Lumen Sites for mine & construction sites

Lightning distance calculator

Count the seconds between the flash and the thunder to work out how far away the lightning struck.

5.2 km away

Within about 10 km: close enough for the next strike to reach you. Get to shelter now.

How it works

The flash reaches you almost instantly. The thunder travels at the speed of sound, about 340 metres every second, a little faster on hot days. So every 3 seconds you count is roughly 1 km.

distance (km) = seconds × speed of sound ÷ 1,000, where the speed of sound in m/s ≈ 331 + 0.6 × air temperature (°C)

Distance at 20 °C, when sound travels about 343 m a second.
Seconds counted	Distance
3	1.0 km
6	2.1 km
9	3.1 km
15	5.2 km
30	10.3 km
45	15.5 km
60	20.6 km

The 30/30 rule

If the count is 30 seconds or less, the lightning is about 10 km away or closer. That is close enough for the next strike to reach you, because lightning can strike well away from the rain. Stop outdoor work and get to shelter.

Then wait 30 minutes after the last thunder before going back outside. Lightning can still strike as a storm moves away, and the trailing edge of a storm is easy to underestimate.

What counts as shelter

Safe: a substantial building with wiring and plumbing, or a fully enclosed hard-top vehicle or machine cab with the doors and windows shut.
Not safe: open-sided sheds and shade structures, tents, open vehicles and plant without an enclosed cab, and anywhere under a tree.
If there is no shelter: get off high ground and away from tall isolated objects, metal fences and open water.

Why work sites don’t rely on counting

Counting is a good habit for one person outdoors. It is not enough to run a site on:

Plant, crushers and haul trucks drown out distant thunder, so a storm can get close before anyone hears it.
Thunder usually carries no more than about 15 km even in quiet conditions, so the first strikes from a growing storm can go unheard.
One person counting covers one spot, and a large operation has pits, plants and camps kilometres apart.
Sites with charged blast holes need to act much earlier than the 30/30 rule, under their explosives procedures.

That is why mines and construction sites use lightning detection, with a warning distance and a stop distance written into their trigger action response plans (TARPs). Lumen Sites watches strikes from the Vaisala global lightning detection network around each site, sets a site to Stop when a strike lands inside its stop distance, and holds it until 30 minutes after the last strike inside the ring.

Limits of counting

The count is a rough guide. Several strikes close together make it hard to match a flash to its thunder, hills and buildings can echo the sound, and the strike you hear is not always the closest one. Treat any thunder you can hear as a reason to stop and check.