Why shouldn't you shelter under a tree
during a lightning storm?
When it starts raining, people head for a tree. For ordinary rain, that's fine. But the moment thunder is rolling, a tree becomes one of the most dangerous places you can stand. Lightning doesn't stop once it hits a tree — it can jump sideways from the tree to you.
A summer evening, a park or playing field. The sky suddenly darkens and fat raindrops start to fall. Thunder rumbles in the distance. There's no building anywhere nearby.
Ahead of you stands a big tree, thick with leaves, dry underneath. Everyone wants to run there. Most people do.
But look through records of lightning injuries and deaths, and "sheltering under a tree" comes up again and again. A tree will keep the rain off you. It will not keep lightning off you. If anything, it draws lightning in.
When lightning strikes a tree, electricity running down the trunk can jump sideways to anyone standing nearby. This is called a side flash. The strike doesn't need to hit you directly — it can reach you from the tree.
In an open area with nothing else around, a tree is the tallest point. Lightning favors tall objects. Standing under a tree means standing right beneath where the strike is most likely to land.
These two dangers stack. You're standing in the spot most likely to be struck ("it's tall"), at a distance close enough to be hit by the jump ("it flashes sideways"). That's exactly what being under a tree means.
Reason 1: lightning looks for the "easier path" and jumps
Lightning is a sudden, massive discharge of electricity that has built up between the sky and the ground. Electricity always tries to take whatever path conducts it best.
A tree trunk, it turns out, isn't that good a conductor. A wet surface helps a little, but the inside is just damp wood. A human body, by contrast, is mostly water and salt — a far better conductor than wood.
If part of the current coming down the trunk finds that "this path is easier," it will jump across tens of centimeters to several meters of air and into a person's body. That's a side flash. It doesn't even need to strike you directly — a strike on the tree right next to you has the same effect.
A lightning rod is safe because, once it catches a strike, it has a thick metal cable built to carry the current reliably down into the ground. Its job isn't to avoid attracting lightning — it's to channel it away safely.
A tree has no such path. It attracts the strike, then the current just finds whatever way it can. That's why it jumps to the nearest living thing.
Reason 2: even once it reaches the ground, it's still dangerous
Current that travels down a tree and into the ground doesn't simply vanish there. It spreads outward through the soil, radiating out from the strike point.
The current is strongest close to the strike point and weakens with distance. In other words, the "strength" of the current varies from spot to spot on the ground.
Now imagine you're standing with one foot forward and one back. The ground under each foot carries a different strength of current. That difference drives current in through one foot, through your body, and out the other foot. This is called step voltage.
This is thought to be why animals like cows and horses — whose front and back legs are far apart — are especially prone to being killed by lightning. The same applies to people: keeping your feet together shrinks the gap between them, and less current passes through your body.
It can jump from a tree, or travel to you through the ground.
So what should you actually do?
- If you hear thunder, you're already at riskIf you can hear thunder, you're within range of a strike. There is no "it's still far off, I'm fine." The moment you hear rumbling, start moving.
- Get into a building or a carA reinforced-concrete building is safest. Next best: a car, bus, or train. Even a wooden house is far better than being outdoors. Indoors, stay a little away from walls, appliances, and taps.
- Stay away from tall trees, utility poles, and towersDon't go near these while thunder is active. Only if there is truly nowhere else to go, move several meters or more away from them (as far as possible), crouch down, and keep your feet together. But understand this is a last resort, not a safe place — treat it as something to do only until you can reach shelter.
- "Rubber boots or a raincoat will protect you" — false. Lightning's voltage punches through a few centimeters of rubber with ease. Against hundreds of millions of volts, rubber is no insulator at all.
- "Wearing metal makes lightning more likely to strike you" — false. Small metal items like jewelry or umbrella ribs are not thought to change where a strike lands. Stopping to remove them wastes time that's far more dangerous. Metal can still cause burns, though, since it heats up quickly.
- "A car is safe because of its rubber tires" — false. What makes a car safe is its metal body, which carries current around the outside. That's why convertibles, motorcycles, and bicycles are not safe.
