I have observed that the power lines buzz louder when there is less moisture in the air.
Why is this?
If it will help the lines are located on the foot hills of a nearby mountain.
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In 1999, the president of the IEEE Power Engineering Society, Robert Dent, noted that:
"The degree or intensity of the corona discharge and the resulting audible noise are affected by the condition of the air--that is, by humidity, air density, wind and water in the form of rain, drizzle and fog. Water increases the conductivity of the air and so increases the intensity of the discharge [..] corona discharges can be produced by water droplets, fog and snow.". (Ref)
The speed of sound is only about 0.1% - 0.6% faster in dry air than in humid air at the same pressure and temperature, but humid air is also less dense than dry air, so my best guess is that the vibration from the wires couples less strongly to air that has moisture in it than it does to denser dry air.