The Science of Keeping Safe from a Lightning Strike

This article was originally posted on on June 10, 2019.

Various news outlets recently reported a fatal accident involving a motorcycle rider being struck by lightning. This is a tragic incident. It comes several days after I watched jaw-dropping video of lightning striking a tree at the Women’s U.S. Open golf tournament. The amazing thing about the video, beyond the lightning itself, was the number of people still milling around outside. It is the season for golf, boating, and other outdoor activities. It is also the season for thunderstorms.

In my 25 years as an atmospheric scientist and professor, I routinely observe that people either don’t fully understand what precautions to take during lightning or underestimate their risks. Here is the science behind strategies to avoid being struck by lightning (indoors or outdoors).

Photo Courtesy: We Love Weather member Wichofilms, Daytona Beach, FL

First of all, the vast majority of lightning flashes (roughly 80% or so)  are intracloud (IC), cloud-to-air (CA) or cloud-to-cloud (CC). It is the cloud-to-ground (CG) lightning flashes that pose a risk to a person standing on a golf course, playing little league baseball, riding a motorcycle, or fishing in a boat. The NOAA National Severe Storms Laboratory website is a good starting point:

Lightning is a giant spark of electricity in the atmosphere between clouds, the air, or the ground. In the early stages of development, air acts as an insulator between the positive and negative charges in the cloud and between the cloud and the ground. When the opposite charges builds up enough, this insulating capacity of the air breaks down and there is a rapid discharge of electricity that we know as lightning. The flash of lightning temporarily equalizes the charged regions in the atmosphere until the opposite charges build up again.

If you are caught outside, there are some key things to remember. Lightning often strikes taller objects like trees, buildings, antennas, and metal poles. Because the atmosphere is a good electrical insulator, lightning is looking for the path of least resistance. Additionally, the physical processes that create a cloud-to-ground lightning stroke depend on the ground itself (graphic below). Most cloud-to-ground lightning is “negatively-charged” so I will succinctly describe that process:

  1. A channel of negatively charged ions surge toward the ground in very distinct steps. This is called a stepped leader. 
  2. As the stepped leader (and its various branches) move toward the ground, it attracts streamers of positive charge.
  3. The electrical potential of the stepped leader connects to the ground, tree, building, or whatever is available in the pathway. The negative charge starts to flow down the pathway.
  4. return stroke explodes “up” the pathway or channel and this is what we actually see as the lightning stroke. It happens so fast that you may not easily discern the direction of propagation.
  5. If there is enough charge left in the cloud, subsequent dart leaders can use the same pathway created by the initial stepped leader. The return strokes associated with dart leaders are why lightning seems to flicker. (This truncated summary is derived from an outstanding National Weather Service educational website called Jet Stream)
5 step cloud-to-ground lightning process (ADAPTATION FROM NWS JET STREAM)

By the way, “positively charged lightning” happens in a similar process except positive ions initially flow downward. It is often associate with “blue sky” or cloud anvils and can be particularly dangerous because often there is no apparent stormy weather evident.

You do not want to be the source of ions for those streamers nor near one. The Centers for Disease Control (CDC) website recommends the following actions if you are caught outside:

If no shelter is available, crouch low, with as little of your body touching the ground as possible. Lightning causes electric currents along the top of the ground that can be deadly over 100 feet away… Stay away from concrete floors or walls. Lightning can travel through any metal wires or bars in concrete walls or flooring.

You might actually be surprised to learn that about one-third of lightning-strike injuries occur indoors. Some tips to remember while inside:

  • Stay away from plumbing or water. Metal piping conducts electricity.
  • Don’t get near electrical equipment like televisions, stereos, or the other fancy smart electronics in homes today.
  • Avoid concrete walls and flooring. Concrete is often supported by metal rods or frames.
  • Don’t use a corded phone, however cellular or cordless phones are fine.