Did you feel the freezing cold wind last night? I think that was a good example of Tsukuba Oroshi (筑波颪), a cold, dry wind that blows in from the north/northwest.
There is disagreement about the meaning of “Tsukuba Oroshi”. Some people think that the name comes from the fact that the wind blows down (吹き下ろしてくる = fuki oroshite kuru) from the top of Mt. Tsukuba. The kanji for “oroshi” consists of the character for “down” (下) sitting on top of the character for “wind” (風). This seems to lend weight to this argument. However, others suggest that oroshi simply means a kind of local wind pattern.
From the bit of research that I did (mainly poking around a few websites), it seems that the word oroshi does usually mean a wind that blows from the top of a mountain, but that the mountain has to be between 1000m and 1500m for this phenomenon to occur. Mt. Fuji, therefore, at over 3000m in height, is too tall to cause an oroshi. Furthermore, Mt. Tsukuba, at 876m, is too short.
So, Tsukuba Oroshi, it seems, is just a local wind pattern that happened to be named after the largest mountain in the area. This is also the case for Haruna Oroshi, which is a wind pattern that was named after Mt. Haruna in Gunma Prefecture.
Regardless of the details of the naming of this wind, Tsukuba’s winters are made extra cold by the existence of Tsukuba Oroshi, so remember to dress warmly, and be sure to have some wind-proof clothes on hand, especially if your main form of transportation involves a bicycle or your own feet.
By the way, it has been said that local farmers planted forests consisting mainly of keyaki (zelcova) trees on their property to protect their homes from the Tsukuba Oroshi. The keyaki tree is now the official tree of Tsukuba. (But the question remains, is Tsukuba Oroshi the official wind of Tsukuba?)