I’ve been reading the guidelines about heat acclimation but can’t see where it says what constitutes a hot environment. Is there a set temperature (taking in to account humidity) or is it relative difference from what temperature you’re used to training at?
It is more about your core temperature rising above normal, rather than temp. My basement is around 70 degrees, but without a fan, my core temperature can easily rise a few degrees and my performance will eventually suffer. And studies show that heat acclimation can improve performance even while performing in a cool environment. Long story short - hard to say there is an exact temperature where heat acclimation is needed. But the warmer the anticipated ride, event, training…the more beneficial heat acclimation will be.
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I don’t know how valid it is but Garmin (at least on the 1030) starts calculating heat acclimatisation at 22deg C (72deg F).
Edge 1030 Owners Manual - Heat and Altitude Performance Acclimation (garmin.com)
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Thanks very much, that’s really helpful 
Thanks HLaB
https://runnersconnect.net/dew-point-effect-running/
A bit of info on core temp and heat/dew point
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Living in north Florida, where we get mild winter temps and horrendous summer temp/humidity, I have to acclimate every spring. We’ve been riding in lower humidity 70s or lower (and a few low 80s days) for months, and last week it flipped to seeing upper 80s and higher humidity and next week will be seeing some 90s. The next couple weeks will be a bit of a struggle, so I’d say low to mid 90s is a definite inflection point, for me. But riding in the morning (higher humidity) at upper 80s can be miserable even in the middle of the summer.
Have seen plenty of people in other places with lower average temps say they struggle to acclimate at much lower temps, though, so maybe a 20 degree swing is an inflection point.
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Thanks MikeMunson
I’m with you @MikeMunson on the 90F being an inflection point for myself, although it’s a dry heat in NorCal.
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Heat stress is best predicted by something called the “wet bulb” temperature. Wet bulb temp is measured in a somewhat hilarious way - you take a thermometer, put it in something like wet pantyhose, then spin it around in circles over your head, then take the temperature reading.
The human body has 4 main mechanisms by which it gains heat from the environment, or tries to eliminate excess heat (say, generated by exercise) to the environment around it:
- Radiation: heat transfer via electromagnetic radiation (IR). This is why you get hot standing out in the direct sun, wearing a black shirt.
- Conduction: heat transfer via direct contact with another object at a different temperature. Imagine holding a cold can of coke against your neck to cool down.
- Convection: heat transfer due to a fluid moving past your body. It’s why a nice breeze cools you down, and you’re at the highest risk of heat illness when riding with a tailwind.
- Evaporation: heat transfer via promoting phase change of substance in contact with your body. ie; sweating, and the subsequent evaporation of that sweat.
Convection and evaporation will predominate as the mode of heat dissipation when you’re riding your bike when it’s hot outside. If it’s cooler, then radiation plays a large role too.
Wet bulb temperatures take into account all of these potential mechanisms of heat transfer, and are why they are the best method to measure potential heat stress. You can occasionally find them reported on specialty weather websites.
The degree of heat stress your body experiences depends on how effective it can remove excess heat from your body.
- If its very hot and humid, you lose much of your ability to eliminate heat via evaporative losses (sweating).
- If you’re in direct sunlight at high-noon, you will gain much more heat via radiation than if you are out in the early morning or evening, even if the air temperature is otherwise the same.
- If you are riding a bike, the air movement you generate will help cool you down more than if you were moving slower. Conversely if you start riding with a tailwind, your convective losses drop dramatically and your ability to eliminate heat is much worse.
The take away point here is that there is a lot of variation based on environmental factors alone (wind, humidity, angle of the the sun, etc.), other than simply ambient temperature (which is measured in the shade, not in direct sunlight on top of a road surface that itself is radiating even more heat back to you from below).
In addition, there is large variation between individuals in their bodies ability to deal with excess heat. This is due to genetic factors, differences in heat acclimatization, medical comorbidities, and medications they may be on.
On average, if you only look at ambient air temperature, healthy athletes start to lose efficiency due to heat at an air temperature of ~25C.
However, this is highly variable. You may do completely fine on a cloudy, dry day day with a light headwind when the ambient air temp is 30C. And you may get significant heat illness when riding at high-noon in the sun on a humid day with a tail wind, when it’s only 22C outside, after you’ve been indoors all winter.
If you want to acclimatize faster, you basically just need to put yourself into a warmer environment than what your bodies used to. What this ambient temperature would be is highly variable. If you feel warm, then that’s probably good.
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I’d expect it’s relative but there’s an inflection point in the low 90s (F, dry bulb). At that point sensible (without sweating/evaporation) heat loss to the environment becomes pretty much negligible (and a few degrees higher, you’re gaining convective heat from the environment).
Below that you can, maybe, balance heat production and shedding by slowing down (a lot, probably). Above that sweating is your body’s only way of shedding heat.
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for myself, it used to be lower, say low 80s. But after 3 years of riding outside in the heat, my inflection point for late afternoon rides seems to have increased to around 90.
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Thanks cnidos! That’s really helpful