How Misunderstanding Lightning Statistics Can Kill You.
It’s a clear, sunny day and Paul is determined to take advantage of it by going hiking. The weather forecast states there is a chance of afternoon thunderstorms, but that is the typical daily forecast for the Pikes Peak region during the summer. Paul is hardly concerned as he read on the National Weather Service web site that his odds of being struck by lightning are only 1 in 1/775,000 per year[i]. It hardly seems worth worrying about. As Paul closes in on his destination he can hear the thunder. Continuing up the mountain Paul feels the rain start and the thunder is getting louder. No problem, Paul thinks, he is still below tree line, and he is certainly not the tallest object in the area. Paul pauses to put on his Gor-tex jacket as a tree 120 feet away explodes in an explosion of light. When Paul awakes his ears are ringing, his left arm and leg are tingling, and he feels dizzy. Paul was lucky. Unknown to Paul lightning has killed people from that distance before.
This fictional event may sound farfetched, but it isn’t. If you are a hiker, a golfer, a soccer player, or just out walking your dog during a lightning storm what happened to Paul could happen to you. Lightning kills more people in the United States than hurricanes, tornadoes, and earthquakes.[ii] Paul relied on the wrong statistic to evaluate his risk of being injured by lightning. If you are hiking in the Pikes Peak/Palmer Divide area, your risk of being struck by lightning is not 1/775,000; your odds are closer to 1/3,300 or less.
A national insurance company might find the National Weather Service lightning statistic useful, but it is totally worthless for outdoor enthusiasts. Why? The NWS statistic uses the estimated number of people reported to have been injured or killed by lightning over a year compared to the total U.S. Population. There are numerous problems with this calculation. Lightning storms are not evenly distributed throughout the United States. Many states don’t have significant lightning storms at all. Washington State has an Average Flash Density of 0-0.1 fl/sq km/yr, while Florida’s is 10-14 Fl/sq km/yr. In addition, thunder storms are seasonal, occurring more frequently during hot summer months. Even within states with frequent lightning storms such as Florida and Colorado the number of lightning strikes varies tremendously on a county by county basis. Another factor that makes the NWS statistic nearly meaningless is that the vast majority of people head for shelter during an approaching lightning storm. People are inside office buildings, homes, or cars where there risk of being struck by lightning is negligible, assuming they are not using a wired telephone, sitting in a bathtub or staring out the window.
The relevant question for an outdoor enthusiast is: What are the odds that you will be injured by lightning if you are outside when the front of a lighting storm passes overhead? As I live in the Pikes Peak/Palmer Divide area, that is the information I will use to answer this question. To calculate this we need to know how close a lightning strike has to be before it can injure or kill you. Most people killed by lightning are not hit directly. In fact, direct lightning strikes only account for about 3% of lightning fatalities.[iii] What frequently injures people are ground current (40-50%) and side flashes (15-30%). Lightning strikes can injure or kill a person who is 40 meters away from the strike zone.[iv] Another important piece of information is the lightning density of local storms. How many cloud to ground lightning strikes occur over a 1 square kilometer area? This number is extremely variable and changes by season and time of day.
So how do we calculate the odds of being injured by lightning? My calculation is: R = D*A/S. The risk is the annual lightning density (D) multiplied by the lightning strike zone (A) and divided by the number of annual storms in the area. The Woodland Park stroke density is 9.9 strokes / km2 yr-1 and the Colorado Springs stroke density is 9.7 strokes / km2 yr-1.[v] There are around 3 strokes per each flash. We know that lightning can injury and kill from 40 meters away from the strike, which gives us a strike zone of 5,024 meters. Recent data shows there are on average 51 storms during a year. That gives us:
R = (3.3/1,000,000 meters/year)(π (40 meters)2)/51 = 0.0003.25
Your odds of being injured by a lightning strike are close to 1/3,300 – ignoring environmental factors. There are a multitude of additional factors that can alter your baseline chance of getting injured by lightning. Your odds are worse if you are: above tree line, in an open field, under a tree, boating, or golfing. To decrease your odds of injury you need to take shelter.
What can you do to remain safe from lightning during the summer? First plan your outdoor activities in the morning. If you are hiking be off any summits by 11 AM. If you find yourself outdoors and hear the approach of thunder ask yourself a question: Do I feel lucky? If you don’t think you are going to win the lotto that day immediately seek shelter as lightning easily strikes up to ten miles away from the storm. If you can hear thunder you can be struck by lightning. If possible find a metal-topped vehicle with the windows up or a substantial building with electricity and plumbing. If those are not available move away from elevated open areas, get out of any water, do not hide under isolated trees, do not lie down on the ground, do not hide under a rocky overhang, and stay away from barbed wire fences, power lines, etc. Colorado is a beautiful place to enjoy the outdoors. With a little planning and knowledge you can prevent yourself becoming a lightning injury statistic.
[i] From NOAA.gov: NWS Lightning Safety (www.lightningsafety.noaa.gov/odds.htm)
[ii] Medscape: Lightning Injuries, Author Mary Ann Cooper, MD; Chief Editor: Rick Kulkarni, MD
[iii] Medscape: Lightning Injuries
[iv] National Lightning Safety Institute: Manifestations of Lightning Deaths and Injuries, Richard Kithi and NWS Lightning Safety
[v] Brandon Vogt, Cafe Scientifique public lecture, 2/11/14, raw lightning data provided by Vaisala, Inc.