Dew point, wet bulb, and reading heat stress.

Temperature isn't the whole story

Air temperature, humidity, dew point, wet bulb, WBGT, solar radiation, wind, rain: they all affect how difficult it is for your body to get rid of heat. For runners this means that two days that have the same temperature on the face of it can actually feel very different, and result in very different race times.

That's why MeteoPace looks at more than just temperature when estimating the heat impact on your run. There are several different ways of describing heat and humidity, and each will tell you something different.

Temperature, humidity and dew point

Most apps lead with air temperature and relative humidity. They are useful, but can be misleading when comparing different days.

Relative humidity is a measure of how close the air is to saturation at its current temperature. Warm air can hold more water vapour than cool air, so the same amount of moisture can create a different humidity reading depending on temperature.

Dew point is generally a more useful number for runners. It is the temperature at which the air would become saturated if it were cooled without changing its water vapour content. If the air temperature is 15°C, for example, and the dew point is 10°C, then the air would need to cool by 5°C before condensation would begin.

The higher the dew point, the more water vapour is present in the air. This is why dew point is often more useful than relative humidity when considering how humid the conditions will feel during a run.

It also has a direct physiological consequence. Your body loses a large amount of heat through sweat; it is an important cooling mechanism for runners. As the water vapour content of the air rises, the gradient driving evaporation decreases, so sweat evaporates less readily and cooling becomes harder.

As a rough guide, a dew point below 10°C will generally feel pleasant for running. Mid teens will start to feel noticeably humid or 'sticky', and high teens and above make cooling while running increasingly difficult. This will have a significant effect on running performance. These are of course only broad guidelines, as other factors will also affect the feel of each run: acclimatisation, wind, solar radiation and exercise intensity will play their part too.

Wet bulb and WBGT

Wet bulb temperature combines air temperature and humidity to give an indication of how effectively the air can cool you through evaporation. The name comes from the way in which it is measured. Imagine a thermometer covered with a wet cloth or wick. As air moves past, water evaporates, cooling the thermometer. The temperature that it eventually settles at is the wet bulb temperature.

The wet bulb temperature depends on the humidity. In dry air evaporation is easier, and the wet bulb temperature falls significantly lower than the actual temperature. In other words, in these conditions a runner would cool more easily through the evaporation of sweat.

In humid conditions, the rate of evaporation is much lower, and the wet bulb temperature stays closer to the actual temperature. Runners in these conditions would find that their sweat stays on their skin rather than evaporating, and cooling is more of a problem. At 100% humidity, no evaporation occurs, and wet bulb temperature is the same as the surrounding temperature.

Wet bulb temperature is a useful way to understand evaporative cooling, but doesn't take into account the heat coming from the sun or surrounding surfaces. This is where wet bulb globe temperature (WBGT) comes in. WBGT combines measurements of wet bulb temperature, air temperature, and radiant heat to provide a broader measure of environmental heat stress.

It is measured using a sensor that includes a thermometer, wet bulb and black bulb (to measure radiant heat).

The American College of Sports Medicine uses WBGT bands for distance-running heat flags. Event medical teams and workplace guidance do too. It's a solid index for how hostile the environment is for keeping core temperature in check. A high WBGT is associated with a higher risk of heat related illnesses, such as heat exhaustion or heat stroke.

Flags for training

In MeteoPace, you'll see WBGT on the training weather view, along with a WBGT safety flag. The colours follow the ACSM distance-running guidance, the same white / green / yellow / red / black chart used for road-race heat flags. We use those colours as a session guide: whether today looks like a day for hard outdoor work, or a day to shorten, slow down, move indoors, or wait.

• White flag (below 10°C): increased risk of hypothermic collapse; wear cold-weather clothing
• Green flag (10-18°C): lower heat-risk level, but collapse can still occur
• Yellow flag (18-22°C): moderate risk of heat illness; ease off if you're at higher risk
• Red flag (23-28°C): high or very high risk of heat stress; slow your pace, and heat-sensitive runners should skip the hard work
• Black flag (above 28°C): extreme risk of heat stroke; avoid hard outdoor work

They can't predict exactly what will happen for an individual runner and should be treated as guidance only. Heat acclimation, fitness, pace and session length all matter too, and if a race organiser or medical professional gives you stricter guidance, follow it.

A flag is not your pace

WBGT is good at heat stress. That's the point of the flags.

A safety flag says the environment is getting nasty. It doesn't spit out your pace cost. Same flag, different answer for how much you'll slow, depending on your body, how long you're out there, how much sun and wind you actually get, how heat builds through the session, and how hard you're going. Heat acclimatisation, fitness, and the rest of the individual differences still count. The flag is the risk colour. Pace still has to consider the rest.

That's why we simplify it to a percentage change, a colour, and the time it costs you. Instead of asking you to add dew point, WBGT, and a flag in your head.

Planning a race? Find your race's conditions and course before settling on a target pace. If wind is part of the picture too, read when drafting helps, and when it can hurt.