weather / T-storms

[DougPaul]

I was the one who used the word "trailer", which confused matters.

IIRC, "trailer" is an older term for the same thing. No significant harm done.

A few things; (not to beat a dead horse, some of this is pertinent to Mountain safety)
*overcoming air resistance. A simple statement used in lay literature, over course the mechanism is ionization. Is the ionization occuring in particles or are electrons getting stipped from O2, N2, CO2?

However the term resistance implies a linear relation (the current is proportional to the voltage). Breakdown is highly nonlinear--minimal current flow (very large resistance) until the field is large enough to cause breakdown which allows a very large current flow (low resistance). IMO, the phrase "overcoming air resistance" is rather misleading--the conduction following breakdown is an entirely differnet mechanism.

Sorry, don't know which molecules break down first. Ultimately the stroke path becomes plasma and I suspect everything becomes ionized. IIRC, the breakdown threshold of air is also affected by humidity and pressure.

*The NOAA link I posted mentioned the role of ice particles in the cloud being central in creating charge within the cloud.

Yes that is one mechanism, but there are others. It is possible to get charge separation without ice. (I carefully used the phrase "not entirely understood" because it is only partially understood.)

I found the reference somewhat misleading--it can be difficult to describe a complex system in a way that is accurate for both lay and expert readers.

* The charge differential exists within a cloud and not between clouds. The positive part of the cloud is the cirrus "anvil" top. The negative is the lowest part of the cloud. The ground becomes positive as you mentioned.

No. Charge differentials can exist between any parts of the (multiple) cloud-ground system. Because the ground can be considered a conductor during the charge buildup phase, essentially all charges in the ground are due to the mirror effect of _any_ nearby cloud charge buildups.

*I recall the "mirroring" of charge that you also mentioned. So if I am right does that mean that the negative charge of the bottom of the cloud would be equal and opposite the sum of the positive charges from the ground and the top of the cloud?

The net ground mirror is the sum of the effects of the charge concentrations in the clouds. (The mirroring effect is also diminished by distance--the force between two charged particles is proportional to the inverse square of the distance) So the ground would be driven positive by one part of the cloud and negative by the other part of the cloud and the net ground charge at any selected point could go either way.

BTW, the charge concentrations in the cloud are attached to particles which may be blown around by the winds and up and down drafts in the storm clouds. Thus the notion that the top of the cloud is positive and the bottom is negative is an idealization and may be true most of the time--but reality can be much more complicated.

Doug

 

[SteveHiker]

I'm waiting for someone to say it, so I'll pre-empt them.

When you are in your car, why is it that you are safe from lightning?

Most people will say it's because you have rubber tires and rubber is an insulator. That is not the correct answer. Your rubber tires aren't thick enough.

The correct answer is because you are surrounded by metal. Lightning is static electricity, and there is no static electric field inside of a conductor. This property of conductors is used as a trick question on most first year physics exams.

 

[MichaelJ]

What Steve said. The principle is called a "Faraday Cage".

 

[DougPaul]

The correct answer is because you are surrounded by metal.

"Faraday Cage".

A rather imperfect Faraday cage--it has big holes (windows) and gaps (seams where the doors open etc). Don't stick your arms or legs out...

The dangerous part of lightning is a short but very intense pulse, not the static fields. (Short pulses can behave very differently from low frequency voltages and currents.) There can be substantial voltages across the edges of the holes and the gaps.

Doug