Atmosphere And Weather

What causes Lightning



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Lightning is a surge of electrical current from an object on the ground to a leader in the sky. Here is how it works:

Thunderstorms form when two or more differently pressured weather fronts collide. High pressure fronts, which bring warmer, clearer weather, try to push over low pressure fronts, which are typically cooler. The collision mixes high and low pressures causing the water molecules in the air to gather together into clouds.

During storms, wind whips the molecules inside the clouds as they travel through the sky. When the rain falls from the clouds in a down draft, it changes the temperature of the cloud. The change in the temperature and the wind cause the molecules inside the cloud to collide.

The bottoms of clouds carry negative charges from the friction of the molecules inside the cloud. As the water and air molecules rub together or collide, electrons (the negative parts of the atoms) are released. They slip down between the molecules and settle at the bottom of the cloud as plasma.

This negativity reaches down from the cloud in a leader. The leader takes "steps" toward the ground, in a zig-zag. These steps are invisible and about 150 feet long.

Positive ions collect in the objects, people and trees on the ground. When a stepped leader reaches within one step (150 feet) of the positive charge, a surge of positive electricity, called a streamer, forms. The streamer reaches up, in an up draft, to meet leader. When they combine, they make a channel.

Electrical current from the object on the ground rushes through the channel, touching off a bright display called the return stroke. The return stroke is the lightning we see. It is where the electrical spark has ionized the air molecules in the channel.

In less than one half second, the leader reaches from the cloud to the streamer. In that tiny time, the air molecules are superheated to five times the temperature on the surface of the sun: 49,000 degrees Fahrenheit (27,000 degrees Celsius). This heating expands the air surrounding the lightning and makes it vibrate. That vibration is the thunder.

All atoms seek to be neutral. Positive and negative ions are drawn to one another. As long as up and down drafts are present during a storm, lightning is possible. The object or person struck by lightning is merely a conduit from the positive charge at the ground level and the negatively charged leader.

Some lightning strikes spark fires. Others leave no visible trace where they actually touched. The human body is an excellent conduit for electricity.

Occasionally, lightning appears in the sky which never touches the ground. The streamer is created from the positive ions at the top of clouds. When a leader comes within one step (150 feet) of the streamer, the channel is created. The electrical current streams through the channel from the lower cloud to the plasma of the higher cloud.

When the leader comes within one step (150 feet) of more than one streamer, the lightning will branch. Each streamer opens a channel with the leader. The channels merge on the way back toward the plasma laden cloud. The return stroke will become stronger as the channels merge. The resulting lightning bolt appears to have small branches at the lower end.

References: http://www.nationalgeographic.com; http://www.science.howstuffworks.com; http://www.mos.org

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