My in laws live on the West side of Novelty Road hill, just East of Redmond on the West facing slope.
We live on the East side of Union Hill above the Snoqualmie valley.
Temperature differences are usually 10F with them hotter. Since the Novelty hill faces West there, I figured it heats up and picks up the heat from all the pavement and buildings in Redmond, particularly with the normal Easterly flow.
It is a mystery to me why we're so much cooler, but there is normally an Easterly flow, so maybe there's some cooling effect from the way the air drops away down to the valley East of us. We're all forested around here for a couple of miles. The Redmond Ridge complex, North of us about 4 miles, is hot as well, likely from the pavement.
My guess is that it's like an airfoil flow over the hill and the lower pressure, faster air is cooler.