I agree with Doremus. I've also noticed that the angle of the sun in the sky makes a much bigger difference on paper negatives than on panchromatic film, and quite a bit more than your light meter will indicate. I think as the sun gets closer to the horizon, the spectrum shifts strongly to the red.
Just a couple weeks ago, I repeated a paper negative pinhole exposure that was perfectly exposed at 3 minutes a few days before. Both days were cloudless and I put the camera in exactly the same spot. I knew it was closer to sunset, so I exposed 7.5 minutes, but the result was very badly underexposed. My camera light meter readings differed by a bit less than 1 stop.