We've been up here for what, almost a year now?
Most of that time, Tom has spent "in the field" (ie: driving around A LOT and working on fixing things). Today, he held both hands out, palms down, and we observed that his right hand is quite a bit darker (tanner) than his left.
I think it's from driving with his right hand on the top of the steering wheel and his left on his thigh. I also think it's kinda funny that he's got his own version of a Farmer's Tan going on. :D