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What Causes Lightning to Strike?

Few things in life are more exhilarating than watching a bolt of lightning flash across the sky, followed by the powerful rumble of thunder. These highly luminous streams of electricity travel at up to 62,000 miles per hour, producing currents of around 300,000 amperes. The intricate details of how lightning is created still has scientists scratching their heads. What is known is that the process involves the interaction of positively and negatively charged ions within a thunderstorm.

The Updraft
- Thunderstorms, known as cumulonimbus clouds, develop strong updrafts. These updrafts can carry water droplets as high as 35,000 to 75,000 feet in the atmosphere. As water droplets rise, they pass through the atmosphere's freeze line and become ice crystals. The area of updraft within a thunderstorm is generally characterized by the darkest clouds.
The Downdraft
- What goes up must come down, and this is true of thunderstorms. The cloud will develop an area of intense downdrafts, generally in close proximity to the updraft. These downdrafts force the ice crystals back down to the surface, where they produce precipitation. The area of downdraft within a thunderstorm is generally characterized by fuzzy, lighter clouds, and is the area of the storm that produces precipitation.
The Collision
- When descending ice crystals (or hail) collide with rising water droplets, the droplets freeze and release heat. This heat causes their surface to remain warmer than the surrounding air, producing graupel. When graupel, also known as soft hail, collides with descending particles again, negatively charged electrons are forced off. These electrons collect on the falling particles, producing a growing negative charge at the base of the storm cloud, and a growing positive charge at the top.
The Bolt
- Opposite charges attract one another, producing an electric field within the cloud. When this field becomes strong enough, a huge "short" occurs, sending an electric current between the two areas of opposite charge. This streak of current produces a lightning bolt. Because air is a good insulator and the distances between charged regions is great, a very large charge must be built up, producing huge lightning bolts -- as opposed to smaller static discharges.
The Ground
- Because opposite charges attract, positively charged ions tend to build up on the ground underneath thunderstorms, attracted to the cloud's negative base. A negatively charged channel, called a step leader, will rapidly extend downwards from the base of the cloud. A positively charged channel, called a streamer, will rapidly extend upward from the ground. When these two channels connect, the electrical circuit is connected and a huge electrical discharge occurs. It is this current that is visible as a bright bolt. Each lightining strike can contain up to 25 individual strokes, or pulses, of current, producing the flicker effect that is common in lightning bolts. However, the strongest electrical fields exist within the thunderstorm, which is why 75 to 80 percent of lightning occurs within the cloud itself, known as cloud-to-cloud lightning.

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