What decides the first "winter wind" of the year?
— When the lines on a weather map crowd together, the north wind starts blowing
One day in late autumn, the news reports: "The first winter wind of the year has blown." This actually comes with clear conditions. And the wind's strength can be worked out almost exactly from how closely packed the lines on a weather map are.
It's late October. Until yesterday it was mild enough to go without a jacket, but this morning, the moment you step outside, a cold wind hits your face.
The leaves on the street trees all lift into the air at once, rattling dryly as they roll along the road. The sky looks oddly blue, and the clouds race by at tremendous speed.
On the evening news, you hear: "Today, the first winter wind of the season blew." Why does the wind suddenly turn wintry on one particular day?
There are only two reasons the north wind starts blowing
As autumn deepens, the continent to Japan's northwest cools rapidly. The cooled air is heavy and piles up near the ground, creating a region of high pressure. Meanwhile, over the ocean to the east, a low-pressure system develops.
Air is pushed from where pressure is high toward where it's low. The narrower the gaps between the lines on a weather map (isobars), the stronger the push. Because the Earth is rotating, the wind gets bent toward the north.
The first day both of these line up is the first winter wind of the season. In other words, it's the signal that "the winter pressure pattern has formed, and the lines have finally crowded together enough."
The "first winter wind" has clear-cut conditions
The first winter wind isn't just a vague seasonal expression. In Tokyo, the Japan Meteorological Agency is said to announce it under roughly these conditions: the period runs from mid-October to the end of November. The weather map must show the winter "high in the west, low in the east" pattern. And the wind must blow from the west-northwest to north, with a maximum speed of 8 metres per second or more.
In the Kinki region too, the Osaka weather bureau announces it under similar conditions. However, only Tokyo and Kinki make this call. In some years, no day meets the conditions, and the announcement never comes before winter sets in.
"High in the west, low in the east" means high pressure to the west and low pressure to the east. On a weather map, the isobars line up like vertical stripes running north-south. Whether these stripes are spaced wide or narrow is what decides the wind's strength.
The wind doesn't blow straight from "high" to "low"
Look at Figure 1. Although the air is being pushed from the "H" on the left toward the "L" on the right, the arrow points downward — from north to south. This is because the Earth is rotating. In the northern hemisphere, moving air keeps being deflected to the right of its direction of travel.
At altitude, where the pushing force and the deflecting force balance out, the wind ends up blowing along the isobars. This wind is called the "geostrophic wind." If the isobars run as vertical stripes, the wind blows from north to south.
Near the ground, friction from buildings and trees weakens the wind. This also weakens the deflecting force, so the wind blows slightly at an angle toward the "low" side. That's why Japan's winter winds usually come from the northwest.
Try moving the slider left to narrow the spacing between the lines. Halve the spacing, and the wind speed doubles. When a weather forecast says "the isobars are crowded, so the wind is strong," this is exactly what it means.
The first winter wind signals the onset of winter. Conversely, the strong wind that blows from the south at winter's end is called the "first spring wind" (haru ichiban). Both can be seen as announcements of the moment the weather map's pattern flips from one season to the next.
On days when the winter wind blows, Tokyo's sky is often clear and dry. That very same wind picks up moisture crossing the Sea of Japan, and drops it as snow or rain on the Sea of Japan side. Having shed its moisture over the mountains, the wind then blows down onto the Pacific side.
Summary
The first winter wind is the strong north wind that blows on the first day cold air has pooled over the continent and the "high in the west, low in the east" pattern has formed. The wind's strength is set by the spacing of the isobars: halve the spacing, and the wind doubles. Because of the Earth's rotation, the wind doesn't blow straight from "high" to "low" — it comes from the north instead.
The winter wind is the sound of cold continental air flowing out to sea.
The more crowded the vertical stripes on the weather map, the louder that sound becomes.
How the same winter wind creates snow on the Sea of Japan side is explained in Why does the Sea of Japan side get so much snow?, the force that bends the wind is covered in Is it true that bathtub whirlpools spin the other way in the southern hemisphere?, and how wind changes after crossing mountains is explained in Why does wind suddenly turn hot after crossing a mountain?
- From late October through November, look at the weather map in the forecast every day. Count how many vertical-stripe isobars cross over Japan.
- On the same day, look up the maximum wind speed for your area, for example on the Japan Meteorological Agency's website.
- Make a table to check whether more stripes meant stronger wind. Also check whether the first winter wind was announced on days when the number of stripes suddenly increased.
Isobars on a weather map are usually drawn every 4 hectopascals. The more lines there are, the narrower the spacing between them.
For those who want to know more — terms, formulas, and textbook connectionsWe've marked which level each part belongs to, from junior-high science to university-level courses
- JHSCovered in junior high school science
- HSCovered in high school "Earth Science Basics / Earth Science"
- HS+Advanced high-school content, or textbook sidebar material
- UnivNot taught in high school — content from university specialist courses (dynamic meteorology, geophysical fluid dynamics)
- ResearchNot even settled as "established theory" at university — something researchers are still investigating
JHSTerms: this phenomenon has a name
- "High in the west, low in the east": Japan's typical winter pressure pattern, with high pressure to the west and low pressure to the east. Also called the winter-type pressure pattern.
- Isobar: a line connecting places of equal pressure. On Japanese weather maps these are usually drawn every 4 hectopascals.
- Seasonal wind (monsoon): wind that reverses direction with the seasons. In Japan it tends to blow from the northwest in winter and from the southeast in summer.
