Flashes of lightning against a dark sky create an otherworldly effect. What is the science behind this beautiful — but dangerous — natural phenomenon? ClassicStock/Getty Images
Lightning is the most beautiful and dangerous aspect of a storm. According to the National Weather Service, between 1989 and 2018, an average of 43 lighting-related deaths were reported in the United States each year. Only about 10 percent of the victims of lightning strikes result in deaths — usually from cardiac arrest or irrevocable brain damage — which is somewhat surprising, considering a lightning bolt is about 50,000 degrees Fahrenheit (27,760 degrees Celsius) — about five times hotter than the surface of the sun.
Although a lightning bolt is only about an inch in diameter (2 to 3 centimeters), it can stretch for miles. While most lightning bolts are 2 to 3 miles (3.2 to 4.8 kilometers) long, the world record flash was observed to stretch 477.2 miles (768 kilometers) across three U.S. states — Mississippi, Louisiana and Texas — in 2020. So the expression "out of the blue" is completely accurate — if you can hear thunder, it's possible, if not very likely, that lightning from a nearby thunderstorm can make it to wherever you are.
Something as powerful as lightning begins with a process that happens in your life all the time: static electricity. If you've ever shocked yourself on a doorknob after walking across a carpet in your socks, you experienced the pop of a tiny lightning bolt. Static electrical charge builds up whenever two objects rub together, and in the case of lightning, the objects are moisture molecules and ice crystals in clouds.
Clouds are one of the places moisture is held and collected in the water cycle. Clouds are formed gradually as terrestrial water absorbs heat from the sun and the surroundings until they collect enough energy to transform from liquid to vapor. A cloud you see is just a collection of water vapor, some dust and other impurities that hangs together in the sky, much like the water in a lake or ocean hangs together on the ground.
Precipitation like rain, snow and sleet occurs comes from clouds, obviously. But it happens as a result of water vapor rising higher and higher, and the temperature of the surrounding air becoming lower and lower. Eventually, the vapor loses enough heat to the surrounding air to allow it to turn back into a liquid. Earth's gravitational pull then causes the liquid to fall back down. If the temperatures in the surrounding air are low enough, the vapor can condense and then freeze into snow or sleet.
In the next section, we'll see what causes electrical storms.
Zap! You just touched a metal doorknob after shuffling your rubber-soled feet across the carpet. Yipes! You've been struck by lightning! Well, not really, but it's the same idea.
Your rubber-soled shoes picked up stray electrons from the carpet. Those electrons built up on your shoes giving them a static charge. (Static means not moving.) Static charges are always "looking" for the first opportunity to "escape," or discharge. Your contact with a metal doorknob—or car handle or anything that conducts electricity—presents that opportunity and the excess electrons jump at the chance.
What causes lightning?
Watch this video to learn about what causes lightning! Click here to download this video (1920x1080, 102 MB, video/mp4).
So, do thunderclouds have rubber shoes? Not exactly, but there is a lot of shuffling going on inside the cloud.
Lightning begins as static charges in a rain cloud. Winds inside the cloud are very turbulent. Water droplets in the bottom part of the cloud are caught in the updrafts and lifted to great heights where the much colder atmosphere freezes them. Meanwhile, downdrafts in the cloud push ice and hail down from the top of the cloud. Where the ice going down meets the water coming up, electrons are stripped off.
It's a little more complicated than that, but what results is a cloud with a negatively charged bottom and a positively charged top. These electrical fields become incredibly strong, with the atmosphere acting as an insulator between them in the cloud.
When the strength of the charge overpowers the insulating properties of the atmosphere, Z-Z-Z-ZAP! Lightning happens.
How does the lightning "know" where to discharge—or strike?
The electric field "looks" for a doorknob. Sort of. It looks for the closest and easiest path to release its charge. Often lightning occurs between clouds or inside a cloud.
But the lightning we usually care about most is the lightning that goes from clouds to ground—because that's us!
As the storm moves over the ground, the strong negative charge in the cloud attracts positive charges in the ground. These positive charges move up into the tallest objects like trees, telephone poles, and houses. A "stepped leader" of negative charge descends from the cloud seeking out a path toward the ground. Although this phase of a lightning strike is too rapid for human eyes, this slow-motion video shows it happening.
As the negative charge gets close to the ground, a positive charge, called a streamer, reaches up to meet the negative charge. The channels connect and we see the lightning stroke. We may see several strokes using the same path, giving the lightning bolt a flickering appearance, before the electrical discharge is complete.
Click for larger animated image!
What causes thunder?
In a fraction of a second, lightning heats the air around it to incredible temperatures—as hot as 54,000 °F (30,000 °C). That's five times hotter than the surface of the Sun!
The heated air expands explosively, creating a shockwave as the surrounding air is rapidly compressed. The air then contracts rapidly as it cools. This creates an initial CRACK sound, followed by rumbles as the column of air continues to vibrate.
If we are watching the sky, we see the lightning before we hear the thunder. That is because light travels much faster than sound waves. We can estimate the distance of the lightning by counting how many seconds it takes until we hear the thunder. It takes approximately 5 seconds for the sound to travel 1 mile. If the thunder follows the lightning almost instantly, you know the lightning is too close for comfort!
What does lightning look like from space?
Lightning observed by the GOES-16 Geostationary Lightning Mapper (GLM) illuminates the storms developing over southeast Texas on the morning of February 14, 2017.
Lightning is an important part of weather forecasting. The Geostationary Lightning Mapper instrument on the GOES-R series satellites can detect lightning activity over nearly the whole Western Hemisphere.
Scientists use data from GOES-R series satellites, along with data from the Lightning Imaging Sensor on NASA's Tropical Rainfall Measuring Mission satellite, to study lightning. This complete picture of lightning at any given time will improve "now-casting" of dangerous thunderstorms, tornadoes, hail, and flash floods.