Lightning is just a big electric spark. It is caused by electricity in a cloud moving through the air to electricity on the earth. The spark may be four or five miles long.
Such a spark is called a lightning flash. When you see a lightning flash, you are just seeing the heating effect of moving electricity on the air. Sometimes, the lightning passes from one part of a cloud to another part, but the explanation is just the same.
How is so much electricity made in a cloud? Even now, scientists are not quite sure of the answer. Perhaps this is the answer: when clouds are made, there is always some hot damp air rising upwards from the ground.
On very hot days (when thunder-storms are likely to happen), this rising air moves quickly up into the middle of the cloud, because of the great heating of the land. As this air moves quickly upwards, it meets falling drops of rain.
This rising air rubs against these falling drops and breaks them up. Because of this, electricity is made in the cloud. In science, it is known that rubbing two things together can sometimes make electricity.
In dry weather some of you have heard a crackling sound in your hair when you were combing it. The comb was rubbing against your hair. This is similar to the making of electricity in a cloud.
As more and more air rises quickly up into the cloud, so more and more drops of rain are broken up to make electricity in the cloud. At last the cloud can hold no more electricity.
When this happen, the positive electricity in the cloud bursts out towards the negative electricity on the earth beneath in the same way as a rier or dam will burst its banks if too full of water after a heavy rain. The cloud becomes completely full of electricity. When the electricity bursts out in this way, it heats the air, and you see a flash of lightning.
In dry weather when you comb your hair, _________.
there are some drops of water
electricity in a cloud is made
you can make electricity
lightning is happened