Why Does Lightning Strike?
Lightning is beautiful and fascinating to watch, it is also very dangerous. One lightning strike carries tens of thousands of amps, hundreds of millions of volts releasing several billion joules of energy in less than a second. There are approximately 4 billion lightning strikes every day on Earth and it kills on average 60 people per year in the United States.
How lightning forms
The day starts out clear and pretty. As the Earth starts to warm this warm air starts to rise. If there is any moisture in the air it also rises with the warm air. This rising air and moisture starts to condense into clouds causing water droplets and frozen pellets to form. As the cloud grows there are updrafts and downdrafts within the cloud. These droplets bump into each other as they move up and down in the clouds. One theory is that the frozen droplets become negative and the unfrozen droplets have a positive charge. The heavier negatively charged droplets fall to the lower parts of the cloud and the updrafts carry the positive charged particles to the upper portions of the cloud. This is called charge separation.
As these clouds grow into a thunderstorm more negative electrons form at the base of the cloud. When the storm clouds move over the earth, the ground underneath these clouds become positively charged since opposite electrons attract their opposite. There is also the repulsion of like electrons. As the negative base of the storm moves over, it actually repulses the negative electrons on the ground away from this area under the storm. The air between the ground and the cloud acts as an insulator between the two opposing charges, for the time being. The negative base of the clouds attract more positive electrons on the ground until finally there is a large enough charge of these opposite electrons to overcome the air insulator and there is a huge release of energy known as lightning.
The lightning strike
A negative charged lightning strike actually happens in several steps. The first part of a lightning strike is when a negatively charged channel starts out of the cloud and is called a step leader. This step leader can branch out into many different channels. The branches of the step leader are actually looking for a place to strike and discharge this huge buildup of electrical charge. At the same time positively charged streamers are moving upwards from trees, buildings and the ground. When the positively charged streamer connects with the negatively charged step leader, a negative charge starts to flow down this channel and at this instant there is what is called a return stroke, which is the actual lightning strike we see. So lightning we actually see is going from the ground up to the cloud, though this happens so fast we cannot see that. If there is enough charge left after the return stroke, there is another leader that is called a dart and this is what gives some lightning strikes that flickering appearance. All of this happens in about 1/20th of a second.
When you feel the hair standing up on end, it is actually the positive electrons flowing upwards and it is imperative you get to safety. You should never wait until this happens though before getting to safety.
There are also positively charged lightning strikes where the step leader is positive and it originates near the top of the storm. These lightning strikes can be more dangerous in the sense that they seem to come out of nowhere, since they aren’t coming from the base of the storm as usual. The science is the same where the positive step leader is looking for a negative streamer to connect with.
We hear thunder after the lightning since sound travels much slower than light. Thunder is the sound of the air being superheated to a temperature of 36,000 degrees F (20,000 C) and expanding, which creates a compression wave that we hear as thunder. You can feel this compression wave hit you if it is close enough.
How far away was that lightning
You can tell how far that lightning strike was by counting the number of seconds between the time you see the lightning and you hear the thunder and divide that number by 5. For example if you see a flash of lightning and hear the thunder 30 seconds later, the lightning was 6 miles away.
Out of the clear blue sky
Lightning can also strike out of what appears to be a clear blue sky. It can strike from a thunderstorm as far away as 10 or more miles. This is rather common in the mountains when there is a thunderstorm on the other side of the mountain and cannot be seen by hikers until all of a sudden there is a lightning strike.
Yes, there can be lightning when it’s snowing. This usually happens when there is a heavy snow shower and an unusually unstable air mass for winter. In fact this is really a pretty sight, the lightning will look like blue flashes in the snow. But this lightning can also hit the ground and be just as dangerous.
is actually lightning that is far away that you can see but never hear the thunder. It got this name because it’s usually seen on hot summer nights. But under that storm that seems so far away is a real thunderstorm going on with actual lightning and thunder.
Other Types of Lightning
There is also cloud-to-cloud lightning, ball lightning, sprites and blue jet lightning. For these mysterious types of lightning you can read The Different Types of Lightning.
© 2009 Sam Montana
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National Weather Service