I asked what it was and it's apparently a snow dome, covering a microwave satellite tracking dish that my Grandfather built in the early 1960's. He helped to construct it when he worked for the Department of Defense at the USAF base nearby.
Once I got back to the house, I checked the family photo archive and sure enough, here it is under construction. It was sometime in 1964. The dome was built to protect the dish from weather, because snow and strong wind gales at this high altitude is a concern.
I know I posted one of these pictures before, but here it is again in its proper context.
It's not clear to me how the dome was set up from these pictures. It looks almost like they draped it over the dish and filled it with air like a hot air balloon.
The trees grew up around it in the last 50 years. But it's still being used by the US Air Force to track satellites.
Pretty cool if you ask me!