Why You Should Never Shelter
Under a Tree in a Storm
When it starts to rain, you head for a tree. That's fine in ordinary rain. But the moment thunder starts, the space under a tree turns into one of the most dangerous places you can be. Lightning doesn't just stop when it hits a tree — it can jump sideways from there.
It's a summer evening at a park or sports field. The sky suddenly darkens and fat raindrops start to fall. Thunder rumbles somewhere in the distance. There are no buildings nearby.
In front of you stands one large tree, its leaves thick enough to keep the rain off underneath. Anyone would want to run there. In fact, most people do.
But look through records of lightning deaths and injuries, and one phrase keeps coming up: "they were sheltering under a tree." A tree protects you from rain, but not from lightning. Worse, it can draw the lightning toward you.
When lightning strikes a tree, the electricity running down the trunk can jump sideways to anyone standing nearby. This is called a side flash. It doesn't need to hit you directly — a hit on the tree is enough.
In an open area with nothing else around, a tree is the tallest point. Lightning tends to strike the highest object it can find. Standing under a tree means standing right beneath lightning's likely landing spot.
These two dangers combine. You're standing at a spot chosen precisely because it's tall enough to attract a strike — and close enough that, if struck, it can jump to you. That's what standing under a tree really means.
Reason 1: Lightning jumps toward the "easier path"
Lightning is a sudden discharge of electricity that has built up between sky and ground. Electricity always tries to take the path of least resistance.
A tree trunk, it turns out, isn't a great conductor. Its surface might be wet, but inside it's just water-laden wood. A human body, on the other hand, is mostly water and salt, and conducts electricity far better than wood.
As current travels down a tree, if a person's body offers an easier path, the electricity can leap across tens of centimetres to several metres of air straight into that person. This is a side flash. You don't need to be struck 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, a thick metal cable gives the current a guaranteed path to the ground. Its job isn't to avoid attracting lightning — it's to carry it away safely.
A tree has no such path. It attracts the strike, then the current flows however it can — often straight into whoever is standing close by.
Reason 2: Current that reaches the ground is still dangerous
Electricity that travels down a tree and reaches the ground doesn't simply vanish. It spreads outward from the strike point in all directions.
The closer you are to the strike point, the stronger the current; the farther away, the weaker it gets. In other words, the "strength" of the electricity varies from spot to spot on the ground.
Now imagine standing with one foot forward and one back. The ground beneath your front foot and the ground beneath your back foot carry different current strengths. 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 set far apart, are especially likely to be killed by nearby strikes. The same logic applies to people: standing with your feet together shrinks that difference and reduces the current passing through your body.
It can arrive by jumping from a tree or spreading through the ground.
So what should you actually do?
- If you hear thunder, you're already at riskIf you can hear thunder, you're already within range of a strike. There's no such thing as "still far away, so it's fine." The moment you hear rumbling, start moving.
- Get inside a building or a carA reinforced-concrete building is safest. Next best: a car, bus, or train. Even a wooden house beats being outdoors. Once inside, stay a little away from walls, appliances, and taps.
- Stay away from tall trees, utility poles, and towersKeep your distance from these while thunder is active. Only if there's truly nowhere else to go, move several metres or more away from them (as far as you can) and crouch with your feet together. But this is a last resort, not a safe place — think of it as something to do only until you can reach shelter.
- "Rubber boots or a raincoat will keep you safe" — false.Lightning's voltage punches through a few centimetres of rubber with ease. Against hundreds of millions of volts, rubber is no insulator at all.
- "Wearing metal makes you more likely to be struck" — false.Small amounts of metal, like jewellery or an umbrella's ribs, aren't thought to change where lightning strikes. Stopping to remove them wastes precious time and is riskier. That said, metal does get very hot and can cause burns.
- "Cars are safe because of their rubber tyres" — false.What actually keeps you safe is the metal body carrying the current around the outside. That's why convertibles, motorcycles, and bicycles offer no such protection.
- "Lightning never strikes the same place twice" — false.Because lightning favours tall objects, a spot with the right conditions can be struck again and again. A place that's already been hit is, if anything, more likely to be hit again. Tall structures like Tokyo Skytree are struck repeatedly, year after year.
- "Standing under a tree is fine as long as you're away from the trunk" — not enough.Being farther from the trunk does help, but the real fix is avoiding standing under a tree at all.
Touching someone who's been struck by lightning will not electrocute you — the body doesn't retain an electrical charge. Don't hesitate to go to them.
