The (Quick & Dirty) Science Behind Lightning
The speed of light is 670,616,629 mph or 299,792,458 meters per 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. This is two to six times the temperature of the surface of the sun! As a result of this heat the surrounding air rapidly expands and then suddenly contracts. This causes the sound wave or thunder.
Distance of a Lightning Storm
If you don’t hear any thunder but see a lightning strike, the lightning is likely over 10 miles away. The miles of atmosphere has absorbed the sound waves. 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.
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
Summary
Use the above equation to calculate a series of distances from a few different strikes. From these calculations you will be able to determine if the thunderstorm is stationary, moving toward you or away. If you don’t calculate any change in the calculated distance then the storm is stationary. However, 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.