The core idea: the map shows air, not weather
Air has weight, and its weight varies from place to place. Where the air column is heavier than its surroundings, the map marks kōkiatsu (高気圧), literally high air pressure, with the single kanji 高. Air in a high sinks and warms as it descends, which dries it out, so highs usually mean clear skies. Teikiatsu (低気圧), marked 低, is the opposite: air rises, cools and forms cloud, so lows bring rain.
Everything else on the map describes how heavy and light air are arranged, which is why the evening news describes the map before the weather.
| Japanese | Romaji | Meaning |
|---|---|---|
| 日本付近は高気圧に覆われています。 | Nihon fukin wa kōkiatsu ni ōwarete imasu. | The area around Japan is covered by high pressure. |
| 明日は低気圧が近づき、天気は下り坂です。 | Ashita wa teikiatsu ga chikazuki, tenki wa kudarizaka desu. | Tomorrow a low approaches and the weather goes downhill. |
| 気圧の谷が通過する見込みです。 | Kiatsu no tani ga tsūka suru mikomi desu. | A trough of low pressure is expected to pass. |
Reading the marks: centres, numbers and isobars
Each high or low has a cross at its centre and a number under it, the central pressure in hectopascals. An open arrow shows which way it is heading, with its speed in kilometres per hour. The curved lines are tōatsusen (等圧線), literally equal-pressure lines. On JMA maps a thin line is drawn every 4 hPa and a thick one every 20 hPa, at 1000, 1020 and so on.
The spacing is the part to read. Pressure lines crowded together mean the pressure changes steeply, and the wind blows hard; lines far apart mean a calm day. Here is how a forecaster, then a colleague checking a phone, might put it.
| Japanese | Romaji | Meaning |
|---|---|---|
| 低気圧は東へ時速三十キロで進んでいます。 | Teikiatsu wa higashi e jisoku sanjukkiro de susunde imasu. | The low is moving east at 30 kilometres an hour. |
| 中心気圧は九百九十六ヘクトパスカルです。 | Chūshin kiatsu wa kyūhyaku kyūjūroku hekutopasukaru desu. | The central pressure is 996 hectopascals. |
| 等圧線が混んでいるから、風が強くなりそう。 | Tōatsusen ga konde iru kara, kaze ga tsuyoku narisō. | The isobars are packed, so it looks like it'll get windy. |
| 熱帯低気圧は天気図では熱低と書きます。 | Nettai teikiatsu wa tenkizu de wa nettei to kakimasu. | On a weather map a tropical depression is written nettei. |
| Mark | Reading | What it shows |
|---|---|---|
| 高 | kō (kōkiatsu) | Centre of high pressure: sinking air, usually fair |
| 低 | tei (teikiatsu) | Centre of low pressure: rising air, cloud and rain |
| 熱低 | nettei | Tropical depression, below typhoon strength |
| 台10号 | taifū jūgō | Typhoon number 10 of the year |
| 1012 | sen jūni | Central pressure in hectopascals |
| ⇒ 20km/h | jisoku nijukkiro | Direction and speed of movement |
| ゆっくり | yukkuri | Moving at 9 km/h or less |
| ほとんど停滞 | hotondo teitai | Barely moving, direction unclear |
The four fronts and their symbols
A zensen (前線), literally front line, borrowed from military language, is the border between a warm air mass and a cold one. JMA sorts fronts into four kinds by movement and structure. The symbols sit on the side the front is advancing towards, so a row of triangles pointing south-east tells you the cold air is pushing south-east.
Ondan (温暖) pairs two kanji for warm, kanrei (寒冷) two for cold. Teitai (停滞) literally means stagnation and heisoku (閉塞) blockage: the occluded front forms where a faster cold front has closed the gap behind the warm one.
| Japanese | Romaji | Meaning |
|---|---|---|
| 寒冷前線が通過したあと、気温が急に下がりました。 | Kanrei zensen ga tsūka shita ato, kion ga kyū ni sagarimashita. | After the cold front passed, the temperature dropped sharply. |
| 温暖前線が近づいて、雨が降りやすくなっています。 | Ondan zensen ga chikazuite, ame ga furiyasuku natte imasu. | With a warm front approaching, rain is becoming likely. |
| 前線が本州の南の海上にのびています。 | Zensen ga Honshū no minami no kaijō ni nobite imasu. | A front stretches over the sea south of Honshu. |
| 梅雨前線の活動が活発になっています。 | Baiu zensen no katsudō ga kappatsu ni natte imasu. | The rainy-season front has become active. |
| 秋雨前線が停滞し、雨の日が続くでしょう。 | Akisame zensen ga teitai shi, ame no hi ga tsuzuku deshō. | The autumn rain front will stall, and rainy days will continue. |
Confusable pairs on the map
Several terms share a kanji or a shape but mean quite different things.
| Pair | Reading | What differs |
|---|---|---|
| 高気圧 / 高温 | kōkiatsu / kōon | High pressure is about the weight of air; kōon is high temperature. A winter high can be freezing |
| 温暖前線 / 寒冷前線 | ondan zensen / kanrei zensen | Warm: long, gentle rain ahead of it, then milder. Cold: a short burst of heavy rain, gusts or thunder, then colder |
| 停滞前線 / 閉塞前線 | teitai zensen / heisoku zensen | Stationary: a long border that barely moves. Occluded: a short front near a mature low |
| 低気圧 / 熱帯低気圧 | teikiatsu / nettai teikiatsu | Plain teikiatsu means a mid-latitude low, usually with fronts; a tropical one has none and becomes a typhoon at about 17 m/s |
| 梅雨 / 梅雨前線 | tsuyu / baiu zensen | Same kanji, two readings: the season is tsuyu, the front is usually read baiu zensen |
Beyond the basics: seikō tōtei, the winter pattern
Seikō tōtei (西高東低) is four kanji that read as a sentence: literally west high, east low. In winter a cold, heavy high builds over Siberia and a deep low sits east of Japan. Between them the isobars run north to south, packed tight: presenters often describe them as tateshima, vertical stripes. JMA also calls it fuyugata no kiatsu haichi, the winter-type pressure pattern.
