Microseason: Occasional Flash Rains

SUMMER

GREAT HEAT

OCCASIONAL FLASH RAINS

2 - 6 August

It’s the last microseason of Summer and the sky becomes volatile once again. Thunderclouds rapidly gather, and sudden downpours, brief but intense, sweep across the countryside. Unlike the long, steady rains of tsuyu, these bursts of precipitation arrive with force, transforming fields, roads, and waterways in a matter of minutes.

Japan's topography of steep mountains, short rivers, and narrow plains has always made it vulnerable to flash floods. Over the centuries, we have developed hydrological systems to accommodate rainfall variability. During the 700s, Kyoto’s grid was not merely aesthetic or political, it also reflected drainage logic, channeling runoff toward Kamo, the main river that runs through the city, and its branching canals. For many of these early urban channels, ishi-zumi, a technique of permeable stone masonry which allows water to seep slowly into the ground while resisting erosion, were put into place. And these are still seen today.

There are also hollowed bamboo culverts used to redirect runoff from thatched roofs into storage tanks or irrigation ditches. The bamboo’s smooth inner surface naturally guides water, while its segmented structure allows for lightweight joinery and quick repair. Eaves were designed with overhangs proportional to regional rainfall, and veranda floors included subtle gradients that directed water outward. The technique of gently sloping wooden floors toward open-air corridors was intended for cleaning, and it served as a passive drainage system anticipating sudden downpours like the ones we experience during this microseason.

Rice paddies, already heavy with ripening grain, must remain flooded but not inundated. Traditional paddies are bounded by hand-compacted earthen ridges that are meticulously maintained to hold water in place even under pressure from heavy rainfall.

Photo credit: Japan Railways