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Japan seems to get a surprising amount of snow in winter, compared to other places that are as cold or colder.

The snow seems to be concentrated on the west side of the country, which suggests the lake effect, only with the sea instead of a lake; the prevailing wind from the west brings air that has picked up moisture going over the sea, and the moisture freezes and falls as snow.

But why then do places like Oregon and Ireland not get so much snow, despite being similarly on the receiving end of moist air?

Those places have an entire ocean to the west, as opposed to Japan which has a narrow sea and then a continent. Conjecture: it's the combination that does the trick. Air blowing over the continent becomes very cold, then blows over the narrow sea and picks up moisture, which then freezes out. So to get maximum snow, you need a continent and then a narrow sea or lake, followed by the land which is to receive the snow.

Is that conjecture correct? Or if not, what is the reason for the amount of snow?

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From my understanding, you are thinking along the right lines but also need to take mountains into account.

Ireland's mountains are lower than Japans. I think that Japan's mountains are a similar height to Oregon's but the high mountains are closer to the coast in Japan. (I don't know either area well, so this statement could be wrong.)

Another thing to consider is that in winter, convective showers are common over the sea. A longer sea track will have more showers and therefore more mixing of the atmosphere, raising the temperature of the air mass.

Winter convection cuts off over land, but showers can penetrate inland a little with the momentum they already have. The closeness of the mountains to the coast in Japan, as well as their height, will give more orographic enhancement to the amount of precipitation as well as possibly reinvigorating the unstable convection by forcing it to rise. Ireland's lower lying status won't have this effect as much.

Oregon has two ranges of mountains, the lower Coastal Range (<1000m) and the higher but 150km inland Cascades (~3000m). The Coastal Range has a similar height to the Irish mountains but are longer in their extent. The orographic enhancement of the Coastal Range will be much less than in Japan. The Cascades will then be in a rain shadow as the air will be less humid downwind of the Coastal Range. While the Cascades still cause orographic enhancement, this is nowhere near as much as if the Cascades were by the coast, which in Japan they effectively are. You can see this effect on the annual average precipitation map of Oregon: Average Annual Precipitation map of Oregon showing higher rates in the Coastal Range than further inland in the Cascades.

As an aside, I had wondered if the Sea of Japan might be warmer than around Oregon and Ireland; this would provide more heat, moisture and instability to the air reaching Japan. I had a quick look at February sea surface temperatures, and they seem broadly comparable to the other areas, at least for the past three years, so this enhancement mechanism seems unlikely. February appears to be the month that Japan gets the most snow, although I'm not sure if that's talking about peak snowfall or peak accumulation.

I found a few articles about Japan's snow online:

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    $\begingroup$ I think the orography matters. For Oregon, there is a low range of hills (the "Coast Range," heights <1000 m and typical heights closer to 400 m) along the coast and the Cascade range is 150 km from shore. On the western, sea-ward sides of the Coast Range, there can be up to 5 m of precipitation annually - mostly rain. For Japan, I wonder if the cold air, coming from East Asia, is generally colder than air coming in to Oregon. That air has been over the Pacific Ocean for thousands of kilometers. $\endgroup$ – John Mar 22 at 19:53
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    $\begingroup$ Rainfall map of ORegon: upload.wikimedia.org/wikipedia/commons/8/80/… $\endgroup$ – John Mar 22 at 20:00
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    $\begingroup$ @John, I agree with both your comments. I didn't know about the 'Coastal Range'. Hopefully this isn't bias speaking, but I feel both your comments support the points I made in my answer: larger mountains closer to the coast cause more rain, and that a longer sea track helps the air each a more stable 'steady state'. Perhaps I should have said that mixing increases the temperature of the air mass. I'll update my answer to work in some of your extra information. $\endgroup$ – Mehmet Mar 23 at 7:44
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    $\begingroup$ Does the rainfall image in the answer work well or would it be better to have it as a link? $\endgroup$ – Mehmet Mar 23 at 7:58
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    $\begingroup$ I agree that my comments support your answer. The image works well. $\endgroup$ – John Mar 23 at 15:35

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