Lightning distance calculator

Seconds between lightning and thunder

The lightning strike was around km away!

Note: We accept no liability for the values given by this calculator; they should always be treated as guidelines only. If in doubt, it’s better to cut your tour short - thunderstorms are not something you should mess around with.

How far away is a thunderstorm?

When you are out in the great outdoors and have no internet access, you can calculate the distance of a storm using the simple 3-second rule: Just count the seconds between when you see the lightning strike and when you hear the thunder clap and divide this number by three. This gives you a rough idea of how many kilometres away the thunderstorm is. If you wait a few minutes and do this calculation again, you can check whether the storm is coming closer or moving away from you.

The idea behind this is that the sound travels one kilometre in around three seconds. This means that you can do the calculation quite easily by using this simple formula.

There are times when you can see the lightning strikes on the horizon, but there is no sound of thunder. This is a sure sign that you don’t need to worry about the storm yet. But how far away is thunder when you can actually hear it? Sound waves are partially absorbed by the atmosphere. So the maximum distance is approximately 18 km.

Formula for calculating the distance of lightnings

Distance (in km) = (Seconds between lightning and thunder x the speed of sound) / 1000

The speed of light can be discounted.

When is a thunderstorm dangerously close?

Now you have a concrete number, and you know how far away the storm is – but how far is far enough? If you think that one, two or three kilometres constitutes a safe distance, you are unfortunately mistaken. Lightning can actually precede a storm front by up to 15 km. This is of course unusual, but still clearly shows how easy it is to misjudge the situation. As a rule you are relatively safe from 20 seconds, as long as the storm is not travelling towards you.

What should I do in case of a thunderstorm?

Well, now you’re right in the middle of it and there is thunder and lightning above you already. There are several things that you should consider to make sure you get out of the situation safely.

Avoid as far as possible:

  • Exposed locations. Where possible descend at least 100 metres from the summit
  • Small caves or overhangs. You need at least one body length of horizontal space and half a body length of vertical space
  • Streams and rivulets which could channel run-off
  • Steel cables. In via ferrata it is imperative that you unhook and move away from the conductive steel cables
  • Isolated trees

If there is no shelter close by, crouch down tightly with both feet close together. It is advisable for groups to spread out.

What do you do during a thunderstorm?

But my grandma always said “Seek the beeches and avoid the oaks”! Unfortunately this is a persistent myth, which we cannot let pass. Lightning can strike any point in open terrain, and certainly has no preference for certain trees. This “wisdom” probably comes from the simple fact that lightning strikes leave much more obvious damage on oak trees, than they do on smooth-barked beeches.

You can tell when a lightning strike is imminent by observing the following signs: There is a gentle crackling in the air, your hair stands on end, metal objects begin to hum and prominent metal objects begin to emit a bluish glow. When you notice these phenomena, you take flight immediately, and follow the instructions described above!

How can I recognise the risk of a thunderstorm in the mountains?

We can distinguish between two fundamental types of storm: Warm and cold front storms.

Heat thunderstorms are generally very easy to recognise and happen on particularly warm days. The clouds typically build up into towering cumulus formations. They tend to form primarily in the afternoon, which means that the morning is calm and can still be used for touring. Though it is still important to take care.

Cold front thunderstorms occur during the passage of a cold front and often trigger a strong change in the weather. The temperature can drop suddenly and high in the mountains it can even begin to snow in summer. Cold front storms are not as easy to identify as warm front storms and so in most cases come as a surprise. Therefore: Check the weather forecast and plan your tours, so that you can always get out of danger.

In general the tendency for thunderstorms is higher in the summer months than in autumn, winter or spring. This is because the land masses are warmed by the sun.

Our tip for mountain touring: check the weather forecast beforehand, keep an eye on weather developments during the tour, and if in doubt turn back or seek shelter quickly, if the weather starts to look treacherous. You should also never head out without warm and waterproof clothing. Thunderstorms are not something you should mess around with, especially in the mountains, and safety should always take priority!

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