Rainy season declared by the JMA

It is official, rainy season has been declared in Japan. Or at least my part of Japan. I am happy to report that it rained on the start day. There is a good chance of rain every day in the forecast.

A chance of rain

I just wish I could find the official JMA announcement of the commencement of rainy season. How do you really know when rainy season starts and when it ends? What is the criteria used, and what is the vehicle for the announcement?

One Reply to “Rainy season declared by the JMA”

  1. Criteria for beginning of rainy season
    When the Okhotsk high pressure in the north wanes and the Pacific anticyclone grows in strength in the south, a stagnated front where the two pressures meet makes weather rainy. This phenomenon is called Rainy Season in Japan.
    Normally Japan Meteorological Agency declares the beginning of rainy season in May or June.
    However there are no clear criteria for declaring beginning of rainy season even in Japan Meteorological Agency.
    The Agency judges it by considering all the various factors together including pressure pattern, type of front, it has been cloudy or rainy in a few days and such weather is expected to continue over the next 7 days.
    For your information, one divide called `beginning of rainy season` is among the system of 24 seasonal divides that originated in ancient China, which was established to roughly indicate the timing like seedtime and reap time for Agricultural reasons. This `beginning of rainy season` falls on around June 11th. This is where the ecliptic longitude of the sun is located at the angle of 80 degrees. This `beginning of rainy season` is different from meteorological `beginning of rainy season`.

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