JHSHSChecking with a formula: getting wind speed from isobar spacing
Assume the isobars are spaced 200 kilometres apart, and estimate the speed of the winter wind. Take the location as latitude 35°N, around Tokyo.
| In symbols | v = ΔP ÷ ( ρ × f × Δn ) where f = 2 × Ω × sin φ |
| In words | Wind speed = pressure difference between isobars ÷ (air density × the Earth's rotational effect × isobar spacing) |
| Where it comes from | It comes from the balance between the "pressure gradient force" pushing the air and the "Coriolis force" that bends it due to the Earth's rotation. Halve the spacing and the speed doubles. |
| Symbol | Meaning and units |
|---|---|
| v | Upper-level wind speed (geostrophic wind). Units: metres per second |
| ΔP | Pressure difference between adjacent isobars. Units: pascals |
| ρ | Air density. Units: kilograms per cubic metre |
| Δn | Isobar spacing. Units: metres |
| Ω, φ | The Earth's rotation rate (radians per second) and the latitude of the location |
| Pressure difference between adjacent isobars (4 hPa) | 400 Pa |
| Isobar spacing | 200 km (200000 m) |
| Air density near the ground | about 1.2 kg/m³ |
| Earth's rotation rate Ω | about 0.0000729 rad/s |
| Sine of latitude 35°N | about 0.574 |
| Ratio of surface wind to upper wind (weakened by friction) | said to be roughly half on land |
| Twice the rotation rate | 2 × 0.0000729 = 0.0001458 |
| Multiply by latitude factor to get f | 0.0001458 × 0.574 ≒ 0.0000837 |
| Density × f | 1.2 × 0.0000837 ≒ 0.0001004 |
| Multiply by the isobar spacing | 0.0001004 × 200000 ≒ 20.1 |
| Upper-level wind speed v (geostrophic) | 400 ÷ 20.1 ≒ 19.9 m/s |
| Surface wind (roughly halved by friction) | 19.9 × 0.5 ≒ 10 m/s |
About 10 metres per second at the surface — above the first-winter-wind threshold of 8 metres per second. If the spacing widens to 300 kilometres, the surface wind drops to around 7 metres per second, missing the threshold. A difference of just 100 kilometres in the gap between the weather map's lines is enough to decide whether the announcement is made at all.
HSHS+The balance between the pushing force and the deflecting force
HSIn high-school earth science, you learn about the "pressure gradient force" and the "Coriolis force" acting on air. The wind that results when these two balance and blows along the isobars is the geostrophic wind. In the northern hemisphere, low pressure sits to the left of the direction the wind is blowing.
HS+Near the ground, "frictional force" also comes into play, making it a balance of three forces. The wind weakens and cuts diagonally across the isobars toward the low-pressure side. The crossing angle is said to be small over the sea, and larger over bumpy land.
UnivThe geostrophic approximation and the boundary layer near the ground
In university-level dynamic meteorology, large-scale winds are treated using the "geostrophic approximation." This holds when the change in wind direction is small compared with the rotational effect — that is, when the "Rossby number" is small. Where isobars curve, centrifugal force is added and the wind is treated as a "gradient wind." The layer near the ground where friction matters is called the "atmospheric boundary layer." The structure within it, where wind direction twists with height, is known as the "Ekman spiral."
📖 For the derivation of the formula and further reading: Geostrophic wind (Japanese Wikipedia) / Coriolis force (Japanese Wikipedia)
ResearchWhat's still not fully understood
- Why does the timing of the winter pattern's onset vary from year to year? The strengthening of the continental high is thought to be linked to upper-level flow over the Arctic and to conditions in tropical sea-surface temperatures. But predicting it weeks in advance is still said to be difficult.
- Will warming push the first winter wind later? If the continental cooling weakens, the onset of the winter pattern could be delayed. But year-to-year swings are large, and there's said to be not yet enough data to state a clear trend.
- How predictable is wind within a city? Between buildings, the effect of friction varies greatly from place to place. Why wind strength differs completely from street to street under the same weather map is something that can only be known through detailed calculation.
In other words, even this article's explanation is "the best we currently understand." The geostrophic formula works well at large scales, but the actual wind at ground level is heavily shaped by terrain and buildings.
Connections to the curriculum (by level)
| Level | Subject / unit | Where it appears in this article |
|---|---|---|
| JHS | 2nd-year science, "Weather and its changes" / "Japan's weather" | High-west-low-east pattern, isobars, seasonal winds |
| HS | Earth Science Basics / Earth Science, "Atmospheric motion" | Pressure gradient force and Coriolis force, geostrophic wind |
| HS+ | Earth Science (advanced) | Surface wind with friction added |
| Univ | Dynamic meteorology / geophysical fluid dynamics | Geostrophic approximation, Rossby number, Ekman spiral |
| Research | Seasonal forecasting, climate change, urban meteorology | Year-to-year variation in the onset of the winter pattern, urban wind |
| — | Connection to daily life | Judging the next day's wind strength from how crowded the vertical stripes on a weather map are |
- Japan Meteorological Agency, explanatory notes on "seasonal phenomena (first winter wind)," and the announcement criteria of the Tokyo and Osaka Regional Meteorological Observatories (気象庁)
- Kogarashi (first winter wind) (Japanese Wikipedia)
- Geostrophic wind (Japanese Wikipedia)
- Ogura, Yoshimitsu, General Meteorology (一般気象学), University of Tokyo Press (東京大学出版会)
- Holton, J. R. "An Introduction to Dynamic Meteorology", Academic Press
※This article is a general-audience science explainer. The figures given are approximations meant to help illustrate the underlying mechanism. The exact conditions for announcing the first winter wind may vary in detail depending on how each weather bureau operates. On days of strong wind, please check official information from the Japan Meteorological Agency.