This last Golden Week, eight climbers died in the Japan Northern Alps. The weather during the holidays was very volatile even in the predominantly flat Kanto area, and except for a day or two when it seemed to be suddenly summer, overall it was pretty cold and gloomy. Some of the deceased climbers were described in the media as being "experienced", yet many or most were wearing only T-shirts and light windbreakers, with no gloves.
It's not charitable to say so, but to me that's gambling with Fate without sufficiently understanding the rules of the game.
I've written about this kind of thing before. It's always sad to hear these stories, and there seem to be more of them in the last few years. Perhaps that's simply because the graying of society is raising the average age of folks on the mountains.
Mountain weather is very changeable, particularly this early in the season, although I've seen some sudden and drastic changes even in mid-summer. It's always better to be over- than under-prepared, if you intend to descend the mountains alive.
I suspect that some of the people who were mentioned as having experience in, for example, the Himalayas may have been taking the Japan Alps too lightly, believing them safe by comparison. I can understand the reluctance to carry a tent or heavy coat on what is planned to be a fairly easy trek, but a good down jacket, while bulky, doesn't weigh that much, and it could save your life. The same is true for warm gloves and a knit ski cap or the like, neither of which add that much weight or bulk. Even a couple of negligible-weight, super-compact "space"/survival blankets added to your rucksack or stuffed into a pocket "just in case" might well be the difference between dying or surviving.
I'd really like, for a change, to start hearing more stories about climbing groups whose "just in case" preparations enabled them to survive and return safely even when the unexpected struck.
From San Francisco CA originally, I'm a long-term resident of Japan with interests in motorcycles, good booze, good food, books, computers, and too many other things--both indoor and outdoor--to mention, even if discretion weren't indicated for some of them.