Why do high voltage power lines hiss when it's raining?

High-voltage partial discharges across the insulators.

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.

Good stuff in the answers so far, but I work with power lines and want to put in my two cents.

This isn't technically a partial discharge; occasionally you may be hearing the crackle of what is usually called a corona discharge. I admit the phenomenon is related, but it is not the same.

See, all uninsulated lines show corona. Its just not a big deal until you're dealing with a pretty high voltage. As the voltage goes from a very big positive to a very big negative, the air around it gets ionized, so about 50 or 60 times a second, it switches direction. This is the normal mains hum discussed in another answer.

Water is much, much heavier than air, and it ionizes just as easily. So on a rainy or humid day, the corona is pulsing with water in it. This gives it momentum, so the heavier water particles travel out farther. But they themselves are ionized, which means they can ionize more air than the line could normally reach on its own, and ionized air is conductive.

And there's almost always 3 of these lines pretty close together. The sound you're hearing is a million teeny tiny electrostatic discharges from all the charged up water particles interacting with each other with nearby lines or grounded objects. This is actually the worst time to be anywhere near them; the air is supposed to be their insulator, and at that moment it isn't working as well.

Occasionally you might see a full corona discharging with the naked eye; it looks like a tiny bit of lighting crawling up the line. If it gets really bad, you'll see a momentary line to line or line to ground short, which looks exactly like a real lighting bolt, just not from the sky.