Lightning kills a few hundred people a year worldwide, and a disproportionate number of them are outdoors, on high ground, in the afternoon. That last detail is the important one, because it means the danger is mostly a scheduling problem. Thunderstorms in the mountains follow a daily rhythm that is remarkably reliable, and a walker who understands that rhythm, knows the 30/30 rule and can recognise the half-dozen places that must not be occupied when the sky turns, has removed most of the risk before leaving the car. Here is how we think about it.
Why mountain storms build in the afternoon
A summer thunderstorm over the hills is usually a convective storm: the sun heats slopes and valley floors, warm air rises, cools, and the moisture in it condenses into cumulus that grow taller through the day. When the updraught is strong enough, and the air aloft cold enough, the cloud towers into a cumulonimbus that can reach 10 to 15 km (33,000 to 49,000 ft) with an anvil top, and the charge separation inside it produces lightning. The whole process takes hours of sunshine, which is why the first flash in the Alps, the Rockies, the Pyrenees or the Tatras so often arrives between 13:00 and 16:00, and why the classic advice is to be off summits and ridges by early afternoon.
Frontal storms are the exception: they ride in on a cold front at any hour, including at night, and the forecast will normally name them. For convective days we manage the clock; for frontal days we manage the route, or stay low.
The 30/30 rule, and what it is actually counting
Light travels effectively instantly; sound travels at about 340 m per second, so a gap of three seconds between flash and thunder means the strike was roughly 1 km (0.6 mi) away, five seconds about 1.6 km (1 mi). The 30/30 rule takes this and turns it into two thresholds. First: if the count from flash to thunder is 30 seconds or less, the storm is within about 10 km (6 mi) and we are already in range, because lightning can strike several kilometres from the rain that seems to mark the cell, and successive strikes from the same storm are commonly that far apart. Second: after the last thunder, wait 30 minutes before considering the danger over, because the trailing edge of a storm produces strikes long after the rain has stopped.
One caveat: in broken terrain thunder is muffled and echoed, so a count is not precise, and if we can hear thunder at all we are close enough to be hit.
Where not to be
Lightning follows the shortest, most conductive path from cloud to ground, and the ground current spreads out from the strike point across the surface. That gives us a short list of places to avoid absolutely: summits and ridge crests, where we are the highest point; isolated trees and rock pinnacles, for the same reason; cave mouths and shallow overhangs, where a current running down the rock jumps the gap through anything standing in it; flat open ground like a plateau or a lake shore, where a standing person is again the tallest thing; and water, or wet gullies that channel ground current. Via ferrata cables, fixed chains and metal footbridges are conductors, and being clipped to one during a storm is one of the worst positions a walker can be in.
The better positions are relative, not safe. A dense stand of trees of similar height, well below the ridge, is far better than a lone tree. A dry slope partway down, away from gullies and not in a depression that will collect water, is better than the crest. Inside a substantial building or a vehicle with a metal roof is the only genuinely safe place, and both are rare above the treeline.
What to do when caught out
If the count is under 30 and there is no shelter, the priorities are: get down off the crest as quickly as is safe, even 50 m of descent on the lee side helps; spread the group out to at least 15 m (50 ft) apart so that a single strike does not injure everyone; drop poles and anything metal a few metres away, not because they attract lightning but because they conduct ground current across our skin; and if the hair stands on end or the air crackles, adopt the lightning position – crouched on the balls of the feet, feet together, head down, hands over ears – on top of a pack or rope to insulate from the ground. Never lie flat: that maximises the body’s contact with ground current. Then keep counting, and move on only after the 30-minute clearance.
Someone struck by lightning is not electrified afterwards and can be touched immediately. Many victims survive if cardiopulmonary resuscitation is started at once, because the usual cause of death is cardiac and respiratory arrest rather than burns.
Timing the day
The single most effective lightning precaution is an alarm clock. On a forecast convective day we aim to be on the highest, most exposed section before 11:00 and heading down by 13:00, which usually means starting in the dark for a long route. The forecast helps in a specific way: it gives the time when the atmosphere becomes unstable enough for storms, sometimes as a “risk of thunderstorms from 14:00” line, sometimes as a probability that rises through the afternoon. We treat the stated start time as an upper bound and work backwards from it with our known pace, roughly 4 km/h on the flat plus an hour for every 600 m of ascent, to find the latest possible departure.
The morning sky confirms or contradicts the forecast. Small cumulus that form before 10:00, grow visibly taller by 11:00 and start to fray and darken at the base are the signature of a storm day; a hard, cauliflower top that flattens into an anvil means the cell is mature and the first strikes are minutes away. Cumulus that stay flat and small, with clear gaps between them, are fair-weather clouds, and the risk is lower though not zero.
A working plan for storm season
- Check the forecast for instability and the earliest storm hour; set a turnaround time an hour before it.
- Choose routes with an escape line: a descent off the ridge that does not go back over the summit.
- Watch cumulus growth from mid-morning; if cloud tops are towering by noon, shorten the day.
- Count flash to thunder; under 30 seconds, act, and do not resume until 30 minutes after the last rumble.
- Leave metal routes and exposed crests before the storm, never during it.
None of this costs more than an early alarm and the willingness to turn back with the summit in sight. The people hurt by lightning in the mountains are rarely surprised by an unforecast storm; they are usually on the ridge an hour or two later than they meant to be.