Brooke Walsh and AJ Kell, of Balclutha Primary School, asks :-


Why is lightning bright and what causes it?

Craig Rodger, a physicist at the University of Otago, responded.

Things which are hot glow, just like the filament in an incandescent light bulb. This becomes hot because a electrical current travels through the filament, heating it such that it gives of light. You can check this by putting your hand near (but not on) an incandescent light bulb - its clearly very warm. Lightning occurs when really big electrical currents pass through the air, heating it to very high levels, and so it glows. But the current is just a short pulse, and so lightning is just a brief flash of light in the sky.

Most household lightbulbs operate with a current of about 0.4 amps. However, a typical lightning flash between a thundercloud and the ground has a current which is about 75 thousand times bigger than this, at 30 thousand amps. Thus lightning flashes are much brighter!

Lightning occurs because charge builds up in thunderclouds, and for this charge to be neutralised, it needs to "leap" through the air to the Earth. This "leaping charge" is the electrical current which causes the lightning flash. Lightning is quite rare in New Zealand by global standards. On average, there are 44 lightning flashes occurring somewhere on Earth every second.