# Impact of Heat on Marathon Performance

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It’s harder to run in hotter weather, something that is easy to forget when toeing the line at a marathon. If you’ve set goals based on cool weather training or performance without taking the heat into account, you will go out too fast and your overall time will suffer. But how much does your performance change with temperature? I found a study[1] that gives some answers.

## Contents

The study showed how finish time changes with temperature. The results are shown in the following graph.

To explain how this works, let’s use a couple of examples. You’ll need to know the heat index//feels like temperature that you expect at the race, and your projected cool weather performance.

• Runner A is expecting to run a 2:40 marathon in cool conditions, but the race is projecting 60 degrees. The 15C/59C line at the 160 minute mark indicates a 3% drop in performance, so the corrected time would be nearly 5 minutes slower.
• Runner B is expecting to run a 3:30 marathon in cool conditions, but the race is projecting 70 degrees. The graph only goes to 3 hours, so we will assume a liner progression, which is likely to be too conservative, but will give us an idea. At 3 hours, the 20C/68C line is 12 percent. If we scale by 4/3, that gives 16%. On 3:30 (210 minutes), 16% gives a corrected time of 4:04, a slowdown of 30 minutes.

## 2 The Details

The study looked at 140 marathon results from 6 races (Boston, New York, Twin Cities, Grandma's, Richmond, Hartford, and Vancouver). Only the first 300 finish times were used as races only started recording all finishers in the 1990s. This gives a pool of 42,000 finish times. The races were divided up into four groups The actual temperature measurement used is Wet Bulb Globe Temperature (WBGT) which is similar to ‘heat index’ given on weather forecasts, but WBGT includes the heat of the sun, which ‘heat index’ does not[2].