noun
cold mountain wind; cold Pacific wind in Tōhoku summer
A cool, moist easterly wind that blows from the Pacific Ocean toward the Tōhoku region in summer, often causing crop damage. The term originally referred to a cold wind descending from mountains, but in modern usage it is strongly associated with the regional summer wind.
今年のやませは特に冷たく、稲の生育が心配だ。
This year's yamase is especially cold, and we're worried about the rice growth.
山背が吹くと、夏でも気温が上がらない。
When the yamase blows, the temperature doesn't rise even in summer.
颪 is a general term for a cold wind blowing down from mountains, while 山背 specifically refers to the cold Pacific wind in the Tōhoku region in summer.
The exact derivation is uncertain; the kanji 山 (mountain) and 背 (back) suggest a wind coming from behind the mountains.