How much will the heat slow me?

Running is a furnace. Only about a quarter of the energy you burn becomes forward motion; the rest comes off as heat. To hold your core temperature steady, your body sweats, and as that sweat evaporatesoff your skin it carries the heat away. Sweat that drips off does nothing, it’s the evaporation that cools you.

That’s why the thermometer only tells half the story. The real driver is the dew point, a direct measure of how much moisture is already in the air. When the air is humid, your sweat can’t evaporate, so it can’t cool you. Your core temperature climbs, your heart rate drifts up to pump blood to the skin, and the pace that felt easy at the start becomes a fight. A 20°C morning at 90% humidity is far harder than a dry 24°C one.

The slowdown isn’t linear, and it isn’t the same for everyone. A cool, crisp day around 10–12°C is close to ideal; from there, every few degrees (and every point of dew point) adds a little more. On a genuinely hot, humid day a well-trained, acclimatized runner might lose a few percent, and a recreational, unacclimatized one closer to double that.

The marathon is hit hardest of any race. The longer you’re out there, the more heat you accumulate and the more dehydrated you become, so the penalty compounds over the back half. That’s why the tool scales the hit by distance: the same conditions cost a 5K far less than a marathon. A few percent sounds small until you do the arithmetic, on a 3:30 marathon, 4% is eight minutes.

Race organisers don’t use air temperature to make safety calls, they use WBGT(wet-bulb globe temperature), a single number that folds together heat, humidity, sun and wind. It’s the standard behind the coloured flags you see at the start, and behind the decision to slow, shorten or cancel a race. Here’s how to read the band the calculator gives you:

Heat is one of the few race-day variables you can train for. Around 10–14 days of deliberate heat exposure, easy runs in the warm part of the day, or finishing sessions overdressed, drives real adaptations: your blood plasma volume rises, you start sweating earlier and more efficiently, and your heart rate and core temperature run lower at the same pace.

The effect is large, acclimatization can roughly halve the penalty, but it fades within a couple of weeks if you stop, so time it into the run-in to your race. If your race is hot and your training has been cool, assume the unacclimatized end of the range and plan accordingly.

• Reset the goal before the gun. Use the adjusted pace, not your cool-weather one. Chasing the old number on a hot day is the single most common way to blow up.
• Bank nothing early. Start at the easier end and let the race come to you. Heat punishes an aggressive first half twice over.
• Drink to thirst, with electrolytes.Take fluid at most stations, but don’t over-drink plain water, pair it with sodium (see the fuelling guide) to protect against cramps and hyponatraemia.
• Cool from the outside. Pour water over your head, neck and forearms; take the sponges; run the shaded line. Evaporative cooling buys real seconds.
• Know when to stop. Chills, goosebumps, dizziness, confusion or stopping sweating are signs of heat illness. Back off or pull out, no finish line is worth it.