The way the weather affects MPG here in the South is based on A/C compressor usage. My Tour is a '96 GL 5-Speed. In the past, it got 31 average summer, and 32-33 in the winter. Of course the compressor runs on humid mornings like today as the windshield would fog up on vent setting.
Gonna just keep enjoying 36 MPG for as long as it lasts.