How to tell if a storm is dangerous when hiking

How to tell if a storm is dangerous when hiking?

Recognise the signs of a dangerous thunderstorm and find out what precautions to take to stay safe in the great outdoors.

Hiking is an exciting activity that lets you reconnect with the elements and nature. As is often the case with outdoor sports, it's essential to remain vigilant in the face of changing weather conditions, particularly thunderstorms.

Damien, a mountain guide in the Hautes-Pyrenees, has taught us how to spot the signs of a dangerous storm and, above all, what precautions to take to stay safe.

The dangers and risks of thunderstorms when hiking

The first danger of a thunderstorm, and the one we all think of first, is lightning. This phenomenon occurs when lightning strikes something (building, tree, object, etc.). The more violent the storm, the more frequent the flashes of lightning and the greater the risk of being struck by lightning.

Lightning can have direct consequences: superficial burns, internal burns, electrification or electrocution and cardiac arrest. There are also indirect dangers and risks associated with thunderstorms: falling materials and trees, heavy showers that can cause flooding, wind and hail.

What are the odds of being struck by lightning?

According to Environment Canada (2021), the probability of being struck by lightning is one in a million. The risks vary depending on where you are: in the mountains or on water, the risk increases considerably.
This probability does not take into account more or less serious lightning-related injuries: trauma from falling objects or trees, hearing or vision damage, etc.

However, by adopting the right attitude and taking the steps listed in our article below, you can minimise the risk of being struck by lightning, either directly or indirectly.

How long does a thunderstorm last?

Thunderstorms are short-lived phenomena lasting a few minutes, or even a few hours in some cases, when thunderstorm zones regenerate continuously in the same place for several hours, with heavy rainfall causing flooding. But in summer, thunderstorms are often brief and not very violent. So you can get back on the road quickly once the storm is over!

Follow the weather forecast: precautions to take

Prevention is the key to avoiding dangerous situations when hiking. Before setting off, always check the local weather forecast to see if there are any weather warnings.

Two hikers in the rain

Where can I find the right weather information?

Mobile apps, specialist websites and local weather stations provide invaluable information for planning your route. Before your hike, check out several weather forecasts to avoid being surprised and, if in doubt, ask at the Tourist Office.

In summer, even if the forecasts look good, especially in the mountains, a thunderstorm can form very quickly. Keep your eyes peeled for signs throughout the day!

Understanding the warning signs of a thunderstorm

Here are the warning signs of an impending thunderstorm:
Cumulonimbus clouds, characterised by their anvil shape and menacing appearance, are often a sign of a severe thunderstorm on the way.
● The formation of a dark, threatening cloud arc.
● You may also notice a rapid increase in wind speed, changes in atmospheric conditions such as a sudden drop in temperature.

If all these signs are present, or if you have any doubts, turn back while there's still time. If you're taken by surprise, here are the right steps to take.

How can you tell if lightning is about to strike? Estimating with lightning and thunder

To estimate how far away you are from a thunderstorm, you need to count the number of seconds between the moment the lightning appears and the moment you hear the thunder.
Let's do some maths: the speed of sound is 337 metres/second and the speed of light is 300,000 kilometres/second. To estimate the approximate distance of the storm, divide the number of seconds between the flash and the thunder by 3. For example, if you count 12 seconds, then the lightning is about four kilometres away.

As you can see, the shorter the time between lightning and thunder, the closer you are to the storm. Try to find a safe haven or shelter while you wait for the bad weather to pass!

Good practice: what to do in the event of a storm?

Once a thunderstorm is in the vicinity, there are some good practices you can follow to limit the dangers. Here are our five tips for staying safe, wherever you are.

Take shelter during the storm: where is the safest place?

If you can take shelter in an enclosed area (hut, cave, etc.), that's ideal. Be patient and wait for the storm to pass before setting off again.

If you are near the car park, take shelter inside your car, avoiding touching any metal parts. There's nothing like your vehicle for shelter: it forms a Faraday cage that electrically insulates the passenger compartment.

Find a place less exposed to lightning

You won't always have a safe, enclosed shelter nearby: so avoid certain areas that are more exposed to lightning than others.

Areas to avoid:
❌ rock faces
❌ bodies of water
❌ ridges and summits
❌ large flat areas
❌ near isolated trees

Areas to take refuge:
✅ steep valleys
✅ dense forests
✅ on a concrete surface

If you spot signs that a storm is approaching, quickly review your options and head for shelter or a safe place.

Keep away from metal objects

Whether you're in a shelter or on open ground (or even in your house!), keep away from metal objects such as hiking poles, tent pegs and fences. Any contact with electrically conductive objects represents a danger in the event of contact electrification (diffusion of the electricity produced by lightning via a conductive object).

As for your mobile phone, keep it in your bag to prevent it from getting wet. As the volume of your mobile phone is minimal, it will have no impact on your risk of being struck by lightning.

NB: On the other hand, electrical appliances plugged in when you’re at home are a danger in the event of a violent storm. That's why it's recommended you unplug everything, especially your smartphones, electrical appliances and any other items connected to the mains before the storm is close.

Adopt a good position

The best position to limit the risks from lightning is to sit with your knees against your chest and your head tucked in, waiting for the storm to pass. By getting closer to the ground in this position, you reduce the risk of electrocution.
Before curling up, place a mattress, your bag, a rope or any other object down to isolate you from the ground. The less direct contact you have with damp ground, the better.

When there are several of you, keep about 3 metres apart to avoid the electricity spreading between you.

Finally, if you have to walk to take shelter, take small steps. The distance between your feet is called "step tension": the further apart your feet are, the greater this tension.

Have the right equipment

When you go hiking, make sure you pack the right equipment to cope with bad weather, including thunderstorms. A waterproof jacket and a change of clothes will be useful for keeping out the cold. A headlamp, a first-aid kit and a detailed map of the region are also essential. It’s also advisable to carry a whistle to attract attention if help is needed.

thunderstorm in the mountains

Hiking requires adequate preparation to cope with changing weather conditions. By being alert to the warning signs of a dangerous storm and taking the appropriate precautions, you can stay safe while you wait for the storm to dissipate, before continuing your adventure.
Take care of yourself and enjoy every step on the trail with care and passion!

photo of the editor


Sports Editor

When I'm not hiking around my native Pyrenees, I’m probably surfing in the Basque Country, my adoptive home. I've always loved sport and I like to try my hand at everything: skiing, mountaineering, climbing, road and mountain biking, trail running, surfing, etc. And I'm going to share with you everything I've learnt over the years.

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