Why shouldn't you shelter under a
tree in a thunderstorm?
When it starts raining, people head for the nearest tree. For ordinary rain, that's fine. But the moment lightning is in the picture, a tree turns into one of the most dangerous places you can stand. Lightning doesn't stop once it strikes a tree — it can jump sideways from the tree to anyone standing nearby.
It's a summer evening in a park or playing field. The sky suddenly darkens and big drops of rain start falling. Thunder rumbles in the distance. There are no buildings around.
In front of you stands one large tree, its leaves thick enough to keep the rain off underneath. Anyone would want to run over there. In fact, most people do.
But if you look through records of lightning deaths and injuries, one phrase keeps coming up: "was sheltering from rain under a tree." A tree will keep the rain off you, but it won't keep the lightning off you. If anything, it draws lightning in.
When lightning strikes a tree, the electricity travelling down the trunk can jump sideways to anyone standing nearby. This is called a side flash. The lightning doesn't need to hit you directly — it can still reach you this way.
In an open space with nothing else around, a tree is the tallest thing sticking up. Lightning tends to strike the tallest point. Standing under a tree means standing right beneath the spot lightning is most likely to hit.
These two dangers combine. You're close enough to be hit by the side flash, standing right under the very spot that's most likely to be struck because it's tall. That's what a tree really is, in a storm.
Reason 1: lightning looks for the "easy path" — and jumps to take it
Lightning is what happens when electric charge built up between sky and ground suddenly discharges all at once. Electricity always tries to take the path that conducts it best.
A tree trunk, it turns out, isn't a particularly good conductor. The surface might conduct a bit if it's wet, but the inside is just wood with some water in it. A human body, on the other hand, is mostly water and salt, and conducts electricity far better than wood does.
If some of the current coming down the tree finds that "this path is easier," it will leap across tens of centimetres to a few metres of air and jump straight into a person's body. That's a side flash. You don't need to be struck directly — standing next to a tree that gets struck is enough.
A lightning rod is safe because, after it catches the strike, a thick metal cable gives the current a guaranteed path down to the ground. Its job isn't to stop lightning from being attracted — it's to carry it away safely.
A tree has no such path. It draws the lightning in, then leaves the current to find its own way through, however that happens. Which is why it jumps to whoever's standing nearby.
Reason 2: even once it's in the ground, it's still dangerous
Current that travels down a tree and reaches the ground doesn't simply vanish there. It spreads outward through the soil in all directions from the strike point.
The current is strongest close to the strike point and weaker further away. That means the ground's electrical strength differs depending on exactly where you're standing.
Now imagine you're standing with one foot forward and one foot back. The ground under your front foot and the ground under your back foot are at different electrical strengths. That difference drives current in through one foot, through your body, and out through the other foot. This is called step voltage.
Animals like cows and horses, whose front and back legs are set far apart, are thought to be especially prone to being knocked down by lightning for this very reason. The same logic applies to people: keeping your feet together shrinks the gap between them, which cuts down the current passing through your body.
It reaches you by jumping from a tree, or by travelling through the ground.
So what should you actually do?
- If you can hear it, you're already in dangerIf thunder is audible, lightning could already strike where you are. There's no such thing as "it's still far off, so I'm fine." The moment you hear rumbling, start moving.
- Get inside a building, or into a carA reinforced-concrete building is safest. Next best are cars, buses, or trains. Even a wooden house is far better than being outdoors. Indoors, stay a little away from walls, appliances, and taps.
- Keep clear of tall trees, utility poles, and steel towersStay away from these while thunder is active. Only if there is truly nowhere else to go should you move at least several metres away from them (as far as you can manage) 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 indoors.
- "Rubber boots or a raincoat will keep me safe" — wrong.Lightning's voltage punches straight 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" — wrong.Small amounts of metal, like jewellery or an umbrella frame, don't change where lightning strikes. Stopping to remove them wastes time you don't have and adds risk. That said, metal does heat up fast and can cause burns.
- "Cars are safe because of their rubber tyres" — wrong.What keeps you safe is the metal body, which channels the current around the outside. That's why convertibles, motorbikes, and bicycles don't offer the same protection.
- "Lightning never strikes the same place twice" — wrong.Lightning picks out the tallest, best-positioned spots, and will strike them repeatedly if conditions are right. A place that's already been struck is, if anything, more likely to be struck again. Tall structures such as Tokyo Skytree are hit by lightning year after year.
- "As long as I stay away from the trunk, being under a tree is fine" — not good enough.Standing farther from the trunk does lower the risk, but the right move is to avoid choosing a tree in the first place.
You will not get an electric shock by touching someone who has just been struck by lightning. No electricity remains in a person's body. Don't hesitate to go to them.
