Why cloud to ground images? The answer is lightning, my obsession with chasing it, and making pictures of it.
Most lightning travels from cloud-to-cloud (CC) or occurs within clouds (intra-cloud or IC). It looks a bit like this:
But 10-20% of lightning reaches the ground. This is the good kind, the kind that makes some photographers chase storms. This is Cloud-to-Ground lightning.
Lightning develops when there are differences between the charged areas of thunderclouds. Channels of negatively ionized air move stepwise downwards towards the earth. When these stepped “leaders” meet channels of positively ionized air, or “streamers”, discharge occurs. Positive current initially flows upwards through the plasma channel to be followed by a massive negative discharge down towards the ground. This return stroke is the most visible part of lightning and the process can repeat itself a few times times, hence why lightning often appears to fluctuate in intensity, or flicker. This is a negative CG.
About 1 in 200 CG’s are a different beast all together, the positive CG. These are the bolts from the blue, cow killers as they are called in the Northern Territory. These originate from the top of the thundercloud, several miles above the negative CG’s, in the area where the positive charge sits. Usually the insulating air and negative charge between the pooled positive charge and the ground prevent discharge. However as a storm is maturing and forming an anvil due to wind shear high in the atmosphere, this “protection” is lost. The lip of the cloud high in the atmosphere extends outwards and there is nothing below it. And a CG can emerge from this when positive ion channels meet with negative channels from the ground. And a humungous discharge occurs, more than 10 times greater than its tame negative cousin.
Positive GC’s are usually over 10 miles in length, they travel at 100,000km a second, have a current of over 300,000 amps, and heat the surrounding air to up to 50,000c. Hotter than the suns surface. The expanding hot air forms a pressure wave, and this is heard as…thunder.
Why wouldn’t you want to chase these?