I have been having this on-going debate with friends at work. I ask - ‘Why does lightning strike the Earth?’ The reply I always get is - ‘It’s completing a circuit - the Earth is the ground.’ I just don’t see it. My confusion is - for it to complete a circuit most of the energy has to travel up into the sky from the Earth the same time or sometime before the lighning strikes. A circuit is circular meaning a ground is only relative to its positive and not one from another system. An analogy - if I take two car batteries and touch the positive of one to the negative of the other - nothing will happen. Even though one post is more negative than other they aren’t in the same system and not relative to each other. All the theories I’ve read on lightning makes reference to electricity being created in the ice layer of clouds. If this is true - why doesn’t the imbalance complete the circuit in the cloud (sheet lightning)? If anyone understands what I’m trying to ask I’d really appreciate an answer. Thanks in advance- B.
Most lightning is up in the clouds like you say. The lightning from the ground is exactly that from the ground to the sky, not like we think of lightning striking the earth from the sky. The earth is always negative, that’s why you better have on your rubbers if you stick your finger in a socket.
Welcome to the SDMB, Cocks
This is from How stuff works
BTW, the water in the cloud travels from the earth to the sky.
Within the cloud, the water is blown all around. Sometimes it goes so high up that it freezes and then falls, gets a new coating of liquid water, then going up and re-freezing. When it’s too heavy, it falls all the way to the ground as hail.
Nonresponsive post from an SC native to welcome SC_Cocks84 to the boards.
Hi all you SC natives, from one living in SC now.
ChooseyBeggar’s link to How Stuff works does not tell the whole story. A lightning bolt from the sky to the ground is not one-way. There is another charge emanating from the ground or, more likely, a tall object on the ground. These two charges are not visible to the naked eye, as they are only charges, until and if they meet forming the lightning bolt.
I’ve seen on TV pictures of the charge emanating from a tree or a telephone pole, and showing it either missing or connecting with the downward charge. I don’t know how the pictures were taken, or don’t remember, but you could not see them with the naked eye.
To complete the circuit, the charge must be routed back to the cloud. If it were just one way, from cloud to ground, there would be no circuit. After the charges meet, there is the return electricity to the cloud, completing the circuit.
No, but the next page of the “How Stuff Works” explanation does, and your explanation doesn’t quite match it.
I didn’t go to the link, but after reading it, how does my description “not quite match it”? The “streamer” is the charge from the ground or an object on the ground. The “step leader” is the charge from the cloud. When they meet, they open a channel for the flow of electricity. Since there has to be a closed circuit, the flow is a loop. If this is not the case, can some one explain to me how you can have the flow without the loop?
Incidentally, it was the purple glows that I saw on TV.
It is not necessary to have a complete circuit in order for current to flow.
For example, a capacitor consists of two metal plates that, while close together, do not actually touch each other. Such a device can be “charged”. You hook one plate up to the (+) side of a battery and the other plate up to the (-) side of the battery. Even though there is no path for the current to flow all the way around, current does flow. The plate attached to the (+) end acquires a positive electrical charge, and the plate attached to the (-) end acquires a negative electrical charge. This continues until the two plates acquire a strong enough charge that the battery cannot charge them any more.
If the charged capacitor is then disconnected from the battery, and the two plates are then connected to each other, there is a sudden discharge of current, a sudden flow of electrons from the (-) plate to the (+) plate, the instant the connection is made. Again, this is not a complete circuit as the capacitor’s plates do not touch each other, yet current flows. You can see the current flowing by charging the capacitor and then connecting it to a sensitive current meter. A really big capacitor will put on a much more impressive display when it suddenly discharges, making a loud “bang” and sending out sparks and such.
The clouds and the Earth are like the 2 plates of a capacitor – they carry opposite charges. The ionized trail through the air is like the wire attaching the two plates of the capacitor to each other. Yes, technically, it is incorrect to call this a “circuit”. But electrical engineers build “circuits” with capacitors in them all the time, and use the term “circuit” in a looser sense to mean any electrical device through which current will flow.
Here’s a good link to FAQ about lightning.
(I used it to prepare when teaching electrostatics this year.)
Also, excellent capacitor analogy, tracer!
From the above link:
Hence, there is a closed circuit. Altho it was referred to as a capacitor in the opening of the link, I don’t think that is the case. A capacitor stores electricity - electricity already produced by the batteries. It doesn’t need a closed circuit because the electricity is already there. In lightning, there is no electricity until the circuit is closed, as there is no pre-produced electricity.
barbitu8 wrote:
Ah, but there is electric charge stored in the clouds – it’s just not provided by an obvious source like a battery. The clouds are charged by friction, the way you can charge up your own body by shuffling your feet along a carpet.
The capacitor analogy, like all analogies, is somewhat limited. The lightning stroke is really not the same as connecting the 2 plates of a charged capacitor with a wire. It’s more like charging the 2 plates of a capacitor so much that their electric potential exceeds the “breakdown voltage” of the air gap between them, and current “jumps” from one plate to the other. (The breakdown voltage for air is pretty high; I once heard a figure of 10,000 Volts per centimeter of air gap. If the gap is filled with something that has a much lower breakdown voltage than air, the effect is easier to achieve and less destructive to the equipment. Neon, for example, has a breakdown voltage of only about 90 Volts; a neon lamp is essentially a capacitor driven beyond its breakdown voltage.)
From my electronics manual it says the dielectric strength (breakdown voltage) for ‘air’ is 75 Volts per mil (.001 of an inch) which would put the value per centimeter 'round 30,000 V.
The capacitor visualization is perfect really. A strong enough charge potential between the cloud and ground and you have breakdown voltage through the atmosphere which is yer dielectric. 
Also, it’s interesting to note that blue lighting carries higher current whereas white lighting carries higher voltage.
I wonder, has there been any new news about the phenomenon of ‘sprites’? - Those little flashes that can be observed miles above a thunderhead. I remember seeing a show a year or so ago where they were trying to decipher them.