Why does Japan's winter coast see so much lightning?
— "Herring-wake" lightning strikes without warning, from low cloud
Lightning brings to mind summer downpours. But on Japan's west coast, in regions like Hokuriku, lightning actually strikes most often in winter. The key is the height of the "minus-10°C layer," where clouds generate electricity. In winter, this layer drops to just above the ground.
Late November, evening in Kanazawa. Cold rain starts mixing with white flecks, pattering against the window.
Then suddenly the sky flashes and a huge crash booms out. There was no distant rumbling beforehand, like you'd get in summer.
In Hokuriku, lightning in this season is called "buri-okoshi" ("herring wake-up"), long taken as the signal that the winter yellowtail fishing season has begun. Why does lightning strike in the depths of winter?
Just two reasons
Cold continental winds blow across the still-warm Sea of Japan. Air warmed from below keeps rising, building clouds that carry an electric charge even though they're low.
A cloud's electric charge forms at heights where the temperature is roughly minus 10°C to minus 20°C. In winter, this layer drops to just 2–4 km, bringing it close to the ground and to tall structures.
Put these two together, and you get lightning with a completely different character from summer's. Let's look at each in turn.
The winter Sea of Japan: a cloud-building hot water bottle
In winter, cold, dry northwesterly winds blow out from Siberia. Warm ocean currents flow into the Sea of Japan that these winds cross. Even in winter, the sea surface is far warmer than the wind blowing over it.
Air warmed from below becomes lighter, and rises carrying moisture upward. This is how the lines of cloud that bring snow to Japan's west coast form. The most vigorous of these become storm clouds.
Kanazawa records lightning on about 40 days a year — among the highest counts in the country. And most of it clusters from November through around January.
How low the charge-generating layer drops in winter
What generates electricity inside a cloud is collisions between ice particles. When hail (small balls of ice) collides with fine ice crystals, charge transfers from one to the other. This is especially active at heights where the temperature runs roughly minus 10°C to minus 20°C.
Temperature drops as you go higher — by about 6.5°C per kilometer of altitude. On a 30°C summer day at ground level, you need to climb above 6 km to reach minus 10°C. On a 5°C winter day, you reach it at just over 2 km.
Look at Figure 1. Turning the dial to change the ground temperature shifts the height of the charge-generating layer up and down. The dashed box on the left marks the summer layer's position.
Why do low storm clouds strike with just "one shot"?
Summer storm clouds grow tall, often topping 10 km. The charge collects high up, so it rumbles repeatedly on its way down to the ground.
Winter storm clouds are low, with their charge pooled right above the ground. So with little warning rumble, a single huge strike can suddenly hit. In Hokuriku this is called "ippatsu-rai" — "one-shot lightning."
Another feature is that lightning can shoot upward from the ground. As shown on the right of Figure 2, a discharge starts from the tip of a tall tower or wind turbine and climbs toward the cloud. Strong seasonal winds can blow the upper part of the cloud sideways, sometimes bringing the usually-hidden positive charge to face the ground directly.
Winter lightning strikes less often, but reports note that individual discharges can last longer and carry more charge. On the west coast, damage to wind turbine blades from winter lightning has become a real problem.
What generates electricity in the cloud is collisions between hail and ice crystals. If white hail falls in winter, it's a sign there may be a charged cloud right overhead.
So what should you do?
- If a lightning advisory is issued, even in winter, avoid open outdoor spacesSince there's little warning rumble, don't assume "it's still far off."
- When it strikes, take shelter inside a building or carStay away from tall trees and towers.
- If someone is struck, call emergency services right awayNo electric charge remains in a struck person's body. Don't hesitate to touch them and begin first aid.
On Japan's winter west coast, lightning strikes even on days with snow or hail. If you hear thunder while shoveling snow, fishing, skiing, or doing any other winter outdoor activity, take shelter inside a building or car right away.
Summary
A cloud's electric charge forms at heights where the temperature is minus 10°C to minus 20°C. On Japan's winter west coast, the warm sea grows storm clouds, while that charge layer drops to just above the ground. That's why lightning runs from low clouds, with little warning, sometimes upward.
What decides lightning isn't the season — it's the height of "minus 10°C."
In winter Hokuriku, that height sits almost within reach.
You can read more about how snow clouds form in "Why does Japan's west coast get so much snow?", and about how the winter seasonal wind begins in the article on the "first kogarashi." For the basics of staying safe from lightning, see "Why shouldn't you shelter under a tree during a thunderstorm?" too.
- Compare the Japan Meteorological Agency's "Lightning Nowcast" on a summer evening versus a day when a strong winter cold front arrives. In winter, you may see lightning markers lined up along the Sea of Japan coast.
- On a day the forecast mentions "strong cold air aloft," check whether a lightning advisory has been issued for Hokuriku.
- On a day hail falls in winter, note whether thunder followed. Multiply the seconds between the flash and the sound by 0.34 to get the rough distance to the lightning (in km).
Always observe from inside a building. Don't go outside while thunder is still audible.
Want to know more? — terms, equations, and textbook connectionsWe mark which level each part belongs to, from junior-high science to university specialist subjects
- JHSCovered in junior-high school science
- HSCovered in high-school "earth science" or "physics"
- HS+High-school advanced content, or textbook sidebar material
- Univ.Not covered in high school — university specialist subject (atmospheric electricity, meteorology)
- ResearchNot yet settled even at university level — something researchers are actively studying
JHSTerminology: this phenomenon has a name
- Winter lightning (tōki-rai): Lightning occurring along Japan's winter west coast. Strikes from low clouds, infrequent but prone to large discharges.
