Why a hot race feels so much harder (and how much it slows you)

I once ran a half marathon on a 78 degree morning with a dew point to match, went out on my cool-weather goal pace, and fell apart by mile 9. I was not unfit. I was ignoring the weather. Heat is not an excuse, it is physics, and it is worth understanding before you toe the line on a warm day.

What heat actually does to you

When you run, most of the energy you produce turns into heat, not motion. On a cool day you shed it easily. On a hot, humid day your body has to send blood to the skin to cool you down, which is blood that is no longer delivering oxygen to your legs. Your heart rate climbs at the same pace, you sweat out more fluid, and everything gets harder. Same effort, slower pace. That is the trade, and you cannot train your way out of all of it.

Dew point is the number to watch

Most runners check the temperature and ignore the dew point, which is backwards. Dew point measures how much moisture is in the air, and moisture is what stops your sweat from evaporating. Sweat that drips off you does nothing. Only sweat that evaporates cools you.

A 70 degree day with a low dew point is pleasant. A 70 degree day with a 68 dew point is a slog, because your main cooling system is jammed. When I plan a race now, I add the temperature and the dew point together and use that combined number to judge the day, which is exactly what the heat tab in the predictor does.

How much slower

It depends on the conditions, but the slowdown is real and it grows fast once the combined temperature and dew point climb. A mild warm day might cost you half a percent. A genuinely hot, sticky one can cost five percent or more, which on a marathon is the difference between 3:30 and 3:40.

How to race in the heat

• Adjust your goal before the start, not at mile 18. Decide your hot-weather pace in advance and run it from the gun.
• Start even more conservatively than usual. Heat punishes a fast start twice as hard.
• Drink to thirst and use the sponges. Pouring water over your head actually helps, because it does the evaporating your sweat cannot.
• Respect the effort, not the watch. On a hot day, run by feel and let the pace be whatever that effort gives you.

The runners who do well in the heat are not tougher, they are just realistic. They look at the dew point, accept the tax, and race the day in front of them instead of the day they wanted.