After reading that Bradley Wiggins raced conservatively in wet conditions at the prologue time trial of Paris-Nice on Sunday, I wondered whether the poor conditions led to average slower times in the time trial.
A scatter plot of start order on the horizontal axis to time behind the leader on the vertical axis shows that there's no discernible upward trend as the day carried on. The green line shows the overall trend. It stays horizontal rather than sloping upward - showing that the times were pretty much consistent throughout the afternoon, though the weather was changing. Why?
There are many reasons why. First, superior time trialists started toward the end of the order. If the weather slowed riders down, time trial specialists' faster times could have evened out the trend. Secondly, weather can have a lot of effects. Heavy rain slows a rider's speed, and wet roads forces a rider to corner conservatively. However, rain also reduces humidity in the air, lowering the air resistance. Wet roads also lead to less rolling resistance - meaning, faster rides. A rider might be fastest right after a rain shower.
A more thorough analysis would shed more light on the question, but based on a quick analysis I feel confident saying that though really foul weather might slow riders down, wet or simply poor weather might not have a significant effect on time trial performance.
Statisticians and others, feel free to chime in.
NB: I removed Nick Nuyens from the data since he crashed and finished over 6 minutes down in a ~12-minute time trial.