You know my neighborhood a bit. The elevation change from the water tower down to the WO&D is 100'. On the clear/calm nights without cold air advection there is cold air drainage and I just can't get that low. Could be worth at least 3 degrees over about a block. That's why to always be wary of point measurements in heterogeneous terrain.
Pretty cool lidar elevation map from Fairfax Co. https://lidar-fairfaxcountygis.opendata.arcgis.com/app/5d4d2187b01f4d62ba53787eea94496b