What is the heat index?

The heat index, or apparent temperature, combines air temperature and humidity into a single feels-like value. High humidity limits sweat evaporation, so the body cools less efficiently and the effective heat stress is higher than the thermometer alone suggests.

How much does heat slow endurance performance?

Performance is roughly stable in cool conditions but degrades as temperatures climb above the high teens Celsius. By the time the heat index reaches the mid 30s, distance runners commonly lose 5-10% of pace, and the risk of heat illness rises sharply.

What model does this use?

It uses the Rothfusz heat index regression to combine temperature and humidity, then maps the resulting apparent temperature to an estimated endurance slowdown based on published marathon-performance-versus-temperature studies.

Does fitness or acclimatization help?

Yes. Heat-acclimatized athletes sweat earlier and more, cool better, and lose less performance for the same conditions. The estimate reflects a typical unacclimatized athlete, so treat it as a cautious planning figure.

When should I stop or slow down?

Treat the risk category as guidance, not a medical limit. In high or extreme categories, slow your target pace, increase fluids and electrolytes, and watch for dizziness, chills, or confusion, which are warning signs of heat illness.

Sport Heat Index Performance Estimator

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Pace for the weather, not the watch

Heat is one of the biggest uncontrolled variables in endurance racing. Hold your cool-weather pace on a hot, humid day and you risk both a blow-up and heat illness. This estimator combines temperature and humidity into a feels-like heat index and translates it into a realistic performance adjustment.

How it works

First the tool computes the heat index using the National Weather Service Rothfusz regression, which accounts for how humidity suppresses sweat evaporation. The familiar form uses Fahrenheit internally, so Celsius inputs are converted, run through the regression, and converted back:

HI = c1 + c2·T + c3·R + c4·T·R + c5·T² + c6·R² + c7·T²·R + c8·T·R² + c9·T²·R²

where T is temperature in Fahrenheit and R is relative humidity in percent. The apparent temperature is then mapped to an estimated endurance slowdown, since studies of marathon times versus weather show performance falling progressively once apparent temperatures climb above the high teens Celsius.

Tips and notes

For example, 30°C at 70% humidity yields a heat index well above 35°C and a meaningful predicted slowdown — a clear signal to dial back goal pace. In cool, dry conditions the regression returns essentially the air temperature and little to no penalty.

Use the result to set a conservative early pace and a hydration plan. Pair it with the sweat-rate tool to size your fluid intake, and remember that acclimatized athletes will outperform this cautious estimate.