Charge balancing in thunderstorms

[Tyro]

IIRC, the charge distribution in clouds is caused by friction of the ice crystals within it. Why should there be any charge distribution in the first place?

First off - how does friction charge up an object?

Then, why should the uneven charge distribution not neutralise itself? The opposite charges in a charged cloud should just attract each other and discharge...not form a coherent structure where the lower side of the cloud is positive and the upper side negative.

Thirdly, when electrons arc up from the Earth to discharge the positive side of the cloud, what about the net positive charge the Earth now possesses and the net negative charge the atmosphere now possesses? Articles about lightning say that it is mostly made of electrons shooting skywards, so what other mechanism discharges the new Earth-positive and sky-negative distribution?

 

[Tyger]

In the Feynman Lectures

you'll find one of the best descriptions of thunderstorm activity. It will answer many of the excellent questions you have raised.

Interestingly, as long as the role of friction in charge transfer has been known, (since the ancient Greeks) there doesn't seem to be much of a theory about why it works.

 

[russ_watters]

Originally posted by Tyro
First off - how does friction charge up an object?

Whether in a cloud or between your shoes and the carpet, it literally rubs electrons off of one surface causing them to collect on the other.

Then, why should the uneven charge distribution not neutralise itself?

It does - lightning. Most lightning is cloud->cloud.
As for the third point, I'm not sure. But I do know lightning can't CAUSE a charge imbalance, it can only relieve one.

 

[Tyger]

Re; Charging the atmosphere

A cloud bottom with a positive charge may be only a couple of miles above the ground, where the cloud top may be ten miles, so the positive charge will be transferred to the to the ground in lightning strokes.

 

[username]

Originally posted by Tyro
IIRC, the charge distribution in clouds is caused by friction of the ice crystals within it. Why should there be any charge distribution in the first place?

First off - how does friction charge up an object?

In the context of lightning it might not be friction at all, there is one theory that as water pellets freeze it causes the outer colder ice shells to become positively charged with a negatively charged warmer liquid interior. When the interior begins to freeze it shatters the outer ice shell. The positive ice shell then gets carried upwards by turbulence to the top of the cloud and the negative liquid interior settles to the base of the cloud.

 

[username]

Originally posted by Tyro
so what other mechanism discharges the new Earth-positive and sky-negative distribution?

I think you got your negative/positive mixed up, anyway what about plain ol rain ?

 

[Tyro]

Originally posted by russ_watters
Whether in a cloud or between your shoes and the carpet, it literally rubs electrons off of one surface causing them to collect on the other. It does - lightning. Most lightning is cloud->cloud.
As for the third point, I'm not sure. But I do know lightning can't CAUSE a charge imbalance, it can only relieve one.

Interesting. I didn't know that most lightning was cloud -> cloud. Presumably its stated that the terrestrial version is more common because ground -> cloud is a lot more visible?

I didn't say that lightning causes a charge imbalance, I only meant to indicate that lightnings job is not yet complete, or that there are still residual charges that need to be neutralised.

 

[Tyro]

Originally posted by username
In the context of lightning it might not be friction at all, there is one theory that as water pellets freeze it causes the outer colder ice shells to become positively charged with a negatively charged warmer liquid interior. When the interior begins to freeze it shatters the outer ice shell. The positive ice shell then gets carried upwards by turbulence to the top of the cloud and the negative liquid interior settles to the base of the cloud.

Very interesting. Since the water molecules have a slightly negative oxygen atom and positive hydrogen atoms, why should ice with hydrogen sticking outwards be more likely to form than ice with a random orientation or the opposite?

Is it because the hydrogen ends occupy a larger volume of space than the oxygen end, so the hydrogen-out, oxygen-in shape makes sense the same way a sphere is structured?

As the interior freezes, the density decreases and the quasi-sphere expands. Correct?

Immediately after shattering, why don't adjacent +ve ice and -ve water neutralise? In any case, their bouyancy difference is not much, so they should not separate too quickly.

IIRC as well, for very fine particles in air, their motion is not governed by bouyant effects, but rather aerodynamic ones. Incidentally, does anyone remember how to find out the critical radius for a bouyancy -> aerodynamic effect predominance switch over?

 

[Tyro]

Originally posted by username
I think you got your negative/positive mixed up, anyway what about plain ol rain ?

Hmm...I meant that the clouds before a ground -> Earth lightning strike are positive on the lower side and negative on the upper side. After a lightning discharge, electrons flow skywards, neutralising the positive lower side. So you get a cloud positive, Earth negative distribution.

About the theory you mentioned, if the ice in a cloud is (lighter) and predominantly negative, why should it discharge as (liquid) rain?

Sorry for the questions, it is not that I am questioning you it is me just trying to form a consistent picture of what is happening.

 

[username]

Originally posted by Tyro
Hmm...I meant that the clouds before a ground -> Earth lightning strike are positive on the lower side and negative on the upper side.

I thought that the clouds are positive on the upper side and negative on the lower side (nearest the earth) which induces a postive charge on the Earth's surface. And that lightning is an electron flow down towards the Earth (positivly induced). see :http://thunder.msfc.nasa.gov/primer/primer2.html [Broken]

Correct me if I am wrong maybe it can work both ways ?.