Why do places have different weather? Air masses
An air mass is a huge body of air, as big as a country, that has about the same warmth and wetness everywhere inside it. It takes its nature from the place where it sat for a long time. Air over cold land is cold and dry. Air over a warm ocean is warm and wet.
Around Japan there are four main air masses: cold and dry from the continent in the north-west, cold and wet from the cool northern sea, warm and wet from the warm Pacific in the south-east, and a warm and dry one from the continent in spring and autumn. Which one is in charge decides the season.
The sea matters too. Warm ocean currents flow past Japan. They heat the air and add water to it, so Japan is wetter than places on the same line of latitude inland on the continent.
Winter: snow on one coast, sun on the other
In winter the continent is very cold. Cold, heavy air sinks there and spreads out as a winter wind. It blows over the island from the north-west.
This wind is dry at first. But it crosses a warm sea, and the sea gives it a lot of water vapour. When the wind reaches the island it hits the mountains and is pushed up. Rising air cools, the vapour turns into cloud, and heavy snow falls on the coast that faces the continent. By the time the air crosses the mountains it has lost its water. The other coast, facing the ocean, gets dry, sunny winter days.
So in one country, on one day, one side can be buried in snow while the other side is sunny.
Early summer rain, hot summer, and typhoons
In late spring the warm wet air grows strong in the south. In June it meets the cold air from the north. The line where two different air masses meet is called a front. Here the two are of nearly equal strength, so the front hardly moves. It sits over the island and gives rain for about a month and a half. This is the early-summer rainy season (called tsuyu or baiu in Japan).
Then the warm air wins and pushes the front away to the north. Summer is hot and sticky because the wind carries wet air from the warm ocean.
From late summer to autumn the warm sea can feed a typhoon: a huge spinning storm. It gets energy from warm sea water. It moves north and may hit the island with strong wind, huge waves and very heavy rain. Spring and autumn are in between: small areas of high and low pressure travel past one after another, so the weather changes every few days.
Fronts, clouds and fog in simple words
Clouds form when air rises and cools. Cool air cannot hold as much water vapour, so the vapour turns into tiny drops of water (or ice). Rising air cools by about 6 degrees Celsius for every 1000 metres.
Fog is just a cloud that touches the ground. It forms when air near the ground cools down, or when warm wet air slides over a cold surface such as cold sea water.
On a cold front, cold air pushes under warm air and lifts it quickly. You get a short, heavy shower, maybe thunder, and then it turns colder. On a warm front, warm air slowly climbs over cold air. You get wide, gentle rain for hours, and then it turns warmer. A stuck (stationary) front is like the rainy season: it brings rain again and again.
Try it: make your own cloud
Ask an adult to help. Put hot water in a clear jar, to about a quarter full. Place a plate with ice cubes on top of the jar. After a minute you will see mist swirling inside, just under the plate. Warm wet air rose, met the cold plate, cooled, and turned into tiny drops: a small cloud. Now tap the season buttons in the 3D above and say aloud which air mass is in charge in each season.
Key formulas and definitions
- Air mass = a huge body of air with the same warmth and wetness
- Cold dry + warm wet meeting = a front = clouds and rain
- Rising air cools by about 6 °C per 1000 m
- Cloud = air cools until vapour turns into drops
- Fog = a cloud on the ground
- Typhoon = a spinning storm that grows over a warm sea
Worked examples
1. Why does the coast facing the continent get heavy snow in winter?
The cold dry wind from the continent crosses a warm sea and takes up water vapour. When it is forced up over the mountains it cools, and the vapour falls as snow on the first coast it meets.
2. Air at sea level is 28 °C. It rises up a mountain to 2000 m. What is its temperature, if it cools 6.5 °C per 1000 m?
Cooling = 6.5 × 2 = 13 °C. Temperature = 28 − 13 = 15 °C.
3. Why does the rainy season last so long?
In early summer, cold air from the north and warm wet air from the south are equally strong. The front between them hardly moves, so it keeps raining over the same place for weeks.
4. A foggy morning appears over a cold bay when warm wet wind blows in. Explain.
The warm wet air is cooled by the cold water. Cool air holds less vapour, so the extra vapour turns into tiny drops near the surface. That is sea fog.
Common mistakes
- Thinking the sea makes the cold wind wet only in summer. The winter wind also picks up water from the warm sea.
- Mixing up the two coasts: the snowy coast is the one that faces the continent, not the ocean.
- Saying a front is a wall or a line of wind. A front is the meeting line of two air masses.
- Thinking a typhoon can form over cold water. It needs a warm sea (about 26 °C or warmer) for energy.