Now that I know what causes thunderstorms here in Southern California (monsoon moisture + heat + topography), and despite this weekend’s return of the marine layer and the subsequent end of the thunderstorm cycle, I wondered: what causes lightning?
Updrafts and downdrafts in the thundercloud cause water and ice particles to collide and take on positive or negative electrical charges, much like a balloon will take on a negative electrical charge when rubbed with a woolen sock. For reasons not well understood, lighter, positively charged ice droplets migrate toward the top of the cloud formation and heavier, negatively charged ice droplets and rain settle toward the bottom of the formation. Eventually this separation creates electrical potential between the negatively charged part of the cloud and the positively charged part of the cloud. Lightning occurs when the electrical charge on both sides overcomes the resistance of the air between them. These “intra-cloud” strikes are the most common type of lightning. “Inter-cloud” strikes, or strikes between clouds, have similar causes. Cloud to ground strikes work somewhat differently.
As the negatively charged part of the cloud moves over the earth, the ground below becomes positively charged by induction. You have seen induction if you have played with magnets and noticed how one side of a magnet attracts its opposite and repels its like, or if you have replaced the batteries in your Wii controllers and noticed the “+” and “-” symbols telling you how to arrange everything. Under the storm cloud, positively charged ions in the earth are attracted to the negative ions accumulating above and negatively charged ions in the ground are pushed away. As with intra-cloud strikes, when the charge on either side becomes strong enough, resistance is overcome, electricity flows and shazzam! lighting strikes.
As for thunder, that’s easy. A bolt of lightning is fantastically energetic and it generates terrific heat, singeing the surrounding air to 30,000 to 50,000 degrees Farenheit. The Crack! you hear is the sound of the explosion made as super-heated lightning strike air expands, rapidly, into the cooler air around it.