What is lightning and thunder ?

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If you have ever jump started a car and accidentally let the two terminals from the jump leads touch, then you will probably seen a flash from the terminals and more than likely heard a "cracking" noise as well. The spark that JUMPED the gap between the terminals is really just like lightning but obviously on a much smaller scale, this is because the voltage in the battery is on a much smaller scale.

During a thunderstorm clouds can build up to a height of about 12km, inside these clouds you can have a mixture of water, hailstones and ice crystals. Air within the cloud is also rising and falling in strong currents. Because the water and hail have different weights they "knock against each other" and as the hail falls through the rising air carrying the lighter water droplets they remove positive electical charge and so over a large area you have an excess in negativly charged hail FALLING through the clouds and an excess of positively charged water RISING in the clouds. This difference in electrical charge causes the huge lightning flash. The thunder is the heat generated by the lightning expanding and then contracting the air around the spark producing a pressure wave experienced as a loud bang or long rumble depending on how far away the lightning is from the observer.

The difference in the size of the spark in a discharging battery, as mentioned earlier, to the size of the spark in lightning is due to the voltages involved. A typical car battery is 12 volts and can produce a spark about 1mm long, where as a lightning bolt can be a billion volts (1 000 000 000) and produce sparks well in excess of 1km.

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