Lightning safety while hiking: leave exposed ground before storms arrive

No place outside is safe when thunderstorms are in the area, according to the National Weather Service. The practical rule for a hiker is to plan the day so that you are off exposed ground when storms are forecast, and to move to safe shelter as soon as you hear thunder. The NWS says that if you hear thunder, lightning is close enough to strike you, and the 30-minute count starts again with every rumble.

What counts as safe shelter

The NWS defines safe shelter as a substantial building with electricity or plumbing, or an enclosed, metal-topped vehicle with the windows up. Those are the two options it gives. Anything outside them, including the structures people usually reach for on a trail, does not qualify.

The NWS brochure names the common cases directly: sheds, picnic shelters, tents and covered porches do not protect you from lightning. A tarp is neither a substantial building nor an enclosed vehicle, so it falls outside the definition for the same reason a tent does.

Once inside, the NWS advises staying away from corded phones, computers and other electrical equipment, avoiding plumbing such as sinks, baths and faucets, and keeping away from windows and doors. It also says not to lie on concrete floors or lean against concrete walls. Cordless and cell phones are safe to use, according to the brochure.

The 30-minute rule runs from the last thunder

The NWS says to stay in safe shelter at least 30 minutes after the last sound of thunder. The clock is tied to the last thunder you hear, not to when the rain stops. The brochure explains why: many lightning deaths occur ahead of storms before any rain arrives, or after storms have seemingly passed and the rain has ended. It also notes that some people were struck after they went back outside too soon.

Work the timing out in minutes. Thunder last heard at 15:20 means the rule keeps you in shelter until at least 15:20 + 30 = 15:50. If a fresh rumble comes at 15:35, the count restarts from that sound, and the wait runs to at least 15:35 + 30 = 16:05. The rain may have stopped by 15:20, and the clock still does not start until the thunder does.

Flash-to-bang: a distance estimate, not a margin

The gap between a flash and its thunder gives a rough distance. Sound travels at the speed given by the ideal-gas relation, c = √(γRT ÷ M), with γ = 1.4 for air, R = 8.314 J/(mol·K), T = 293.15 K for 20 °C, and M = 0.02896 kg/mol for dry air. That gives:

c = √(1.4 × 8.314 × 293.15 ÷ 0.02896) = √117,823 = 343 m/s

1 km ÷ 343 m/s = 2.9 s
1 mile (1,609 m) ÷ 343 m/s = 4.7 s

So each 3 seconds between flash and thunder is roughly 1 km, and each 5 seconds is roughly 1.7 km. The count is only a distance estimate. It does not give a safe margin. The brochure says that if you can hear thunder, you are in danger, and warns not to be fooled by blue skies, because lightning is close enough to pose an immediate threat whenever thunder is audible.

Planning the route around storm timing

Thunderstorm timing varies by region and season, so the forecast for your own area is the starting point. The NWS brochure advises considering postponing activities if thunderstorms are forecast, and monitoring the sky once you are out: towering clouds, darkening skies and flashes of lightning are the signs it lists. Plan to be off exposed high ground before storms are expected, and set a turnaround time that is earlier than the forecast window.

Use the trail time formula to find how long the climb takes. For an 8 km route with 600 m of climb, the tool's default pace of 5 km/h and a climb rate of 10 minutes per 100 m give the moving time below. Descent is ignored with the "none" setting, so the figure covers the climb and flat sections only.

flat time  = 8 km ÷ 5 km/h = 1.6 h = 96 min
climb time = 600 m × 10 min ÷ 100 m = 60 min
moving time = 96 + 60 = 156 min (2 h 36 min)

If the route needs 2 h 36 min to reach the high point, the return and any breaks have to fit in the time before the storm window. For setting the stop point, see the turnaround time guide. The Sun and daylight tool gives sunrise and sunset in UTC, so convert to local time before comparing them with the forecast, and the sunset and remaining daylight guide covers daylight planning.

The NWS brochure also notes that lightning often strikes outside the area of heavy rain and may strike as far as 10 miles (16 km) from any rainfall. A clear patch between cells is therefore not a safe interval. For where to pitch once a storm is possible, the choosing a campsite guide covers site selection.

Last-resort steps when no shelter is near

If you are caught outside with no safe shelter anywhere nearby, the NWS lists steps that may reduce your risk. The NWS does not describe any of them as making you safe outdoors. They are:

• Get off high ground such as hills, mountain ridges or peaks, immediately.
• Never lie flat on the ground.
• Never shelter under an isolated tree, and avoid isolated tall trees, towers and utility poles, because lightning tends to strike the taller objects in an area.
• Never use a cliff or rocky overhang for shelter.
• Get out of and away from ponds, lakes and other bodies of water.
• Stay away from objects that conduct electricity, such as barbed wire fences, power lines and windmills.

The NWS brochure adds two points for groups and open ground. Metal does not attract lightning, but lightning can travel long distances through wires or fences, so keep clear of them. If you are with a group, spreading out may increase the chance that someone is struck, but it tends to prevent multiple casualties and leaves people able to help.

After a strike

People struck by lightning do not carry an electrical charge. The NWS brochure says they may need first aid immediately. Its first step is to call for help, and it tells people to give first aid if they are trained and to move the person to a safer place if possible. The brochure's fuller first-aid steps are a matter for a recognised first-aid course rather than a hiking guide.

Do not wait for the storm to pass before calling for help. The brochure's first instruction is "Call for help. Call 9-1-1" in the United States, and the local emergency number applies elsewhere.

What the site does not model

Trailwright has no lightning model and does not forecast storms. Storm timing, cloud development and local terrain all change the risk on a given day. For what to save and carry before a trip, see the offline trip planning checklist and the ten essentials.