Weather and how it changes

Air holds only so much water vapor

A cubic meter of air can hold only so much water vapor. That limit is the saturation amount, and it rises with temperature.
Humidity is how much vapor the air holds now, as a percentage of the saturation amount at that temperature.
Cool the air and it reaches saturation. That temperature is the dew point, and below it the vapor turns into water droplets.

The most water vapor a cubic meter of air can hold

Air warmed at the ground grows lighter and rises as an updraft.
Pressure falls with height, so the rising air expands, and expanding cools it.
Once it has cooled to the dew point, the vapor turns into fine water droplets and ice. Gathered together, these are cloud.
The height where cloud begins is the cloud base. The wider the gap between ground temperature and dew point, the higher it sits.

A weather map draws isobars through points of equal pressure, one every 4hPa.
Where pressure stands higher than around it there is a high, and where it stands lower, a low.
Wind runs from high to low and, because the earth turns, veers to the right of its path in the northern hemisphere. The closer the isobars, the stronger the wind.

Where warm air meets cold air there is a front, and a low near Japan carries two of them.
At the warm front on the east side, warm air climbs over the cold air. Nimbostratus spreads and light rain falls for a long time.
At the cold front on the west side, cold air shoves the warm air upward. Cumulonimbus builds, bringing heavy rain and thunder for a short time.
Once the cold front has passed, the temperature drops and the wind turns from southerly to northerly.

Cold front Cold air shoves warm air up. Cumulonimbus builds and heavy rain falls for a short time
Warm front Warm air climbs over cold air. Nimbostratus spreads and light rain falls for a long time
Stationary front Cold and warm air balance and the front stops moving. The early summer and autumn rain fronts
Occluded front The cold front has caught the warm front, and the low begins to fade

Four air masses of differing character sit around Japan, and which one holds sway changes with the season.
In winter the Siberian air mass pushes out, setting up high west, low east and bringing snow to the Sea of Japan side.
In summer the Ogasawara air mass covers the country, holding a run of humid, clear days.

Siberian air mass Cold and dry. Grows in winter
Okhotsk air mass Cold and damp. Appears in the early summer and autumn rains
Ogasawara air mass Warm and damp. Pushes out in summer
Yangtze air mass Warm and dry. Sends traveling highs across in spring and autumn

Winter High west, low east The Siberian air mass pushes out. Snow on the Sea of Japan side, dry clear skies on the Pacific side
Rainy season Stationary front The Okhotsk and Ogasawara air masses meet, and the front between them stops moving
Summer High south, low north The Ogasawara air mass covers the country. Humid, with showers in the afternoon
Autumn Traveling highs Highs and lows cross in turn, so the weather changes on a cycle