- "Lightning never strikes the same place twice" — false. Lightning favors tall, exposed spots, so it can strike a good location again and again. A place that's been struck once is, if anything, more likely to be struck again. Tall structures like Tokyo Skytree are hit by lightning repeatedly, year after year.
- "Standing away from the trunk under a tree is fine" — not enough. It helps, but avoid choosing to stand under a tree at all.
Touching someone who has been struck by lightning will not shock you. The body doesn't retain electricity. Go to them without hesitation.
- Call emergency services immediately and report a lightning strike
- If there's no breathing or response, start chest compressions (CPR). Use an AED if one is available
- Even if the heart or breathing has stopped, prompt treatment can save someone struck by lightning
- But if the area is still dangerous, move to safety first — the next strike can hit the same spot
Light and sound tell you the distance
- Start counting seconds the instant you see the flash
- Stop counting when you hear the thunder
- Seconds counted ÷ 3 gives the rough distance in kilometers. 3 seconds ≈ 1 km, 9 seconds ≈ 3 km
Light reaches you almost instantly, but sound travels only about 340 meters per second. This method uses that gap. But don't use it to decide "it's far, so I'm safe." Storm clouds can move in minutes, and a strike can land well outside the edge of the visible cloud. Use the count not to measure distance but to tell whether the storm is getting closer. If the seconds are shrinking, head indoors fast.
Summary
A tree isn't dangerous simply because "it's a bad place to dodge rain." Three things stack up in the same spot: ① it's the tallest thing around, so it attracts the strike; ② the current can jump from the tree to a person; and ③ the current then spreads further through the ground.
The moment you hear thunder, head for a building or a car — not a tree.
That alone prevents most lightning accidents.
Lightning doesn't fall in a straight line but zigzags because the current is carving its path as it goes — which is also why no one can predict exactly where it will land. We go into this in "Why does lightning fall in jagged zigzags instead of a straight line?"
Want to go deeper? ― terms, formulas, and how this connects to the classroomWe mark which level each topic belongs to, from middle-school science to open research questions
- MSCovered in middle-school science
- HSCovered in high-school "Physics Basics"
- HS+High-school "Physics," or an advanced/sidebar topic in textbooks
- UnivUniversity-level specialist content (atmospheric electricity, high-voltage engineering) not taught in high school
- ResearchNot settled fact even at university level — an active research question
MSTerms: lightning vocabulary
- Side flash: the phenomenon described above, where current jumps from a tree to a nearby person. Thought to be the main cause of accidents under trees.
- Step voltage: the voltage difference between two feet created by current spreading through the ground.
- Direct strike: lightning that hits a person or object directly.
- Zone of protection: an area near a tall object considered relatively less likely to be struck directly. But this requires being sufficiently far from that object — directly beneath a tree does not count.
- Lightning rod: equipment that catches a strike and carries it safely to the ground via a thick conductor. It doesn't prevent lightning — it provides it a path.
MSHSLightning is really just "very large static electricity"
The static shock that makes your hair stand up when you rub it with a plastic ruler and lightning belong to the same family. Only the scale differs. That smaller version of static can even ignite things in the right conditions (see static electricity while refueling).
Inside a storm cloud, updrafts send ice particles and hail crashing into each other violently. These collisions exchange electric charge between particles, carrying lighter particles upward and heavier ones downward. The result is that opposite charges build up near the top and bottom of the cloud — the same relationship as the ruler and your hair, just happening inside a cloud.
Once the built-up charge exceeds a limit, it forces its way through the air all at once. Air normally doesn't conduct electricity, but at a high enough voltage, it does. This is called dielectric breakdown.
HSChecking with a formula: hearing thunder means you're already within range
The most useful lightning calculation is measuring how close a strike actually is, yourself. No clock or equipment needed.
distance = speed of sound × elapsed time
| distance | distance to the lightning [m] |
| speed of sound | about 340 m/s |
| elapsed time | seconds from flash to thunder |
Light arrives almost instantly, so the moment you see the flash can be treated as the moment the strike happened. All you need is to count the seconds until the sound arrives.
