How to tell if a storm is dangerous 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.

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.

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!

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.

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!

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.

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!

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.

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.

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.

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.

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.

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.