Dive deeper · 2 min read
Lightning strikes when the sky above you is still blue ☀️
The bolt comes from a storm too far away to see, and the thunder is just air exploding outward from its heat. So much for waiting for the rain 🌧️
Lightning can reach you from a thunderstorm that is tens of kilometres away, even when the sky directly overhead is clear blue. The bolt is not coming from the clouds you can see; it is coming from a storm hidden below the horizon. This is why the first sign of danger is often not rain, but a distant flash or a low rumble. [1][2]
How far can a bolt travel?
A typical lightning channel is only a few centimetres wide, but its length can be enormous. Most cloud-to-ground strikes travel a few kilometres, yet horizontal bolts can stretch more than 100 kilometres across the sky. In 2020, the World Meteorological Organization confirmed a single flash over southern Brazil that spanned about 709 kilometres, the longest ever recorded. [3][4]
Why thunder is not a warning
Thunder is the sound of air exploding outward from the lightning channel. A bolt heats the surrounding air to around 30,000 °C in a fraction of a second, and that air expands faster than the speed of sound. The result is a shock wave that we hear as a crack or a rumble. Because sound travels much slower than light, you see the flash first and hear the thunder later. [2][5]
- Count the seconds between flash and thunder, then divide by three to estimate the distance in kilometres. [2]
- If you can hear thunder, you are already within striking range of the storm. [1]
- Lightning can strike more than 15 kilometres sideways from the parent cloud. [1]
The blue-sky bolt
A bolt from the blue happens when a storm is far enough away that its clouds are below your horizon, but a long horizontal channel or a branching leader reaches out toward you. The sky above you can be completely clear, yet the storm is still close enough to send a strike. This is why safety rules say to seek shelter as soon as you hear thunder, not when you feel rain. [1][2]
“When thunder roars, go indoors.”
— U.S. National Weather Service lightning safety campaign
What to do
If you hear thunder, move inside a substantial building or a hard-topped vehicle and stay there for at least 30 minutes after the last rumble. Open shelters, trees, and picnic pavilions do not protect you from a strike. The storm does not need to be overhead to be dangerous. [1][2]
Sources
- Lightning Safety Tips and Resources National Weather Service
- Lightning Science: Frequently Asked Questions National Weather Service
- WMO certifies two megaflash lightning records World Meteorological Organization
- Longest lightning flash: 709 km World Meteorological Organization
- Thunder and Lightning NOAA National Severe Storms Laboratory
