|Cutnell, John D. & Kenneth W. Johnson. Physics. 3rd ed. New York: Wiley, 1995: 608.||"Consider for example, a lightning bolt in which 25 C of charge moves through a potential difference of 1.2 × 108 V"||120 MV|
|Lide, David R. Handbook of Chemistry and Physics.New York: CRC Press. 1996-1997: 14-33.||"The electric potential … with respect to the ground has a magnitude in excess of 107 V"||10 MV|
|Adams, Charles K. Nature's Electricity. Pennsylvania: Tab Books, 1987: 132.||"the difference … becomes great enough to overcome the resistance of the insulating air … the voltage in these cases can be as much as 100 million volts."||100 MV|
Lightning is formed form a buildup of charge in the atmosphere. It build up charge from the static from water droplets. This causes a charged cloud. This charged cloud induces an oppositely charged surface on the earth. Ions between these two sources flow through the air and form a "path"in the air. This path is lower in resistance than the surrounding air and charge flows through this channel, creating lightning.
Electricity is the flow of charge. The current is the amount of charge that flows through a cross-sectional area in one second. Voltage is the work done per charge. The voltage of lightning varies with certain altitudes and thickness of the bolt. These fluctuating values are caused by the fact that the cloud and earth act as capacitors. A capacitor is a device used to store charge. When the spacing between them increases, the voltage needed to produce lightning increases. Benjamin Franklin's experiment during a thunderstorm helped to reveal that lightning is electricity and that the charges can be conducted to the ground without causing much damage using a lightning rod.
It seems from these values that the most definitive answer is from 10 to 120 million volts.
Mathieu Lo -- 1998