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(so that the ice pellets acquire a negative charge and the ice crystals become positively charged.)

so that the ice pellets acquire a negative charge and the ice crystals become positively charged.

(Then updraft wind currents carry the light, positively charged ice crystals up to the top of the cloud.)

Then updraft wind currents carry the light, positively charged ice crystals up to the top of the cloud.

(The heavier, negatively charged ice pellets are left to concentrate in the center.)

The heavier, negatively charged ice pellets are left to concentrate in the center.

(This process)

This process

(explains why the top of the cloud becomes positively charged, while the center becomes negatively charged.)

explains why the top of the cloud becomes positively charged, while the center becomes negatively charged.

(The negatively charged region is large: several hundred meters thick and several kilometers in diameter.)

The negatively charged region is large: several hundred meters thick and several kilometers in diameter.

(Below this large, cold, negatively charged region, the cloud is warmer than -15 degrees Celsius,)

Below this large, cold, negatively charged region, the cloud is warmer than -15 degrees Celsius,

(and at these temperatures, collisions)

and at these temperatures, collisions

(between ice crystals and falling ice pellets produce positively charged ice pellets)

between ice crystals and falling ice pellets produce positively charged ice pellets

(that then populate a small region at the base of the cloud.)

that then populate a small region at the base of the cloud.

(Most lightning takes place within a cloud when the charge separation within the cloud collapses.)

Most lightning takes place within a cloud when the charge separation within the cloud collapses.

(However, as the storm cloud develops, the ground beneath the cloud becomes positively charged)

However, as the storm cloud develops, the ground beneath the cloud becomes positively charged

(and lightning can take place in the form of an electrical discharge)

and lightning can take place in the form of an electrical discharge

(between the negative charge of the cloud and the positively charged ground.)

between the negative charge of the cloud and the positively charged ground.

(Lightning that strikes the ground is the most likely to be destructive,)

Lightning that strikes the ground is the most likely to be destructive,

(so even though it represents only 20 percent of all lightning,)

so even though it represents only 20 percent of all lightning,

(it has received a lot of scientific attention.)

it has received a lot of scientific attention.

(Using high-speed photography, scientists have determined)

Using high-speed photography, scientists have determined

(that there are two steps to the occurrence of lightning from a cloud to the ground.)

that there are two steps to the occurrence of lightning from a cloud to the ground.

(First, a channel, or path, is formed that connects the cloud and the ground.)

First, a channel, or path, is formed that connects the cloud and the ground.

(Then a strong current of electrons follows that path from the cloud to the ground,)

Then a strong current of electrons follows that path from the cloud to the ground,

(and it is that current that illuminates the channel as the lightning we see.)

and it is that current that illuminates the channel as the lightning we see.

(The formation of the channel is initiated when electrons surge from the cloud base toward the ground.)

The formation of the channel is initiated when electrons surge from the cloud base toward the ground.

(When a stream of these negatively charged electrons comes within 100 meters of the ground,)

When a stream of these negatively charged electrons comes within 100 meters of the ground,

(it is met by a stream of positively charged particles that comes up from the ground.)

it is met by a stream of positively charged particles that comes up from the ground.

(When the negatively and positively charged streams meet,)

When the negatively and positively charged streams meet,

(a complete channel connecting the cloud and the ground is formed.)

a complete channel connecting the cloud and the ground is formed.

(The channel is only a few centimeters in diameter,)

The channel is only a few centimeters in diameter,

(but that is wide enough for electrons to follow the channel)

but that is wide enough for electrons to follow the channel

(to the ground in the visible form of a flash of lightning.)

to the ground in the visible form of a flash of lightning.

(The stream of positive particles that meets the surge of electrons from the cloud often arises from a tall,)

The stream of positive particles that meets the surge of electrons from the cloud often arises from a tall,

(pointed structure such as a metal flagpole or a tower.)

pointed structure such as a metal flagpole or a tower.

(That is why the subsequent lightning that follows the completed channel often strikes a tall structure.)

That is why the subsequent lightning that follows the completed channel often strikes a tall structure.

(Once a channel has been formed, it is usually used by several lightning discharges,)

Once a channel has been formed, it is usually used by several lightning discharges,

(each of them consisting of a stream of electrons)

each of them consisting of a stream of electrons

(from the cloud meeting a stream of positive particles along the established path.)

from the cloud meeting a stream of positive particles along the established path.

(Sometimes, however, a stream of electrons following an established channel)

Sometimes, however, a stream of electrons following an established channel

(is met by a positive stream making a new path up from the ground.)

is met by a positive stream making a new path up from the ground.

(The result is a forked lightning that strikes the ground in two places.)

The result is a forked lightning that strikes the ground in two places.