For many people, the only explanation needed for lightning is that it's a bad thing. Hearing the thunder it creates is bad, seeing it is bad, and getting hit by it is DEFINITELY bad. Other, more curious folk may actually want to know how lightning is created, however, and complex scientific explanations don't help shed light on this seemingly supernatural phenomenon.
Fortunately, lightning is neither supernatural nor that difficult to understand - at least not in theory, anyway. And it all begins with the water cycle.
Lightning is usually the result of a storm forming in an area, and in order for a storm to happen you need rain, which needs clouds. The water cycle is what creates these clouds. Though there are several ways of doing this, most water is taken up into the sky when the sun, ever toasty, causes large amounts of water to change from a liquid into a gas. This process, known as evaporation, turns the water into water vapor that's carried away into the air.
Warm air naturally rises, and so this water vapor is taken up into the sky. As it goes up the vapor begins to cool, for the higher you go the cooler the surrounding air gets. Once the air gets cool enough the water vapor begins to change back into water, though once it's in the sky it's not quite heavy enough to fall as rain. The air has to become saturated with water for that to happen. Instead, that water begins to form clouds.
If there's enough water in the air, it will become supersaturated - think of it as the air overflowing - and the clouds will begin to release that water as rain. Down it goes.
Now comes the tricky part. Though much of the water vapor rises up and becomes drops of water again, some of it may travel up so high that it becomes ice crystals. This elongates the cloud, making it stretch vertically as well as horizontally. When this happens the water and the ice begin to intermingle, and it's believed that the two elements interacting with one another generates electricity.
And what happens when you get electricity in the sky? Yep, lightning. But how does it come down to the ground?
As a matter of fact, it doesn't always strike the ground. If you've ever watched a lightning storm you've probably noticed that sometimes the clouds will flash, but you won't see a lightning bolt hitting the ground. This means that the lightning is traveling from one cloud to another. This is known as sheet lightning, and is harmless to anyone on the ground. Lightning that goes FOR the ground, however, is anything but harmless.
Why does lightning hit the ground? It depends on the charge carried by the electricity. Lightning doesn't hit randomly: most of it has acquired a negative charge from being in the cloud and is looking for something with a positive charge. Opposites attract, as they say. When it finds something the lightning will leap down and strike whatever is positively charged. This doesn't mean lightning is going to hit everything that's positively charged, of course: the electrical charge of the cloud is just trying to balance itself out by getting rid of the excess energy.
Lightning is dangerous. It can deliver over a million volts of electricity, which can be deadly if it strikes a person. Fortunately the chances of a lightning striking a person are fairly low, so long as that person is safely indoors during the storm. If you are trapped outside, get rid of any metallic objects that might attract lightning and look for shelter in the nearest building.