- Lapse rate: The rate at which temperature falls with altitude. On average, about 6.5°C per km.
- Hail (arare): White ice balls a few millimeters across, formed when water droplets freeze onto ice particles in a cloud. The main driver of charge generation in storm clouds.
- Buri-okoshi: Hokuriku's name for lightning that strikes from late autumn into winter, named for its overlap with the winter yellowtail (buri) fishing season.
JHSHSCheck with the equation: how high is the charge layer, summer versus winter?
Let's find the height at which the cloud reaches minus 10°C, using ground temperature and the lapse rate. We'll compare summer and winter.
| In symbols | h = ( T0 − T ) ÷ Γ |
| In words | Height at target temperature = (ground temperature − target temperature) ÷ temperature drop per km |
| Where it comes from | The approximation that temperature aloft falls in proportion to height (T = T0 − Γ × h), solved for height |
| Symbol | Meaning and unit |
| h | Height at target temperature (km) |
| T0 | Ground temperature (°C) |
| T | Target temperature (here, minus 10°C) |
| Γ | Lapse rate (°C/km). Using the average value, 6.5 |
| Lapse rate | 6.5°C per km |
| Summer ground temperature | 30°C |
| Winter ground temperature on the Sea of Japan coast | 5°C |
| Temperature at which charge generation is most active | minus 10°C to minus 20°C |
| Summer: temperature difference from ground to minus 10°C | 30 + 10 = 40 °C |
| Summer: height at minus 10°C | 40 ÷ 6.5 ≈ 6.2 km |
| Winter: temperature difference from ground to minus 10°C | 5 + 10 = 15 °C |
| Winter: height at minus 10°C | 15 ÷ 6.5 ≈ 2.3 km |
| Summer height as a multiple of winter's | 6.2 ÷ 2.3 ≈ 2.7 times |
In winter, the base of the charge layer sits at only about a third the height it does in summer. 2.3 km is roughly the height of Mt. Fuji's 5th station — a trailhead you can drive to. That's how close to the ground a cloud's charge really is.
HSHS+How does a cloud's charge separate?
HSAs air rises, it expands and cools (adiabatic expansion), so temperature falls with altitude. Inside a cloud, water droplets can stay liquid even below minus 10°C (supercooling), and hail grows as these droplets collide and freeze onto ice particles.
HS+When hail collides with ice crystals, charge transfers between them. Below roughly minus 10°C, hail is thought to charge negative and ice crystals positive. The light ice crystals get carried upward, while the heavier hail stays lower. This is how positive charge collects at the top of the cloud and negative charge in the middle.
Univ.Non-inductive charging and the character of winter lightning
In atmospheric electricity, charging from hail-ice crystal collisions is called non-inductive charge separation (the ice-collision charging mechanism). The temperature at which the sign of charging flips is called the charge-reversal temperature, thought to be around minus 10°C. Winter lightning is known to have a higher proportion of positive-polarity strikes (where a cloud's positive charge flows to the ground) than summer lightning. It also produces more upward lightning from tall structures (discharges beginning with an upward leader), and discharges with long continuing currents have been observed.
📖 For the derivation of the equation and further reading: Lapse rate (Japanese Wikipedia) ／ Lightning (Japanese Wikipedia)
ResearchWhat's still not fully understood
- The fine detail of charging. Why charge transfers in a particular direction when hail and ice crystals collide is thought to involve how ice grows on each surface, but it isn't fully explained.
- Radiation from winter clouds. Gamma rays lasting several tens of seconds have been observed coming from winter storm clouds in Hokuriku. The cloud's electric field is thought to accelerate electrons, but the precise conditions are still under study.
- How upward lightning occurs. Under what conditions, and how often, upward lightning strikes from wind turbines or towers remains an active research question, tied closely to lightning protection for wind turbines.
In other words, this article too describes things only "as currently understood." Winter Hokuriku is also a site that draws attention from lightning researchers worldwide.
Textbook connections, by level
| Level | Subject / unit | Where in this article |
|---|---|---|
| JHS | 2nd-year science, "static electricity," "weather change and Japan's climate" | Winter seasonal winds and west-coast clouds; charge separation from ice collisions |
| HS | Earth science, "atmospheric structure"; physics, "electric fields" | Temperature falling with altitude; how charge accumulates in a cloud |
| HS+ | Earth science advanced, "cloud physics" | Supercooled droplets and hail; temperature-dependent direction of charging |
| Univ. | Atmospheric electricity, meteorology | Non-inductive charge separation, charge-reversal temperature, positive-polarity and upward lightning |
| Research | High-energy phenomena in storm clouds; lightning observation | Elementary charging processes, gamma rays from storm clouds, conditions for upward lightning |
| — | Everyday relevance | Lightning advisories on Japan's west coast; sheltering while snow-shoveling, fishing, or at ski resorts |
- Japan Meteorological Agency, "Lightning Observation and Statistics" (気象庁)
- Kitagawa Shin'ichirō, *The Science of Lightning and Storm Clouds — From Lightning Discharge to Storm Cloud Electricity* (雷と雷雲の科学 ― 雷放電から雷雲の電気まで), Morikita Publishing (森北出版)
- Dōmoto Kōichirō, *The Science of Winter Lightning* (冬季雷の科学), Corona Publishing (コロナ社)
- Lapse rate (気温減率, Japanese Wikipedia)
※This article is a general-audience science explainer. The figures given are approximations meant to aid understanding of the underlying mechanism. When a lightning advisory or other weather warning is in effect, please follow the instructions of the Japan Meteorological Agency, your local government, and fire/emergency services.