Some people call it nature's fireworks. "Electricity out of control" might be more appropriate. Lightning is one of the most exciting parts of weather. But it also can be scary and dangerous. Did you know that lightning can heat the air to more than 50,000 degrees Fahrenheit? That's much hotter than the surface of the sun! Summer is the time for thunderstorms. And since this is Lightning Safety Week, meteorologist Dan Stillman answers some questions about storms.
What causes lightning?
Most molecules of air are neutral -- that is, they are made up of an equal number of protons and electrons. But some are "charged." Molecules that have fewer electrons than protons have a positive charge. Those that have more electrons than protons have a negative charge.
Lightning is the flash you seen when electricity travels between areas of positive and negative charges that are separated from each other. Lightning can flash within a cloud, between two clouds or from a cloud to the ground. Scientists don't know how the positive and negative charges get separated within a cloud, but they have a hunch: When ice crystals and hailstones in a cloud collide, the ice crystals become positive, while the hailstones become negative. Since ice crystals are lighter, they float to the top of the cloud; the heavier hailstones sink to the bottom. This makes the upper part of the cloud positively charged and the lower part negatively charged. [See graphic.]
So, how does lightning get from a cloud to the ground?
Normally, there's a mixture of positive and negative charges on the ground. But things are different during a thunderstorm. Since opposites attract, the negatively charged lower part of a thunderstorm cloud attracts the positive charges on the ground. When the difference between the negative and positive charges gets big enough, electricity rushes between the cloud and the ground. That rushing is visible as a stroke of lightning.
How are lightning and thunder related?
First comes lightning, then comes thunder. The extreme heat generated by a flash of lightning causes air to quickly expand and contract. This creates a giant wave in the atmosphere that travels away from the flash in all directions. Thunder is the sound we hear when the wave reaches our ears.
Light travels much faster than sound. So there's a delay between when we see lightning and hear thunder. Sound travels at a speed of about one mile per five seconds. That means, for example, that if there's 20 seconds between the flash and the boom, the storm is four miles away.
What safety rules should people follow?
A lot of people don't realize that lightning can strike well in advance of a storm and well after a storm passes. That's why the "30-30 rule" is so important. The first "30" stands for 30 seconds -- a thunderstorm is close enough to be dangerous if the time between lightning flash and thunder is 30 seconds or less (which means the storm is within six miles). The second "30" stands for 30 minutes -- the amount of time you should wait after the last rumble of thunder before going back outside.
The safest place to be during a lightning storm is in a building, away from a window. It's a good idea to stay off corded phones and not use the shower or bathtub. The next best place is inside a car. Stuck outside? Stay away from trees, poles and metal objects because they can attract lightning. Squat low to the ground to make yourself as small a target as possible.