Counting "one, two, three" slowly gives a rough number of seconds. This formula needs no equipment at all. That's the most important thing about this calculation.
| 3 seconds from flash to thunder | 340 × 3 = 1020 m (about 1 km) |
| 10 seconds from flash to thunder | 340 × 10 = 3400 m |
| Converted to kilometers | 3400 ÷ 1000 = 3.4 km |
| 30 seconds from flash to thunder | 340 × 30 = 10200 m (about 10 km) |
As a rough rule, "seconds × 340 m," or about 1 km per 3 seconds, is good enough.
It's tempting to think "10 seconds means it's 3.4 km away, so I'm fine." This is the single biggest mistake this article wants to correct.
The next strike won't necessarily hit the same spot as the last one. A storm cloud spans several to over ten kilometers, and a strike can land anywhere within that span. If one just struck 3.4 km away, that means you're already under that same cloud, or right next to it.
| Range implied by hearing thunder | up to roughly 340 × 30 = 10200 m |
| Span of a storm cloud's strike zone | on the order of 10 km |
| What this means | hearing thunder = already within range |
This calculation isn't for working out "how many minutes do I have left." It's for recognizing "this is already dangerous." The moment you hear thunder, get indoors or into a car, regardless of distance.
It's also widely recommended to stay inside for about 30 minutes after the last clap of thunder. Even once the sound stops, the cloud may still be overhead.
The scale involved explains why standing under a tree is so dangerous. We can estimate it with energy = voltage × current × time.
| Voltage | about 100 million V = 10⁸ V |
| Current | about 30,000 A = 3 × 10⁴ A |
| Duration of flow | about 0.0001 seconds = 10⁻⁴ s |
| Multiplying together | 10⁸ × 3 × 10⁴ × 10⁻⁴ = 3 × 10⁸ J |
| Heat to raise 1 tonne (1000 kg) of water 1°C | 4.2 × 1,000,000 = 4,200,000 J |
| How much can 1 tonne of water be heated? | 300,000,000 ÷ 4,200,000 ≈ 71°C |
That's enough heat to raise 1 tonne of water by 71°C, released in one ten-thousandth of a second. Hitting a tree, it flashes the moisture inside the trunk instantly into steam, splitting the wood apart.
And on its way down through the tree toward the ground, that current can jump to a nearby person (side flash). A tree isn't a place to choose just because it keeps the rain off. Get into a building or a car first. If that's not possible, the next section covers what to think about instead.
* Voltage, current, and duration vary enormously between strikes; the figures here are representative order-of-magnitude values.
HSLightning by the numbers
| Voltage | around 100 million volts (household power is 100 V) |
| Current | around 30,000 amperes (a household breaker trips around 30 A) |
| Duration of flow | roughly a thousandth of a second |
| Temperature of the discharge channel | around 30,000°C (about 5 times the sun's surface temperature) |
| Speed sound travels | about 340 m/s (light travels about 300,000 km/s) |
* Lightning varies enormously strike to strike. The figures above are rough guides.
These numbers also explain what thunder actually is. When the discharge channel is instantly heated to 30,000°C, the air around it expands explosively. The shock wave that results, traveling as sound, is thunder. The long rumbling happens because the discharge channel stretches several kilometers, so sound from the near part arrives before sound from the far part, spreading the noise out over time.
HS+Why rubber "doesn't work"
Whether an insulator can block electricity comes down to how much voltage it can withstand. Air breaks down at roughly 30,000 volts per centimeter (a rough figure for dry conditions). Rubber withstands more than that, but not by an overwhelming margin.
Lightning's voltage is on the order of 100 million volts — far beyond what a few centimeters of rubber boot can withstand. For current that has just punched through several kilometers of air, a few centimeters of rubber is no obstacle at all.
A car, on the other hand, is safe not because of insulation but because it's surrounded by a conductor. Inside a metal enclosure, current flows along the outer surface and barely affects the interior. This property is called electrostatic shielding, known more widely as the Faraday cage. Rubber tires have nothing to do with it.
UnivStep voltage in formula form
When a current I spreads through the ground, the potential at distance r from the strike point is approximately V(r) = ρI / (2πr), where ρ is the ground's resistivity (assuming roughly uniform hemispherical spreading).