- Call emergency services immediately and say they've been struck by lightning
- If they're not breathing or responsive, start chest compressions (CPR). Use an AED if one is available
- Even when a lightning strike stops the heart or breathing, prompt first aid can save the person
- But only once the area itself is safe — move them to safety first if needed. The next strike can hit the same spot
Light and sound can tell you the distance
- The moment you see the flash, start counting seconds
- Stop counting the instant you hear the thunder
- Seconds counted ÷ 3 ≈ distance in kilometres. 3 seconds ≈ about 1 km, 9 seconds ≈ about 3 km
Light reaches you almost instantly, but sound travels only about 340 metres per second — this calculation uses that gap. But don't use it to decide "it's far enough to be safe." Storm clouds move within minutes, and strikes can land away from the edge of the cloud too. Use the count not to learn the distance, but to tell whether the storm is getting closer. If the count keeps shrinking, get indoors fast.
Summary
A tree isn't dangerous because it's "a bad place to shelter from rain." It's dangerous because three things coincide in the same spot: ① it's the tallest thing around, so lightning picks it; ② the current it carries can jump to a nearby person; and ③ once it reaches the ground, it keeps spreading outward.
The moment you hear thunder, head for a building or a car — not a tree.
That alone prevents most accidents.
The reason lightning bolts don't fall in a straight line but bend in jagged zigzags is that the current is forging its own path as it goes — which is also why the strike point can't be predicted beforehand. We cover this in detail in "Why does lightning fall in jagged zigzags instead of a straight line?"
Want to go deeper? ― Terminology, equations, and how this connects to the textbooksFrom middle-school science to topics still being researched, each level is labelled
- Middle schoolcovered in middle-school science
- High schoolcovered in high-school "Basic Physics"
- High school+from high-school "Physics," or treated as advanced/sidebar material in textbooks
- Universitynot covered in high school — from university-level subjects (atmospheric electricity, high-voltage engineering)
- Researchnot yet settled even at university level — an active research question
Middle schoolTerms related to lightning
- Side flash: the phenomenon in the main text where current jumps from a tree to a person. Considered a major cause of lightning accidents under trees.
- Step voltage: the voltage difference created between your two feet 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 is said to be relatively less likely to be struck. However, this requires being far enough from that object — it does not include the area directly beneath a tree.
- Lightning rod: equipment that catches a lightning strike and carries it safely to the ground through a thick conductor. It doesn't prevent lightning — it provides a route for it.
Middle schoolHigh schoolLightning is really just "very large static electricity"
The static that makes your hair stand on end after rubbing it with a plastic ruler is the same family of phenomenon as lightning — only the scale differs. That smaller version of static electricity can also ignite fires in the right place (see static electricity while refuelling).
Inside a storm cloud, updrafts send ice particles and soft hail crashing into each other violently. These collisions exchange charge between particles, with lighter particles carried upward and heavier ones pulled down. The result is that opposite charges accumulate near the top and bottom of the cloud — the same mechanism as the ruler and your hair, just happening inside a cloud.
Once the accumulated 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 anyway. This is called dielectric breakdown.
High schoolChecking the maths: if you can hear it, you're already within range
The single most useful lightning calculation is measuring "how close is it right now" 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 the flash until the thunder arrives |
Light arrives essentially instantly, so you can treat the instant you see the flash as the instant the lightning struck. All you need to do is count the seconds until the sound reaches you.
Counting "one, two, three" slowly gives you a rough number of seconds. This calculation needs no equipment at all — that's the whole point of it.
| 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 good enough.
It's tempting to think "10 seconds means it's 3.4 km away, so I'm still fine." This is the single biggest mistake this article wants you to avoid.
The next strike won't land in the same place as the last one. A storm cloud spans several to over ten kilometres, and a strike can land anywhere within that area. If one just struck 3.4 km away, that means you're already under that same cloud, or right next to it.
| hearing thunder ＝ distance to the lightning | at most around 340 × 30 = 10200 m |
| the area over which a storm cloud can produce strikes | said to be roughly 10 km across |
| what this means | hearing thunder ＝ you're already within range |
The point of this calculation isn't to tell you "how many minutes you have left." It's to tell you "this is already dangerous." Once you hear thunder, get indoors or into a car right away, regardless of distance.
And it's widely recommended that you stay indoors for about 30 minutes after the last rumble of thunder. Even once the sound stops, the cloud may still be overhead.
The scale of a lightning strike explains why a tree is so dangerous. It can be roughly estimated as 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 these | 10⁸ × 3 × 10⁴ × 10⁻⁴ = 3 × 10⁸ J |
| heat needed to raise 1 tonne (1000 kg) of water by 1℃ | 4.2 × 1,000,000 = 4,200,000 J |
| how many degrees that heats 1 tonne of water | 300,000,000 ÷ 4,200,000 ≒ 71 ℃ |
Enough heat to raise 1 tonne of water by 71℃ all at once is released in one ten-thousandth of a second. When it hits a tree, the moisture inside the trunk flashes instantly to steam, and the trunk itself can split apart.
