silent lightning

[mikan]

I didn't see this covered in the "How fast is lightning" thread (or maybe my powers of extrapolation just ain't up to task), so let me ask it here:
why is it that some lightning is silent, without any audible thunder?
I've seen this kind of lightning twice, and both times it was lightning that appeared to be "contained" within the cloud(s). On one occasion, there was a single (but pretty huge) cloud off in the distance that was flickering with "internal" lightning, almost non-stop (quite a light show).
The other time was with a thick bank of clouds (news reports the following day said the clouds rose to 40,000 feet), where the lightning flashed every few seconds but almost always danced along the bottom of the clouds; occasionally you'd see a bolt that headed earthward, and only then would there be a loud clap of thunder. (In this latter case, the clouds were directly overhead, so I can't imagine that distance from lighting to ear was a factor here.)

Someone in the other thread said that the speed of lightning can significantly vary, so is silent lightning simply a result of (slow) speed? Ideas, anyone?

[JimFox]

I remember as kid, growing up in Connecticut, we would have heat lightning, that would only happen on the evenings of hot days. There would be no thunder, just flashes of lightning. It was a common accurance.

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[falcon2]

I believe CA has covered heat lightning, http://www.straightdope.com/classics/a1_170a.html.
But the speed of the lighning bolt is not going to effect the sound that it makes.
Heres a few thing that might.

The air up there is not as dense, and the ligning would generate less sound enegry than at ground level.

Air turbulance could deflect and scatter the sound wave so they are nout perceivable by the time the reach gound level.

Sound energy can be partialy reflected by pressure and temperature gradiants on the way down, just like light reflecting off of glass.

[funneefarmer]

Just because the flashes were seen in a cloud overhead doesn't mean that those flashes were from that cloud. At night these clouds could be reflecting from other storms farther off. I think I remember hearing on the news ( I hate it when I say that) that the usaul distance of travel for the sound of thunder is 15 miles. The light at night could travel much further than the sound of the thunder.

[Zyada]

One of my favorite childhood memories...
Early summer and I was outside, the sun was low but it was still light outside. There was exactly one cloud in the sky. This cloud had lightening flashing constantly. No sound at all. In fact, you couldn't see the lightening at all, just the flashes lighting up the contours of the cloud.

It was a rather magical experience for me.

[mikan]

Hmmm. Well, in the second case I described above, not only were the clouds overhead, but many of the lightning flashes were as well. They were hugging the *bottom* of the clouds, and you could clearly see the "bolts" as they streaked across the sky, often branching out as they spread across the sky (kinda like sideways-growing roots). So, anyway, I don't think distance had much to do with it.