The pattern explains Japan's winter split. A cold, dry north-west wind crosses the relatively warm Sea of Japan and picks up moisture, then meets the mountains that run down the middle of the country. The moisture falls as snow on the Sea of Japan side, and the air that crosses the mountains arrives dry. So the mountains of Niigata dig out while Tokyo has blue skies and cracked lips.
| Japanese | Romaji | Meaning |
|---|---|---|
| 冬型の気圧配置が強まっています。 | Fuyugata no kiatsu haichi ga tsuyomatte imasu. | The winter pressure pattern is strengthening. |
| 等圧線が縦じまになっていますね。 | Tōatsusen ga tateshima ni natte imasu ne. | The isobars are running in vertical stripes. |
| 北西の風が強く吹くでしょう。 | Hokusei no kaze ga tsuyoku fuku deshō. | Strong north-west winds will blow. |
| 日本海側では大雪となるおそれがあります。 | Nihonkai-gawa de wa ōyuki to naru osore ga arimasu. | There is a risk of heavy snow on the Sea of Japan side. |
| 太平洋側は乾燥した晴れの日が続きます。 | Taiheiyō-gawa wa kansō shita hare no hi ga tsuzukimasu. | On the Pacific side, dry, sunny days continue. |
| 明日から冬型だって。 | Ashita kara fuyugata datte. | I heard the winter pattern sets in tomorrow. (casual) |
The pressure patterns of the year
Japan's seasons can be read as a sequence of named pressure patterns. In spring and autumn, highs and lows march east in turn, so the weather changes every few days. In early summer a front stalls across Japan; in high summer the Pacific high takes over; in autumn a second rain front appears before winter's seikō tōtei. A presenter in each season might say one of these.
| Japanese | Romaji | Meaning |
|---|---|---|
| 春は天気が周期的に変わります。 | Haru wa tenki ga shūkiteki ni kawarimasu. | In spring the weather changes in cycles. |
| 太平洋高気圧が張り出して、蒸し暑くなるでしょう。 | Taiheiyō kōkiatsu ga haridashite, mushiatsuku naru deshō. | The Pacific high will extend, making it hot and humid. |
| 梅雨明けはまだ先になりそうです。 | Tsuyuake wa mada saki ni narisō desu. | The end of the rainy season looks some way off. |
| Pattern | Reading | Season | What it usually brings |
|---|---|---|---|
| 移動性高気圧 | idōsei kōkiatsu | Spring, autumn | Fine spells alternating with lows every few days |
| 梅雨前線 | baiu zensen | June to July | Weeks of rain along a front that creeps north |
| 南高北低 | nankō hokutei | Summer | The Pacific high covers Japan: hot, humid and sunny |
| 秋雨前線 | akisame zensen | September to October | A second spell of long rain |
| 西高東低 | seikō tōtei | Winter | Snow on the Sea of Japan side, dry sun on the Pacific side |
| 南岸低気圧 | nangan teikiatsu | Late winter, early spring | Lows along the south coast that can bring snow to Tokyo |
From map to forecast: how presenters and people talk
Forecasters speak in a fixed, polite register: deshō for predictions, mikomi desu for expectations, osore ga arimasu for risks, and set phrases such as zensen no katsudō ga kappatsu, the front is active, and taiki no jōtai ga fuantei, the atmosphere is unstable. JMA's published style guide even lists terms to avoid: it prefers fuyugata no kiatsu haichi to plain fuyugata.
When people pass the forecast on, the register drops. The deshō becomes datte or tte, meaning so they say, and the terms shrink: fuyugata, zensen, teikiatsu. Two friends are planning a hike for the weekend.
| Japanese | Romaji | Meaning |
|---|---|---|
| 大気の状態が不安定になるでしょう。 | Taiki no jōtai ga fuantei ni naru deshō. | The atmosphere will become unstable. |
| 移動性高気圧に覆われて、穏やかに晴れるでしょう。 | Idōsei kōkiatsu ni ōwarete, odayaka ni hareru deshō. | Covered by a migratory high, it will be calm and sunny. |
| 南岸低気圧の影響で、関東でも雪が降るおそれがあります。 | Nangan teikiatsu no eikyō de, Kantō de mo yuki ga furu osore ga arimasu. | Because of a south-coast low, snow may fall even in Kanto. |
| 天気図を見ると、日曜は崩れそうだね。 | Tenkizu o miru to, nichiyō wa kuzuresō da ne. | Looking at the weather map, Sunday looks like it'll turn bad. |
| 前線が抜けたら晴れるって。 | Zensen ga nuketara hareru tte. | They say it'll clear up once the front has passed. |
Common mistakes reading a Japanese weather map
Reading 高 as hot and 低 as cold. The cause is that the kanji also mean high and low in words like kōon, high temperature. On the map they are pressure. A winter high over Siberia is bitterly cold; read 高 as heavy air and fair weather.
Taking the isobars for temperature lines or rain bands. The cause is that some foreign maps colour temperature. On a Japanese surface map the curves are pressure only, and their spacing tells you about wind.
Reading the front symbols as marking the cold side. The cause is that blue triangles look as if they belong to the cold air. In fact they face the direction the front is moving, towards the air it is about to replace.
Expecting the low itself to be where it rains hardest. The cause is picturing a low as one rain cloud. The heaviest rain often falls along its trailing fronts, sometimes hundreds of kilometres away.