Sometimes people will say something like, "I just ran in 100% humidity and it was rough." If it were 100% humidity at 70F, then that's a 70F dew point and 140 T+D. Those are "bad" conditions for racing. But 100% humidity at 40F real temp is 40F dew point, and isn't that bad at all (T+D of 80). The T+D calculation takes into account the real temp and the humidity (as dew point) for conditions for running. Then, I use this adjustment chart for my pacing.
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So if it is 60F with a 60F dew point, or 70F with a 50F dew point, they're both T+D 120 to me. In both cases, I adjust by 1%. So if the goal pace was 7:03 under ideal conditions, then under a T+D of 120 it would be 7:07. Things really add up towards the warmer end when you get into "bad" racing conditions. I personally prefer not to race when the T+D is much greater than 130, which is something like 65F temp with a 65 dew point. While the adjustments are not perfect, they are a good starting point for finding your own personal adjustment data.
Back in 2018, I had a brief window of two weeks where we experienced a wide range of T+D and absent of fitness changes could see with my own data the changes in pace based on the T+D.
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Hope that helps explain some of the benefits of the methodology.