Prius in 100 Degree Heat

I noticed that when we had the heat wave (around 98 degrees) i actually was getting better gas mileage. I had all the windows down and it seemed my engine wouldn't go on as much. I was able to get to speeds of 40 mph still in electric mode. I was astonished. Then I had to ruin my tank of 58mpg by driving up to NY to visit the ex. but still I love the heat waves. I just have to carry an extra shirt to change into once I reach my destination. lol