Interesting fuel economy drop with defroster

On saturday I drove to a 5K foot race 18 miles away on the new F1 Circuit of the Americas. Its 18 miles away and I thought I would get great mileage out there, so I zeroed the trip B and drove.

We had thick fog that morning, and I spent a lot of time between 40 and 55 mph because of poor visibility. I also had to use my defroster most of the trip. I was at about 45 mpg indicated, before hitting the traffic jam to the track. Its going to be awful when the real race happens and they get car race instead of running race traffic. When I parked I was at 42 mpg. The battery was at 2 bars.

Coming back without the defroster, and going much faster I got 54 mpg indicated. It would have been higher, but I was at about 30mpg after 2 miles as the car used gas to charge the battery.

Did most of the mileage hit come from the defroster? There isn't much elevation change, but hills in between, so its not from uphill versus downhill. Temperatures were in the low 60s going there, low 70s coming back. The car sat for 2 hours and it seemed to go through the full start up dance.