One of these air masses is called the Ogasawara air mass. This air mass is located to the south of Japan, and it is warm and full of moisture. The Ogasawara air mass always arrives near Japan from the south around June.
On the other hand, the Okhotsk Sea air mass is located in the northern part of the Okhotsk Sea, which is still cold at that time. This air mass is cold and full of moisture and covers northern Japan.
This Okhotsk Sea air mass and the Ogasawara air mass mentioned earlier begin to push against each other over Japan around June. The two air masses then form a boundary between the two air masses called the east-west rainy season front and become stagnant. The cold air and the warm air push against each other, and this is what causes a lot of rain to fall over Japan. This is the rainy season.