Glossary
Dew point
The temperature at which air becomes saturated. Above about 18 °C (64 °F), sweat evaporates poorly and running pace costs rise sharply.
What it is
The temperature at which air can hold no more water. It is a better guide to how a run will feel than relative humidity, because it does not change as the air warms through the day.
Cooling depends on sweat evaporating. When the dew point is high, it does not evaporate well, so you keep the heat.
| Dew point | How it runs |
|---|---|
| under 10 °C (50 °F) | no noticeable cost |
| 13-16 °C (55-61 °F) | slightly sticky |
| 16-18 °C (61-64 °F) | pace starts to cost |
| 18-21 °C (64-70 °F) | hard efforts get much harder |
| over 21 °C (70 °F) | reconsider the session |
What it is not
It is not humidity. A relative humidity of 90% at 8 °C (46 °F) is a pleasant morning. The same figure at 28 °C (82 °F) is not.
It is also not the whole story. Ely and colleagues found that slower runners lose more to heat than faster ones over a marathon.
What moves it
- Air mass, not your route. It is a property of the weather.
- Time of day, a little. It moves far less than temperature does.
- Sun and wind. Both change what a given dew point feels like.
When to ignore it
Ignore it for easy running. Slow down and the day is fine.
Ignore the exact number in favour of the sum of temperature and dew point, which is the index most heat adjusters use.
Sources
- El Helou N et al. Impact of environmental parameters on marathon running performance. PLoS ONE 7(5):e37407, 2012 primary
- Ely MR et al. Impact of weather on marathon-running performance. Med Sci Sports Exerc 39(3):487-493, 2007 primary
- Vihma T. Effects of weather on the performance of marathon runners. Int J Biometeorol 54(3):297-306, 2010 primary