Understanding Lightning & Thunder on the Trail

When hiking, particularly above treeline, the importance of lightning awareness cannot be understated. Understanding more about lightning and thunder can help keep you safe.

The (Quick & Dirty) Science Behind Lightning

The speed of light is 670,616,629 mph (299,792,458 meters/second), so when you see a lightning strike you’re perceiving it practically instantaneously. Every strike heats up the surrounding air to between 18,000 to 60,000 degrees Fahrenheit (two to six times the temperature of the surface of the sun), which causes the surrounding air to rapidly expand and then suddenly contract, resulting in a sound wave – thunder.

A lightning strike on a dark mountain.

Distance of a Storm

If you don’t hear any thunder but see a lightning strike, the lightning is likely over 10 miles away from you. The sound wave is being absorbed by the atmosphere before reaching you. As such, if you see a lightning strike but don’t hear the corresponding thunder you are likely not in immediate danger. Storms move quickly, however, and it’s important to hike off of exposed ridges, passes, and summits and to prepare for the storm to potentially reach you soon.  

If you do hear thunder, you are within about 10 miles of the lightning strike and should seek immediate protection from a lightning strike.

The American Hiking Society has a downloadable pdf titled Lightning Safety that outlines specific precautions that should be taken.

Lightning Safety Guidelines

Prepare:

Always check the weather forecast before leaving town, if conditions are not safe on trail find a safe place to shelter in town.

If a storm is approaching:

Always descend from peaks, ridges or elevated areas when in a storm or when one is approaching.

Seek protection in town:

A safe structure is a building that is fully enclosed and has electricity and plumbing. A public library would be a great place to temporarily shelter in town if you can’t find a hostel or motel. Structures like trail shelters, patios, tents, or sheds are not safe during a lightning storm. 

Seek protection on trail:

If you can not find a suitable shelter on trail seek protection by immediately reducing elevation, find a valley or depression in the area and avoid isolated trees or tall structures. 

Position yourself correctly:

Never lie flat on the ground during a lightning storm. You should crouch on the ground with your feet together and with your weight on the balls of your feet.

The tips above are not comprehensive. Please review detailed information from the National Oceanic and Atmospheric Administration (NOAA) and the American Hiking Society before your next outdoor adventure.

Day Lightning
Lightning Strike

To determine the approximate distance (in miles) of lightning from you, count the number of seconds between the observed lightning flash and the sound of thunder, then divide this number by five. (See equation below.)

Approximate Distance Away (miles) = number of seconds between flash and sound / 5

Use the above equation to calculate a series of distances from a few different strikes and you will be able to determine if the thunderstorm is stationary, moving toward you or away from you. If the calculated distance doesn’t change between lightning strikes you can assume the storm is stationary. If the calculated distance gets larger between lightning strikes you can assume the storm is moving away from you. If the distance gets shorter between lightning strikes the storm is moving closer to you and storm precautions should be observed.