- Call emergency services immediately and tell them a lightning strike occurred
- If there's no breathing or response, begin chest compressions (CPR). Use an AED if one is available
- Even if a strike stops the heart or breathing, prompt treatment can save a life
- But only if the area is safe — move to shelter first if it isn't. The next strike can hit the same spot
Light and sound tell you the distance
- The moment you see a flash, start counting seconds
- Stop counting the instant you hear the rumble
- Divide the count by 3 to get the rough distance in kilometres. 3 seconds ≈ 1 km, 9 seconds ≈ 3 km
Light reaches you almost instantly, but sound travels only about 340 metres per second — that gap is what this trick relies on. But don't use it to decide "it's far away, so I'm safe." Storm clouds move fast, in minutes, and a strike can land well away from the edge of the visible cloud. Use it not to measure distance but to check whether the storm is getting closer. If the count keeps shrinking, get inside quickly.
The takeaway
A tree isn't dangerous merely because it's a poor rain shelter. It's dangerous because three things stack up in exactly the same spot: ① it's the tallest thing around, so lightning picks it; ② a strike can jump from it to a person; and ③ current that reaches the ground keeps spreading outward.
When you hear thunder, head for a building or car — not a tree.
That single habit prevents most lightning accidents.
The reason a lightning bolt zigzags instead of falling in a straight line is that the electricity is carving its own path as it goes — which is also why nobody can predict exactly where the next strike will land. We cover this in more detail in "Why does lightning zigzag instead of falling straight down?"
Want to go deeper? — terms, formulas, and how this connects to the classroomFrom middle-school science to open research questions — each section is labelled by level
- MSCovered in middle-school science
- HSCovered in high-school "Physics Basics"
- HS+Covered in high-school "Physics," or treated as advanced/optional in textbooks
- UnivNot covered in high school — university-level material (atmospheric electricity, high-voltage engineering)
- ResearchNot yet settled even at university level — an active research question
MSTerms: lightning vocabulary
- Side flash: the phenomenon described above, where a strike jumps from a tree to a person. Thought to be a leading cause of injuries suffered under trees.
- Step voltage: the voltage difference between two feet, caused by current spreading through the ground.
- Direct strike: lightning that hits a person or object directly.
- Zone of protection: the area near a tall object that's considered relatively less likely to be struck. This only applies if you're far enough from that object — it does not include the space directly beneath a tree.
- Lightning rod: a device that catches a strike and carries it safely to the ground through a thick conductor. It doesn't prevent lightning — it gives it a safe route.
MSHSLightning is really just "very large static electricity"
Lightning is the same family of phenomenon as the static shock that makes your hair stand up after rubbing a plastic ruler on it — the only difference is scale. That smaller version of static electricity can even ignite fires in the right conditions (see static electricity at the petrol pump).
Inside a storm cloud, updrafts send ice crystals and soft hail crashing into each other violently. These collisions transfer charge between particles, and lighter particles get carried upward while heavier ones sink. As a result, 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 threshold, it forces its way through the air in one sudden discharge. Air normally doesn't conduct electricity, but at a high enough voltage, it does. This is called dielectric breakdown.
HSDo the maths: if you can hear it, you're already within range
The single most useful calculation for lightning safety is measuring how close a storm actually is, using nothing but your own senses.
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 |
Because light arrives almost instantly, the moment you see the flash can be treated as the moment the lightning struck. All that's left is counting how long the sound takes to catch up.
Counting slowly — "one, two, three" — gives you a rough number of seconds. This formula needs no equipment at all. That's exactly what makes it useful.
| 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 kilometres | 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 roughly 1 km per 3 seconds, is all you need to remember.
It's tempting to think "10 seconds means 3.4 km away, so I'm still fine." That's the exact mistake this article most wants to correct.
The next strike doesn't have to land where the last one did. A storm cloud spans several kilometres to over ten kilometres, and a strike can land anywhere within that span. If a strike just landed 3.4 km away, you're already underneath that same cloud, or right next to it.
| Distance implied by audible thunder | At most, roughly 340 × 30 = 10200 m |
| Span in which a storm cloud can produce strikes | Roughly 10 km |
| What this means | Hearing thunder = you're already within range |
This calculation isn't for figuring out how many minutes you have left — it's for recognising that you're already in danger. The moment you hear thunder, get to a building or car, regardless of the distance.
And it's widely recommended to stay indoors for about 30 minutes after the last rumble of thunder. Even once the sound stops, the cloud may still be overhead.
Understanding the scale of a strike 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 | About 0.0001 s = 10⁻⁴ s |
| Multiplying it out | 10⁸ × 3 × 10⁴ × 10⁻⁴ = 3 × 10⁸ J |
| Heat to raise 1 tonne (1000 kg) of water by 1°C | 4.2 × 1,000,000 = 4,200,000 J |
| Temperature rise possible for 1 tonne of water | 300,000,000 ÷ 4,200,000 ≈ 71°C |
That's enough heat to raise the temperature of a tonne of water by 71°C, released in one ten-thousandth of a second. When lightning hits a tree, the moisture inside the trunk flashes to steam instantly, and the wood can split apart.
And on its way down through the tree to the ground, that current can jump to anyone standing nearby (a side flash). Standing under a tree just to stay dry isn't worth the risk. Get into a building or car first. The next section covers what to do if that's not possible.
