ELI5: Rainy season…

11 comments

1. [deleted]

2. Here’s the ELI5 explanation:

   Not rainy everywhere all the time even in rainy season.

3. Yeah, about 15 years ago, i mentioned to my student that ‘wow, rainy season seems really long this year’ because it had rained at least a bit every single day for well over a month, deep into July. He got angry, slapping the table saying ‘No! Rainy season ended Monday last week. It was announced!’

   So yeah. There is the official announcements and there is reality

4. IIRC it has to do with the movement of the Pacific high and how it pushes warm wet air into Japan causing rain. Once it “shifts” which is in normal years surprisingly predictable rainy season is over and we move into the hellscape season. Or course with the Jet Stream having shifted due to the combined forces of climate change and an El Nino year all bets are off.

   Better explaination and actual science rather than old man memory here:

   https://www.data.jma.go.jp/gmd/cpd/longfcst/en/tourist_japan.html#:~:text=Summer%20(June%2DJuly%2DAugust)&text=Early%20summer%20is%20the%20rainy,cool%20polar%20maritime%20air%20mass.

5. Tsuyu refers to a weather front where moist hot air from the pacific and cold air from the Asian continent, meet that rests over or around the Japanese mainland during June and July. The meeting point of these air masses allows the formation of rain clouds.

   The air meeting point moves northwards once the it becomes hot enough near the equator (aka summer) and those air masses push the Tsuyu air mass further north.

   ​

   Does it matter? well Tsuyu rain tends to be days long whereas summer rain is usually only a day unless its a typhoon. Tsuyu often prevents Typhoons reaching mainland Japan so now any new Typhoons that form can impact Japan.

   ​

   In recent years the summers have been quite rainy, but pre Covid we had long hot summers and cold winters. We had mild winters during the last few years with these wetter summers. We had snow this winter in Nagoya, the first time in a few years, so I think we will be having a long hot dry summer this year.

6. I’ve wondered the same thing. Personally, I don’t care what TV people say, if it’s raining a bunch – to me, it’s rainy season.

7. Just think of “Rainy season” as a misleading translation. It’s the position of an anticyclone (so very scientific and very measurable, and per se nothing to do with “rain”) which expands during the summer.

   When the anticyclone (which is located in the Pacific) is over a specific place, then the Tsuyu(“=rainy season”) is over for that place.

8. tl;dr i think it mostly has to do with how large the Pacific High really is and weather the rain is caused by a “stationary rain front” instead of migrating lows & swinging typhoons

   from my observation, the “rainy season has ended/begun” announcements are mostly related to the evolution over time of the strength & size of the North Pacific High (pressure)

   as the Sun heats the air near the equator, it rises up and then moves northward/southwards. as it cools, the air starts to sink again, and swirls clockwise (due to the Earth’s rotation) creating a high pressure area

   as summer begins, the Pacific High grows and then warm, moist air scooped up from the equatorial waters collide with the cooler continental air, causing a very long “rain front/line” that stretches from China to Japan

   given enough time, the Pacific High should be large enough to push the “rain front” northwards and then later dissipates

   a more calmer, stable weather should prevail under this high pressure regime giving the characteristic summer weather

   but occasionally, you have typhoons that form in the Northwest Pacific basin that meanders towards Asia and also sometimes migrating low pressure cyclones (low pressure, counter-clockwise) that move from mainland Asia and then across Japan. both these can still bring considerable rain given the right conditions

9. Someone correct me if I’m wrong.

   I think it also has to do with how fast solids and liquids heat up. Solids heat up and lose heat faster than liquids, that in turn transfer that heat to the air above them, but at different rates.

   Part of me thinks that we get the evening rains（夕立）because the continent got colder faster than the ocean we are in, so the colder wind comes down toward Japan (wind goes from colder places to warmer places because colder things are more condensed and thus creates pressure to fill the space the warmer places are vacating, as warmer air goes up). Then the colder wind meets the moist air around Japan and condenses that moist air, making it rain.

   I don’t know if this explanation works for Japan because we are not on a tiny island in the middle of the Ocean, so Japan’s own heat gain and loss probably has a big influence.

   I’m just procrastinating before going to bed. Thought it would be fun to brainstorm this one.

10. it seems pretty simple – we call it “summer” but summer doesn’t always have the first of hot days that end on the last day of summer. flowers don’t magically show up on the first day of spring. &c &c

11. Rainy season does not conform with the traditional Japanese concept of 4 seasons and thus just communist propaganda, please don’t listen to those pushing for an agenda. Japan is unique and has only 4 seasons, and now we are in summer, which is hot and sunny and humid, none of those “rainy season” nonsense.