Why is June the 'month of no water?'

The start of tsuyu, or the rainy season, has already started in Japan. I’ve always wondered how the beginning of the rainy season was determined, for no sooner is the season announced than we experience a week of beautiful, sunny weather. 

It’s as if mother nature is snubbing her nose at all the meteorologists and saying, “Think ye can pin me down, d’ye? Well, take that!” 

The answer to this question lies in first understanding why it rains. 

The baiu zensen

From about May to July, there is a stationary front over Japan known as the baiu zensen (梅雨前線, the seasonal rain front). Check a weather map of Japan at this time of year, and you’ll find a flat front with semi-circles on the northern side and triangles on the southern side. 

This indicates a stationary front or a non-moving boundary between two differing air masses. Because of the baiu zensen, long continuous rainy periods can linger in the area affected.

In East Asia, air masses of differing temperatures and humidity from areas of high atmospheric pressure over the Sea of Okhotsk and the Pacific Ocean meet along the baiu zensen, creating clouds and rain. The strength of these two regions bumping up against each other prevents the front from dissipating.

When there have been several days of unsettled weather, and it looks likely to continue to be rainy or cloudy for several successive days, the Japanese Meteorological Agency will announce that a particular region of Japan has entered tsuyu iri (梅雨入り, the rainy season). 

Similarly, tsuyu ake (梅雨明け, the end of the rainy season) is announced when the rains have stopped, and there have been consecutive sunny days. If the past is anything to go by, the rainy season will end in the northern part of Kyushu around mid-July, or just in time for the Hakata Gion Yamakasa festival.

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