Why the 30/30 Rule Is Not Enough

Lightning is one of the fastest and most destructive natural forces. In the United States alone lightning kills an average of 49 people a year1 and costs businesses tens of millions of dollars in losses annually2. It goes without saying that detecting lightning and related convective activity presents a costly challenge for any business operating most metal facilities, with ample hazardous materials and human operators present. Yet most facilities use a simple lightning-to-thunder distance calculation based on the methods of the early 20th century3. The rule originally named flash-to-bang is now simply known as the 30/30 rule, it consists of two parts:

  1. If you hear thunder within 30 seconds of seeing a lightning flash, seek shelter.
  2. Remain inside until 30 minutes after you hear the last clap of thunder.

Thunder is a result of the air vibrations caused by lightning, since the speed of sound is known, a simple calculation can estimate the distance of the lightning strike. This is the most basic method of lightning detection and ranging and is often used for public outings, sports events and million dollar enterprises in lieu of a more accurate system.


There are two inherent weakness in the 30/30 rule, the first is that the calculation is based on the belief that lightning strikes can travel an average of six miles, although instances of lightning striking ten and even twenty miles away from a storm are common4. A 1998 study by Holle, Lopez, & Zimmerman found that six miles was the minimum recommended safe distance5. The study also found that up to 80 percent deaths and injuries related to lightning occurred outside of the immediate storm area3. The second finding invalidates the second part of the 30/30 rule whereby you are instructed to shelter 30 minutes after hearing the last clap of thunder. Though the storm and the lightning have passed, the potential for a lightning strike can still exist.


The second weakness of the 30/30 rule, is the reliance on human observations. Even in ideal conditions thunder can only be heard up to ten miles away4, meaning that if something obstructs the view of faraway lightning storm, lives can be in danger long before anyone sees or hears the first signs of lightning. This is especially dangerous to those working on top of mostly metal structures, especially if those facilities are involved in hazardous chemical processing. Even optical based sensors can only detect lightning re-actively, by the time an evacuation signal is triggered, some personnel may not have enough time to get to safety before the storm and the lightning are on top of them.


A truly predictive lightning detection solution would have to integrate an electric field meter to offer a proactive lightning detection system. Using an electric field meter to measure the atmospheric electric potential, allows to measure the potential of lightning before and after a storm. Electric field meters are not only capable of detecting the likelihood of lightning before it occurs, they also require less signal filtering and trigger fewer false alarms3. Field meters however lack the distance of the optical lightning detection sensors.


A comprehensive lightning detection solution should therefore be comprised of optical lightning sensors complemented with atmospheric electric field sensors. That way if a turnaround operation involving some welding, triggers the optical lightning sensors a look at the electric field meter can determine that it’s an anomaly. Conversely, if an optical sensor detects lightning outside of the range of the field meter, it can trigger a warning or evacuation alarms to give personnel time to evacuate to safety. Data driven lightning detection takes the guess work and the human element out of safety. With the ability to identify when lightning is detected, probable and imminent facilities can stay smarter and safer.


For information about SAFER Systems Lightning Detection Solutions contact sales via email at sales@safersystem.com or call 805.383.9711.



  1. National Weather Service
  2. Lightning Protection Institute
  3. Private University Products and News
  4. The National Severe Storms Laboratory
  5. Bulletin of the American Meteorological Society

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