Outdoor temperature significantly effects your running performance. In cooler temperatures, your body has to expend energy to stay warm, by increasing your metabolism. In warmer temperatures, it has to expend energy to stay cool, by diverting blood flow to the skin and increasing perspiration.

Research shows that the average runner performs best at 43°F (men) and 44°F (women).

For every degree warmer or cooler, you’ll either run at a slower pace, or expend more energy at a given pace.

Here’s a simplified chart (data based on researching 10 years of marathon results. Coldest and warmest entries are estimates:)

Interestingly, women are slightly more effected by colder temperatures, while men are significantly more effected by warmer temperatures.
Temperature Highs And Lows Table.png

Interestingly, women are slightly more effected by colder temperatures, while men are significantly more effected by warmer temperatures.

Other Factors

These are critical factors that are not taken into account by the chart above:

Sun: your perceived temperature can easily be 10–15° higher in direct sun than in full shade.

Wind: generally, wind will cool you off, by carrying heat away from the body and encourage sweat to evaporate faster. Be aware that in extremely warm and humid conditions, wind can have the opposite effect. Remember that a convection oven uses wind to cook food more quickly!

Humidity: the moisture content of the air has a profound effect on your comfort and performance. This is because your body cools itself by evaporative heat loss. The drier the air, the more efficient this process. Runners perform best in lower humidity conditions, but humidity’s effects are most pronounced in warmer temperatures. There are a few ways you’ll see humidity reported:

Actual Humidity: this is simply the moisture content of the air, represented as a percentage. There are so many other variables that this figure is difficult to interpret

The complication is that the warmer the temperature, the more moisture the air can hold. So a moisture level that feels comfortable at a warmer temperature might feel extremely damp and saturated at a cooler temperature.

Relative Humidity: this is humidity expressed as percentage of the amount of water the air can hold at its current temperature. It’s a more useful representation of how dry or moist the air will feel.

Temperature / Humidity Index: an attempt to estimate the subjective temperature. In hot, humid weather, this number will be higher than the actual temperature

Dew Point: the temperature at which the air, with its current moisture content, will be saturated, leading to dew forming. This measurement isn’t well understood, but gives a very succinct sense of how humid the air will feel. If the dew point is 20° below the actual temperature, the air will feel dry. If it’s 2° below the actual temperature, the air will feel like a swamp!

Wet Bulb Temperature: this is the scientist’s preferred measure. It’s obtained by shrouding the bulb of a thermometer in wet fabric. Evaporative heat loss will cool the bulb.

The higher the humidity, the higher the wet bulb temperature. When the wet bulb and dry bulb temperatures are the same, the relative humidity is 100%.

Wet bulb temperature is the most useful metric for making safety judgments.

69° wet bulb: running becomes dangerous. Many marathons will be cancelled.

80° wet bulb: all vigorous activity is dangerous.

92° wet bulb: any longterm exposure is dangerous.

96° wet bulb: lethal within a few hours, even at rest and in the shade.

How to Handle Temperature Extremes

Running in the cold is much easier to manage than running in extreme heat and humidity. We have a marvelous variety of cold-weather gear for runners, from base layers to insulation to wind shells. Because your body creates so much heat while running, you generally won’t need more than a thin layer of insulation, but you’ll need coverage of all your exposed skin, especially at the extremities.

In heat and humidity, your best bet is to avoid the middle of the day, and to prefer shaded routes (a path through the trees is rather than a desert road!). Hydrate as much as you reasonably can, make sure you’re getting adequate electrolytes, and keep your distance and intensity moderate.

Alternatively, there’s always the treadmill!