Humidity is also an important factor. With extreme humidity, an 85 degree day can be a lot worse than a 90 degree day with low or no humidity.
When riding a portion of the Trans Am in Montana in late June a few years ago I rant into a few people who had started in the east in early May. They had stayed ahead of the extreme heat in the south and midwest and hit the west after any major threat of lingering snow storms. They would also likely cross eastern and central Oregon before the crispy critter hot days of late July and beyond. (I once did much of the Oregon portion of the Trans Am starting the second week of September as part of an organized event. We had a couple of days east of McKenzie Pass that were downright roasting. The high on the rest day in Sisters was in the mid to upper 90s.) Interestingly, I did the same Montana stretch last year but started a week earlier than the previous time. I encountered only one person riding the Trans Am east to west.