Something I have learned- it is not all the heat, it is not all the sun, it is not all the humidity, but it is the mixture- known as the dew point. This is what affects me the most. I got this chart from Runner’s World and I find it to be helpful. Hopefully, it will help you out too.
|DEW POINT (°F)||RUNNER’S PERCEPTION||HOW TO HANDLE|
|50–54||Very comfortable||PR conditions|
|55–59||Comfortable||Hard efforts likely not affected|
|60–64||Uncomfortable for some people||Expect race times to be slower than in optimal conditions|
|65–69||Uncomfortable for most people||Easy training runs might feel OK but difficult to race well or do hard efforts|
|70–74||Very humid and uncomfortable||Expect pace to suffer greatly|
|75 or greater||Extremely oppressive||Skip it or drama|