Respect lightning: how it works, what you need to know

Tornadoes get the most attention during severe weather, but lightning can be equally destructive. We saw that recently in the news with several house fires from lightning strikes.

Lightning is also an underrated hazard for people even during garden variety thunderstorms. It tends to kill or injure people, sometimes severely, just one or two at a time, but not provide the dramatic video of other weather disasters.

In the United States, lightning killed more than 200 people per year back in the 1940s and early 1950s with a peak of 432 deaths in 1943.

This is Lightning Safety Awareness Week and the the good news is that these numbers have decreased dramatically in recent decades in spite of population growth. Last year there were 23 lightning-caused deaths in the US, the fewest of record.

The decrease certainly has multiple causes such as peoples' increased awareness of the danger. There also are fewer people working out in the open without protection or nearby shelter. For example, many tractors have metal cabs that would route the strike around the driver and to the ground.

Lightning is a giant spark of static electricity generated by a thunderstorm. All thunderstorms produce lightning, but about ninety percent of it stays in the clouds. Researchers estimate that the remaining ten percent amount to 25 million cloud-to-ground strikes in the U.S. each year.

Exactly how the cloud gets charged up to produce the stroke and how that charging can occur so quickly and repeatedly are not fully understood. It seems that the rain drops, ice crystals and soft hail in the cumulonimbus cloud collide frequently and exchange electrical charges.

The ice crystals take on positive charge and, because they are lightweight, are carried to the top of the cloud by updrafts. The soft hail become negatively charged and, because they are heavier, stay lower in the cloud and give the part of the cloud closest to the ground a negative charge.

The animation below shows this process. This animation and the others to follow are all from the National Oceanic and Atmospheric Administration and can be found along with other information and educational tools at NOAA's lightning safety site.

When sufficient negative charge has built, stepped leaders race toward Earth in steps about 150 feet long.

As the stepped leader gets close to Earth, upward positive streamers will form, often from high points locally, until one of them connects with a stepped leader.

Once a connection has been made, the lightning stroke will discharge the negative charge downward through that one lightning channel. If sufficient charge remains in the cloud, multiple return strokes can occur down the same channel.

The whole stepped leader/lightning stroke process takes much less than a second.

In super-slow-motion photography the lightning stroke seems to go upwards because the channel lights up from bottom to top. That is because the charges nearest the ground move downward first and light up as they go, followed by those above.

It's sort of like watching cars stopped at a red light. After the light turns green, the first car moves followed by the second and then the third, etc., with the motion rippling backward from the traffic light along the line of cars. Here is an animation.

Charge goes downward to Earth, starting from near the ground.