CAST Practice Test 8

Where Lightning Comes From

¹A thunderstorm is a huge natural electrical machine. Inside a tall storm cloud, strong currents of rising air carry water droplets high into the sky, where the temperature is far below freezing. There, the droplets turn into small ice crystals and into larger, heavier pellets of hail. As the air churns, these particles crash into one another again and again.

²Each collision can move a tiny electrical charge from one particle to another. In general, the lighter ice crystals become positively charged and are carried by the rising air to the top of the cloud. The heavier hail becomes negatively charged and stays in the middle and lower parts of the cloud. In this way, the cloud becomes separated into regions of opposite charge. The negative charge in the cloud also attracts positive charge on the ground below, which gathers on trees, buildings, and other tall objects.

³Air is normally a good insulator, which means that it resists the flow of electricity. As the separated charges grow larger, though, the air can no longer hold them apart. When a cloud-to-ground flash begins, a faint channel of charged air called a stepped leader works its way down from the cloud in a series of short jumps, each roughly 150 feet long. As it nears the ground, a tall object may send up a streamer of positive charge to meet it. When the two connect, a powerful current called the return stroke races up the channel. This is the brilliant flash that people see. A single flash often contains three or four strokes in quick succession, which is why lightning sometimes seems to flicker.

⁴The return stroke heats the air around it to as much as 50,000 degrees Fahrenheit, about five times hotter than the surface of the sun. Heated so suddenly, the air expands violently and sends out the sound wave we hear as thunder. Because light travels much faster than sound, people see the flash before they hear the thunder.

⁵An old saying claims that lightning never strikes the same place twice. In fact, tall, pointed, isolated objects are struck again and again, because they offer lightning an easy path. The Empire State Building in New York City, for example, is hit an average of about 23 times each year.