And on its way down through the trunk to the ground, that current can jump sideways to anyone standing nearby (a side flash). A tree is not a place to choose just because it keeps the rain off. Get into a building or car first. What to do if that's not possible is covered in the next section.
※ Voltage, current, and duration vary widely between strikes — the figures here are representative, illustrative values only.
High schoolLightning by the numbers
| voltage | roughly 100 million volts (household supply is 100 V) |
| current | roughly 30,000 amps (a household breaker trips around 30 A) |
| duration | roughly one thousandth of a second |
| temperature of the discharge channel | around 30,000℃ (about 5 times the sun's surface) |
| speed sound travels | about 340 m/s (light travels at about 300,000 km/s) |
※ Lightning strikes vary widely in scale. The figures above are illustrative.
The rumble of thunder can also be explained with these numbers. When the discharge channel is heated to 30,000℃ in an instant, the surrounding air expands explosively. The shockwave that results travels outward and reaches us as sound. The long, rolling rumble happens because the discharge channel stretches several kilometres, and the sound from nearer parts arrives before the sound from farther parts.
High school+Why rubber can't stop it
Whether an insulator can block electricity comes down to how much voltage it can withstand. Air breaks down (stops insulating) at around 30,000 volts per centimetre (a rough figure for dry conditions). Rubber withstands more than that, but not by an order of magnitude.
Lightning's voltage is on the order of 100 million volts — far beyond what a few centimetres of rubber boot can withstand. To current that has already punched through several kilometres of air, a few centimetres of rubber is no obstacle at all.
A car, on the other hand, 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 on the interior. This property is called electrostatic shielding, and the enclosure is known as a Faraday cage. The rubber tyres have nothing to do with it.
UniversityStep voltage, written as an equation
When current I spreads through the ground, the electric potential at distance r from the strike point can be approximated, with ρ as the ground's resistivity, as V(r) = ρI / (2πr) (assuming uniform spread over a hemisphere).
If your two feet sit at 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 gets smaller — i.e., 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 idea is formalised as "allowable touch voltage and step voltage" in grounding design standards for power transmission equipment and substations — material covered in electrical and high-voltage engineering.
ResearchWhat's still unknown
- Why does lightning start in the first place? This is a famous unsolved problem in atmospheric electricity. Breaking down air requires roughly 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 happen anyway. Theories such as cosmic rays acting as a trigger (runaway breakdown) have been proposed, but the question remains unresolved. This happens every day, all over the world, yet exactly how it begins is still not settled.
- Where a strike will land still can't be predicted. We can observe a storm cloud approaching, but we cannot tell in advance exactly where the next strike will hit. That's why safety guidance isn't about "prediction" — it's about getting away from the danger zone.
- The "lightning crouch" posture is being reconsidered. It used to be taught as standard practice, but the US weather agency dropped it from its recommendations around 2008, reasoning that "it gives no meaningful protection, while creating a false sense of safety." In Japan it's still presented as a last resort, but it's more accurate to understand it as "something to do only when nothing else is possible, more a consolation than a safety measure," not "a posture that will save you."
- Ball lightning remains unexplained. Reports of a glowing floating sphere during thunderstorms exist all over the world, and there are even a few chance observational records, but it has never been reproduced or satisfactorily explained.
- Winter lightning on the Sea of Japan coast is unusual even by world standards. Discharges with more energy than typical summer lightning have been recorded there, and research is ongoing, partly driven by the damage it causes to wind turbines.
In short, lightning is a familiar phenomenon that still sits at the frontier of research. Treat this article's content as "what's understood so far."
How this connects to the textbooks (by level)
| Level | Subject/unit | Where it appears in this article |
|---|---|---|
| Middle school | Science – static electricity and current / properties of sound / weather | What lightning is, calculating distance from thunder, cumulonimbus clouds |
| High school | Basic Physics – current and voltage / heat / waves (speed of sound) | Voltage and current figures, why thunder rumbles for a long time |
| High school+ | Physics – electric field and potential / electrostatic shielding (advanced material in many textbooks) | Dielectric breakdown, why rubber doesn't help, why cars are safe |
| University | Atmospheric electricity / high-voltage engineering / power engineering | Step voltage equation, grounding design, lightning protection equipment |
| Research | Physics of lightning discharge (unsolved) | The lightning-initiation problem, predicting strike locations, reassessing the crouch posture, ball lightning, winter lightning |
| ― | Disaster prevention / safety education | Move once you hear thunder, go indoors or to a car, calling emergency services 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), including distance from trees and utility poles and the concept of zones of protection.
- U.S. National Weather Service, Lightning Safety (including the background to 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 equipment and grounding design standards (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 so on vary considerably depending on the scale and measurement conditions of a given strike. This article presents commonly cited representative values.
※ This article is a general-audience science explainer. For real decisions about how to act, follow announcements from the Japan Meteorological Agency and instructions from local authorities, fire services, or site managers. The figures given are approximate values meant to help you understand the underlying mechanisms.