These are some interesting questions that you have asked. I'll try to answer all four of them for you and hopefully they'll give you some more insight to lightning.

First, lighting occurs due to an accumulation of charge, electrons if you like, in the air. As a storm grows, electrical charges build up in the clouds. At the same time, oppositely charged particles are growing in number on the Earth's surface. As you know opposite charges attract and due to the large number of charges this attraction grows quickly. At some point the attraction becomes large enough to overcome air's resistance to electrical flow, in this case the flow of charges between the ground and the clouds. These particles move toward each other at incredible speeds and when they meet they complete an electrical circuit. Charge from the ground then surges upward at nearly one-third the speed of light and we see a bright flash of lightning.

Now to answer your final question, the lightning starts at the ground and moves upward, it is just moving too fast to be able to tell this.

If you have a stream of weakly charged particles moving through the air, sometimes the particles will split from each other trying to find the path of least resistance through the air. When this happens, branches will form and this is what people call branch or forked lightning. Now does lightning only occur in storms? I am not sure. I would think that lightning could occur any time there was enough charge built up in the clouds. However, I would guess that only during storms would this occur.

Lightning is, simply put, a giant spark of electricity. As you probably know, even matter that is not electrically charged is made up of components which are both positively and negatively charged. For some reason I don't understand (and, I am told, is not well understood) these charges will separate in a storm cloud -- resulting in different regions of the cloud having different electrical charges.

Usually what happens is that electrons (which have negative charge) build up on the bottom surface while protons (having positive charge) are at the top. The negative charges on the bottom of the cloud can pull on the positive charges in the ground and repel the negative ones. The result is that the ground surface becomes positively charged also. As the charges build up more and more, the situation eventually becomes very unfavorable and one of two things can happen. Either the electrons in the cloud will suddenly jump toward the protons at the top of the cloud (known as "sheet lightning") or the electrons will find their way to the ground.

The electrons flowing to the ground are so energetic that they excite the air which causes it to glow. That is what you see as lightning. Apparently, the air can get as hot as 55000 degrees Fahrenheit! Also, not every strike of lightning hits the ground. Often there are lightning strikes between clouds or between one part of a cloud to another. Sometimes lightning can start from the ground -- these strikes appear to have the branches pointing upward instead of downward. I believe they are very rare and usually occur from tall buildings. By the way, there is more to lightning than just lightning. In 1990, it was discovered that during lightning strikes, large red flashes (called "red sprites") can be seen from above the thunder cloud. And in 1995, tall cones of blue light were discovered (called "blue jets") above thunderclouds also. As far as I know, nobody understands what these are or how exactly they relate to lightning. Lightning is on the forefront of scientific knowledge!

Lightnings, are caused by an electric discharge between the clouds and the earth (in most cases) . Electric charges accumulate in the clouds through the friction of water molecules which are present in great numbers in the clouds. This reservoir of electric charges has to eventually empty itself somewhere. It will do so by establishing an electric discharge through the atmosphere and towards the earth and this is the path followed by the lightning you see.

The electric potential (energy associated with all the electric charges) of the cloud is larger than that of the earth and therefore charges will flow from the high potential reservoir (the cloud ) to the earth (at lower potential. Much like water will fall from a high water lake to a lower basin by forming a cascade or a river. This electric discharge path ionizes the molecules (molecules which have lost of number of electrons) in the atmosphere between the clouds and the earth surface. When a molecule in the air is ionized it will glow and this is why you see light associated with the lightning.

Have you also noticed that sometimes when you are in an airplane and fly over a thunderstorm that there are also lightnings that do not go towards the earth but instead go upwards? Have you also noticed that there are lightning in the sky during summer nights even though there is no thunderstorm? . These lightnings also do not go towards the earth but propagate between clouds. Try to guess why and give an explanation.

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