You can see why Eastern Washington is called Big Sky Country in this photo of a Palouse farm around 1940. Source:Progress Commission Photographs, 1937-1945.

Washington State is host to a stunning variety of terrain and climate. West of the Cascade Mountains it is rainy and lush with mild seasonal weather. East of the mountains, however, the weather varies widely from season to season, and sparsely treed, rolling hills stretch out to a vast horizon. This iconic terrain was formed by cataclysmic floods that occurred at end of the last Ice Age, which swept massive amounts of soil and organic debris through Eastern Washington on its way to the Pacific Ocean, creating the same undulating surface one sees at the bottom of a river but on a colossal scale. The grain harvester trundling over these fertile hillsides is a common sight in this part of the state. In a few months, this same landscape will be blanketed in snow, and the hills will merge with the white sky until spring.

You can see this image and many others in the Progress Commission Photographs, 1937-1945.