry. By the last few laps of the race, I would be hard pressed to describe it as any more slippery than the regular non-weather track.
One thing... in many test sessions, once rain stopped, it never started again even after 1-1/2 hours. So I have no clue how long you need to have the track up & running for the rain to stop, and then start again.
It seems to me like if the track from loading to end of race is going to be less than 2 hours - if it stops raining & the sun comes out - the rain will likely not come back.
I have never done any testing with sunny non-fixed.
And I have not done any tests on any other weather change tracks, so they may vary as well.