A fixed site monitoring station recorded the potential gradient disturbances near to two high voltage power lines during 2008. The full year's results show that the electrical environment downwind of power lines is modified compared to that upwind. Potential gradient disturbance was greater on days when there was rainfall. Humidity was inversely correlated with mean potential gradient when the station was both downwind and upwind of both power lines. Wind speed is weakly correlated with the standard deviation of a 10 minute sample of potential gradient downwind of both power lines, but not upwind. The distributions of mean and standard deviation of potential gradient in 10 minute samples showed that the field was more negative overnight and on days where there was rain, but less variable at night and on dry days. Upwind of the power lines, the average 24 hour trace exhibits the natural background Carnegie curve, with peaks corresponding to increased global thunderstorm activity, while local effects mask this trace when the FSMS is downwind of the power lines. The results show that corona ions can cause potential gradient disturbances downwind of high voltage power lines, most particularly during rain and high humidity, and overnight.