Summer in the mountains brings magnificent days — and the worst of the weather's fury. Thermal storms are common there, violent, and sometimes develop in under an hour: a blue sky at 10am can turn into an electric hell by 2pm. The cause is warm air rising from the valleys and meeting cold air at altitude, forming storm cells that grow visibly before your eyes. Lightning, hypothermia from a sudden temperature drop, and falls on soaked terrain become real threats — ones that hikers in a hurry to reach a summit often underestimate.
The warning signs to watch for
The sky speaks to those who know how to read it. Watch for cumulus congestus — those cauliflower-shaped clouds that build vertically around midday, often as early as 11am in summer. A top that flattens into an anvil shape signals that the storm cell has reached maturity and could discharge at any moment. If your hair stands on end, your skin tingles, or you hear a continuous metallic buzzing (the corona effect, also known as St. Elmo's Fire), lightning is imminent: you are literally standing inside an electric field that is charging up.
The 30-second rule for gauging the threat
You don't need a barometer to know whether the storm is far away or right on top of you. As soon as you see a flash of lightning, count the seconds until you hear thunder. Sound travels at roughly 340 meters per second, so every 3 seconds corresponds to about 1 kilometer of distance. If less than 30 seconds separate the flash from the thunder, the storm is within 10 kilometers — the threshold at which lightning can already strike you, even without rain overhead. This is what's known, across the Atlantic, as the "30-30 rule": under 30 seconds, you seek shelter; and once sheltered, you wait at least 30 minutes after the last thunderclap before heading back out.
The crisis checklist for exposed terrain
If you can't get back down to the valley or reach a staffed refuge in time, apply these strict rules immediately:
- Leave summits and ridgelines: descend as low as possible, away from prominent features and ridgelines that naturally attract a discharge.
- Move away from metal objects: set your trekking poles, ice axe, or frame backpack at least 30 meters away from you, and remove any metal jewelry.
- Don't shelter under an isolated tree: it's the perfect lightning rod. Also avoid small, shallow rock overhangs, where the current can arc across the gap above you.
- Take the safety position: crouch on your sleeping pad or backpack to insulate yourself from the ground, feet together, head tucked into your shoulders. Never lie flat on the ground — a stretched-out body offers a larger contact surface to the current traveling through the ground after a nearby strike.
- Spread the group out: if you're hiking with others, stay a few meters apart rather than clustering together, so a single strike doesn't hit everyone at once.
"You don't negotiate with a storm: you outrun it, or you take shelter. The summit, for its part, will still be there tomorrow."
If lightning strikes someone near you
Contrary to popular belief, a person struck by lightning holds no electrical charge — you can touch them safely and must act fast. First check whether they are breathing and responsive. If they are unconscious and not breathing normally, begin CPR immediately and call for help (112 from a mobile phone in the mountains, even without your own carrier's signal) — cardiac arrest is the leading cause of death after a lightning strike, and early resuscitation changes everything. If the victim is breathing, place them in the recovery position, cover them to limit hypothermia, and monitor for skin burns or altered consciousness while waiting for rescue. Don't stay put in an exposed area to give aid: first move both the victim and yourself out of danger before starting first aid, or you both risk a second strike.
Planning ahead: reading a reliable forecast
Anticipation remains your best ally, long before you ever need the crisis checklist. Check specialized mountain weather forecasts (not just the valley forecast) the day before and the morning of, paying particular attention to the thunderstorm risk index and the predicted onset time of the first cells. Apps that show real-time lightning strikes also let you track a storm system's progress over neighboring ranges before it reaches yours. Plan to start your hikes at dawn: most summer mountain storms form in the early afternoon, so an early start gives you a comfortable margin to be back down — or at least out of exposed areas — before the critical hours.
Once the storm has passed, stay alert
The silence after the last thunderclap doesn't mean the danger has passed. Storm cells can regenerate, or hide a second one right behind the first — which is why you should wait about thirty minutes without any new lightning or thunder before setting off again. Use this pause to also assess the state of the terrain: rain may have soaked the trails, made rock slabs slippery, or triggered small rockslides on steep slopes. Change into dry clothes if you can to limit the risk of hypothermia, which lurks just as much as lightning once the adrenaline wears off.