* Voltage, current, and duration vary widely between individual strikes; the figures here are representative rough estimates for grasping the scale involved.
HSLightning by the numbers
| Voltage | Around 100 million volts (household supply: 100 V) |
| Current | Around 30,000 amps (a home circuit breaker trips around 30 A) |
| Duration | Roughly one thousandth of a second |
| Channel temperature | Around 30,000°C (about 5 times the Sun's surface) |
| Speed of sound | About 340 m/s (light: about 300,000 km/s) |
* The scale of lightning varies greatly from strike to strike. The figures above are rough estimates.
What thunder actually is can also be explained with these numbers. When the discharge channel is instantly heated to around 30,000°C, the surrounding air expands explosively. The shockwave that results travels as sound — that's thunder. The long rumble happens because the discharge channel can stretch several kilometres, and sound from the near part arrives before sound from the far part.
HS+Why rubber can't stop it
Whether an insulator can block electricity comes down to how much voltage it can withstand. Air breaks down (loses its insulating property) at roughly 30,000 volts per centimetre in dry conditions. Rubber can withstand more than that, but not by an overwhelming margin.
Lightning operates at around 100 million volts — far beyond what a few centimetres of rubber boot can withstand. For current that has just punched through several kilometres of air, a few centimetres of rubber is no obstacle at all.
A car, meanwhile, is safe not because of insulation but because it's enclosed in a conductor. Inside a metal box, current flows along the outer surface and has almost no effect inside. This property is called electrostatic shielding, and the structure is known as a Faraday cage. Rubber tyres have nothing to do with it.
UnivStep voltage, expressed as a formula
When current I spreads through the ground, the potential at a distance r from the strike point can be approximated as V(r) = ρI / (2πr), where ρ is the soil's resistivity (assuming uniform hemispherical spreading).
If your two feet sit at distances r and r + d (where d is your 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 shrinks — that is, the closer you are to the strike point. This is also the basis for the advice to keep your feet together: shrinking the stride d shrinks ΔV.
This same reasoning is codified as "allowable touch voltage and step voltage" limits in grounding design for power transmission equipment and substations — material covered in electrical and high-voltage engineering.
ResearchWhat we still don't know
- Why does lightning start in the first place?This is a famous unsolved problem in atmospheric electricity. Breaking down air is thought to require about 3 million volts per metre, but the electric fields actually measured inside storm clouds are nearly an order of magnitude smaller than that — and yet discharges still happen. One proposed trigger involves cosmic rays (runaway breakdown), but the question remains unresolved. Lightning happens somewhere on Earth every single day, and we still don't know exactly how it begins.
- We still can't predict exactly where a strike will land.We can track an approaching storm cloud, but not the precise spot where its next strike will hit. That's why safety advice focuses on "get away from risky spots" rather than "predict the strike."
- The crouching posture is being reassessed.The "lightning crouch" used to be taught as standard practice, but around 2008 the U.S. weather agency dropped it from its official recommendations, reasoning that it offers no meaningful protection while creating a false sense of safety. It's still presented as a last resort in Japan, 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 handful have even been caught on camera by chance, but the phenomenon has never been reproduced or fully explained.
- Winter lightning on Japan's Sea of Japan coast is globally unusual.Some of these strikes carry more discharge energy than typical summer lightning, and research is ongoing, partly driven by the damage they cause to wind turbines.
In short, lightning is a familiar phenomenon that's still, in some respects, an open research frontier. Treat everything in this article as reflecting current understanding, not a closed case.
How this connects to the classroom, by level
| Level | Subject/unit | Where it appears in this article |
|---|---|---|
| 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 so long |
| HS+ | Physics: electric field and potential / electrostatic shielding (often an advanced topic in textbooks) | Dielectric breakdown, why rubber doesn't help, why cars are safe |
| Univ | Atmospheric electricity / high-voltage engineering / power engineering | 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 prevention / safety education | Move when you hear thunder, go indoors or to a car, emergency calls and first aid |
- Japan Meteorological Agency (気象庁), explanatory material on lightning and guidance on using its lightning nowcast service.
- Society of Atmospheric Electricity of Japan (日本大気電気学会), "How to Protect Yourself from Lightning" (safety guidelines), covering distance from trees and utility poles and the concept of protection zones.
- U.S. National Weather Service, Lightning Safety (including the background on dropping the "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).
- Japanese technical standards for electrical equipment and grounding design (treatment of touch voltage and step voltage).
- Explanatory material from the Japanese Association for Acute Medicine (日本救急医学会) and others on first aid for lightning-strike casualties.
* Figures for voltage, current, temperature, and similar values vary widely depending on the scale of the strike and how it was measured. This article presents commonly cited representative estimates.
*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, or site managers. The figures given here are meant to illustrate the underlying mechanisms, not to serve as precise thresholds.