What heat and humidity actually take off your race time.
Worked out from temperature and dew point, then scaled to your goal pace, because slower runners lose far more time to heat than fast ones do. Calibrated against published marathon data, including the wet bulb globe temperature performance studies and El Helou's analysis of 1.8 million finishers. It can't see direct sun or wind, both of which make a hot day worse than the numbers suggest. 12°C and below is treated as a green light.
The number is the start. The Heat Penalty is my guide to what heat actually does to your training, the 2 numbers to check every morning and the pace correction to apply at each temperature, with the table.
If you train in heat and sub-3 is the goal, reach out. Send me the word HEAT on Instagram, tell me where you are with it, and I'll send you the guide.