Hi Aitch,
TBR stands for Temporary Basal Reduction. When I'm in the sun I find that by blood sugars also drop down quite drastically so I decrease my basal rates for the time I'm going to be in the sun. For example, I knew I was going to be outside in the very hot weather on Saturday so I lowered my basal rate from 0.80 units an hour to 0.60 units for the 6 hours I was out in the heat. It's trial and error but give it a go.
Hope this helps?
Katie