I recently drove to Montreal and back in my Model S (I've done this many times in the Roadster, but this was my first in the Model S). Driving out in the afternoon, it was raining steadily. I was getting 245 Wh/km, going right on 100 kph the whole way. That's quite a bit more than normal. Usually you can get Rated Range at around 90-95 kph. Driving back at night, the rain had stopped but it was a bit cooler. I was getting under 200 Wh/km while going 100 kph. I actually sped up to 110 kph and it still consumed less power than the outbound trip. In previous trips with the Roadster I found the power consumption was the other way 'round - a little more going westbound than eastbound. Probably due to prevailing winds. No significant elevation changes. Was only mildly breezy, and since the weather was coming from the West I doubt I was hitting headwinds. Was that ~ 20% energy difference due to the rain? I've not done a whole trip in the rain before, but neither do I recall seeing that much effect.