The amount that your core temperature rises during training is generally influenced more by your workout than by the weather. Athletes training with CORE know that their core temp can rise above 38.9°C/102°F even in very cool weather. That’s because almost all of the heat that causes core temperature rise is generated by the body itself, rather than being absorbed from the surrounding environment.

Heat generated inside the body

The human body is quite inefficient at converting chemical potential energy (calories) into mechanical energy (useful work). For example, cyclists convert only about 20% of their calories burned into power to the pedals. The other 80% of the calories expended becomes heat energy.

This means that a cyclist who outputs 200 watts at the pedals is actually generating a total of 1000 watts, 800 watts of which are in the form of heat energy. If you raise the power at the pedals to 300 watts, heat energy rises to 1200 watts. Consider that a typical electric space heater outputs 1500 watts of heat. So, pedalling a bike is like having a small space heater inside your body!

When at rest, humans generate about 100 watts total, mostly as heat energy. So, basic exercise generates 8–10 times the heat content of a body at rest.

Heat absorbed from the environment

The amount of heat we gain from our surrounding environment is generally a fraction of what we generate ourselves. For example, the heat gained from being outside on a warm sunny day is similar to that generated by only 100 watts pushed at the pedals.

Dispersing internal heat

Whether your core temp has risen due to intense training or from absorbing environmental heat, your body needs to disperse that heat. The ease at which it does so is based largely on your skin temperature. A low skin temperature makes it easy to disperse heat. A high skin temperature makes it more difficult.

Skin temperature is in turn highly influenced by air temperature, relative humidity, wind speed, and clothing worn. This effect on skin temperature is the main influence of weather on core temperature while training.

CORE’s Heat Strain Index

CORE’s new Heat Strain Index gives an objective value to show how hard your thermoregulatory system is working. Its formula uses both core temp and skin temp to arrive at a value between 0 and 10 (10 represents an exceptionally high strain).

On a cool, cloudy day, pedalling at 200 watts, your Heat Strain Index may be at 0 or 1, even if your core temp is at, say, 38.4°C/101°F. While you’re likely sweating during this workout, you probably feel quite comfortable because you’re not working hard to stay cool.

That same workout, even on a 22°C/72°F sunny day with low humidity, will likely feel comfortable. You’re only gaining an additional 500 watts of heat from the environment, and your skin stays cool because sweat evaporates quickly in low humidity and the wind generated by your cycling speed. Your Heat Strain Index is still likely below 3 or 4.

However, that same workout on a 22°C/72°F sunny day with high humidity may feel very uncomfortable. The high humidity means sweat is not evaporating, which makes skin temperature high. Your Heat Strain Index may be at 4 or more, and your core temp will likely keep climbing if you don’t take additional cooling measures.

But a 35°C/95°F sunny day with low humidity? Even though you’re gaining additional heat from the environment, the low humidity makes it easy for sweat to evaporate and the skin to stay cool. Your heat strain index may still be at 3 or 4, and you may even feel reasonably comfortable.

Conclusions

The amount of heat generated by training generally far exceeds the amount of heat absorbed from the environment, even on very warm days. This means core temp can rise quite high even on cool days if doing high intensity training. Likewise, a recovery-level effort may generate so little heat that your core temp will not rise much, even on a very warm day.

The weather conditions and your clothing choices do, however, influence how easily your body can shed that heat. CORE’s Heat Strain Index shows how hard your thermoregulatory system is working by comparing core temp to skin temp. When skin temperature is low, the Heat Strain Index is low, and you feel comfortable. When core temp and skin temp are high, the Heat Strain Index is high and your body is working hard to shed heat.

High skin temperature can be caused by high ambient temperature, high humidity, low wind speed, strong sun exposure, heat-trapping clothing, or water-proof clothing.

References

Heat Balance in the Human Body. The University of British Columbia.

Kenny, Natasha & Warland, Jon & Brown, Robert & Gillespie, Terry. (2008). Estimating the radiation absorbed by a human. International journal of biometeorology. 52. 491-503. 10.1007/s00484-008-0145-8.