If the two feet sit at r and r + d (d being the stride length), the potential difference between them is ΔV = ρI/(2π) × [1/r − 1/(r+d)]. Because this takes the form of a difference in 1/r, it grows sharply as r gets smaller — that is, the closer you are to the strike point. This is also the basis for the advice to keep feet together: shrinking the stride d shrinks ΔV.
This same reasoning is codified as "permissible touch voltage / step voltage" in grounding design standards for power transmission equipment and substations, part of electrical and high-voltage engineering.
ResearchWhat's still unknown
- Why does lightning start at all? This is a famous unsolved problem in atmospheric electricity. Dielectric breakdown of air is thought to require roughly 3 million volts per meter, but the electric fields actually measured inside storm clouds are nearly an order of magnitude smaller. Yet discharges happen anyway. Theories involving cosmic rays as a trigger (runaway breakdown) have been proposed, but the question remains unsettled. Even though it happens constantly, all over the world, every day, exactly how it begins is still not established.
- Where a strike will land still can't be predicted. We can observe a storm cloud approaching, but not pinpoint where its next strike will hit. That's why safety advice isn't about "predicting" — it's about getting away from the exposed spot.
- The crouching posture is being reconsidered. "Lightning crouch" used to be taught as standard advice, but the U.S. National Weather Service dropped it from its recommendations around 2008. The reasoning: it "offers no meaningful protection while giving a false sense of safety." Japan still presents it as a last resort, but it's more accurate to think of it as "something to do when nothing else is possible," not "a posture that will save you."
- Ball lightning remains unexplained. Reports of glowing, floating orbs during thunderstorms exist worldwide, and a few have even been caught on camera by chance, but the phenomenon has never been reproduced or fully explained.
- Winter lightning on Japan's west coast is globally unusual. Some discharges observed there carry more energy than typical summer lightning, and research continues, driven partly by damage to wind turbines.
In short, lightning is something we encounter constantly, yet it remains an active research frontier. Treat everything in this article as "what's currently understood."
How this maps onto the curriculum (by level)
| Level | Subject / unit | Where it appears here |
|---|---|---|
| MS | Science: static electricity and current / properties of sound / weather | What lightning is, the thunder-distance calculation, storm clouds |
| HS | Physics Basics: current and voltage / heat / waves (speed of sound) | Voltage and current figures, why thunder rumbles for a long time |
| HS+ | Physics: electric field and potential / electrostatic shielding (an advanced topic in many textbooks) | Dielectric breakdown, why rubber doesn't help, why cars are safe |
| Univ | Atmospheric electricity / high-voltage engineering / power engineering | The step-voltage formula, grounding design, lightning protection equipment |
| Research | Physics of lightning discharge (unsolved) | The initiation problem, predicting strike locations, reassessing the crouch, ball lightning, winter lightning |
| ― | Disaster preparedness / safety education | Move when you hear thunder, go indoors or to a car, calling for help and giving first aid |
- Japan Meteorological Agency explanatory materials on lightning, and guidance on using the lightning nowcast service.
- Society of Atmospheric Electricity of Japan, "How to protect yourself from lightning" (safety guidelines; distance from trees and utility poles, the concept of zones of protection).
- U.S. National Weather Service, Lightning Safety (including the history of dropping the so-called "lightning crouch" from its recommendations).
- Dwyer, J. R. & Uman, M. A., The physics of lightning, Physics Reports 534(4), 147–241, 2014 (the lightning initiation problem, runaway breakdown).
- Technical standards for electrical facilities and grounding-design standards (treatment of touch voltage and step voltage).
- Japanese Association for Acute Medicine and related bodies, explanatory material on first aid for lightning-strike casualties.
* Values for voltage, current, temperature, and similar quantities vary widely depending on the scale of the strike and measurement conditions. This article presents commonly cited representative figures.
※ This article is a general-audience science explainer. For actual safety decisions, follow announcements from the Japan Meteorological Agency and instructions from local authorities, fire departments, and site managers. The figures given here are approximations meant to illustrate the underlying mechanisms.