Why does lightning strike?

Lightning is one of nature's most dramatic displays: a sudden, blinding flash followed by the rumble of thunder. It is powerful, fast, and can be dangerous. People have watched lightning with awe and fear throughout history. At its core, lightning is a giant spark of electricity, and understanding it begins with understanding how electric charge builds up inside a storm.

Lightning is born inside towering thunderstorm clouds. Inside these clouds, strong air currents push water droplets and ice crystals up and down, and these particles constantly collide with one another. These collisions cause tiny electric charges to be transferred between particles. Over time, this sorts the charges within the cloud: lighter, positively charged particles tend to gather near the top of the cloud, while heavier, negatively charged particles collect near the bottom.

Air is normally a good insulator, meaning it resists the flow of electricity. So the separated charges cannot move easily at first. But as the storm continues, the difference in charge keeps growing, becoming enormous. Eventually the electrical force becomes so strong that it overpowers the air's resistance. At that point, the built-up charge is suddenly released as a powerful flow of electricity. This rapid discharge is the lightning we see, and it acts to balance out the charges.

Lightning can leap between differently charged regions within a single cloud, between two clouds, or between a cloud and the ground. In a cloud-to-ground strike, the negative charge at the bottom of the cloud is drawn toward positive charge that gathers on the surface below. The lightning channel is incredibly hot, heating the surrounding air so quickly that the air expands explosively. That violent expansion creates a shock wave, and the shock wave is what we hear as thunder. We see the flash before we hear the thunder simply because light travels much faster than sound.

This article was written using information from Wikipedia.