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I noticed when walking home the other day that the high voltage (200kv I believe) lines running through here were hissing in the rain. What is causing them to hiss?

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Buzz or tssss? The lines are usually quite warm, so they might rapidly evaporate the water that falls on them. – Nick T Dec 2 '10 at 18:48
You're right, it was more of a 'hissing'. Edited – pfyon Dec 2 '10 at 19:09
those birds must have some tough feet then – Matt Williamson Dec 2 '10 at 19:49
are these the big bad cross-country lines? how many conductors? – JustJeff Dec 2 '10 at 22:16
They do? I never noticed. – muntoo Dec 3 '10 at 1:19
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4 Answers

up vote 10 down vote accepted

High-voltage partial discharges across the insulators.

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What is a partial discharge? – pingswept Dec 2 '10 at 19:26
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@Nick T - nice link! – JustJeff Dec 2 '10 at 22:18
That is some weird stuff. Thanks for the link. – pingswept Dec 2 '10 at 23:17
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I'm not going to upvote an answer consisting of only 7 words, however correct. A bit more explanation would be welcome. – stevenvh Jul 15 '11 at 17:24
up vote 8 down vote

I was taught that this is due to the Corona effect. Basically, the power lines ionize the air around them, causing audible hum, along with havoc in the EM spectrum. This is why really high voltage lines and transformers will sometimes have a slight aura around them.

Generally, the effect is undesired, because it robs the transmission lines of energy (the hum/light/heat dissipates energy), so a lot of equipment is manufactured to try and stop this effect.

The Wikipedia article will do this subject much more justice than I can.

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If it's due to this, wouldn't I hear the hum/buzz whether it was raining or not? – pfyon Dec 8 '10 at 16:43
I seem to recall that it was more likely in humid air, but I'm not particularly sure. – mjcarroll Dec 8 '10 at 17:56
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They'll actually do it when it's not raining too. It's called mains hum. Power lines carry AC voltage at either 50 or 60 Hz which is at the low end of the audible range of most humans. In the presence of an electromagnetic field (like the one generated by AC power), the molecules of ferromagnetic materials (the metallic conductors inside of power lines) will not only try and align themselves with the field but sometimes change or distort their shape if the applied potential is strong enough. This alignment/change in shape can cause collisions between the molecules comprising the power-lines which, given enough of them, can be heard by an observer.

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...but if it gets annoying, you could just think of it as a million electrons clapping their hands & cheering you on in your journey home – Joel B Dec 3 '10 at 1:11
as per my comment to mjcarroll, why do I hear the hum/buzz only when it's raining? – pfyon Dec 8 '10 at 16:44
@pfyon - I might guess that though rain makes some noise it also drowns out much more ambient noise such as rustling leaves or tall grass by weighing down and suppressing otherwise noisy things. A sound of a bunch of dry leaves can integrate into one loud ambient noise. I go hiking on the Appalachian Trail and there is always one spot that we go under power lines and I can hear them even when it's not raining. Also, when it rains, the air is more saturated with water vapor (making it less of an empty vacuum) and can most likely carry sound further. – Joel B Feb 10 '11 at 19:51
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My wild guess would be that the hanging water droplets might be causing corona discharges. Corona is usually worst around points of sharper curvature, where the electric field gradient is most intense. Higher voltages, like the 200kV you mention, would make this more pronounced.

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You beat me to it by one minute. Cheers. – mjcarroll Dec 2 '10 at 22:05

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