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in case it was not plain enough to me that heat + humidity was a bad combination: https://www.psu.edu/news/research/story/humans-cant-endure-temperatures-and-humidities-high-previously-thought/ it gets worse with age.

Heat input + Heat generated = Heat output (evaporation + conduction/convection) to maintain temperature. Neglecting heat input and conduction/convection.... If you're doing nothing, just sitting around and are an average male, you'll be burning 2000-2500kcal a day. Assuming the latter, that means 104kcal per hour. At 540cal/g, you'd need to vaporize 192g of water (192mL) to maintain temp. If you are riding tempo, you're likely going to be burning 600-800kcal/hr. Assuming the latter, that means you need to evaporate 1.48 liters of water per hour to not overheat. If the humidity doesn't allow you to evaporate that much water, you will either overheat or (more likely) your body will force you to slow down. I need to research convection cooling. I am assuming it's very small compared to evap, but I'd like to find out for sure.

Yep, it's all about the airflow. When you stop for a light, it feels horrible, and the sweat starts accumulating, but the payoff is when you start moving again. It feels like AC. On days when evap is not great, I also take an insulated water bottle with lots of ice and splash a little bit in the crooks of my arms and on my head if I hit a red light.

now the question is: what trail should we just nickname "Air Fryer" now? I don't think I am capable of creating enough "air flow" to make a different. maybe that's why road rides feel slightly less horrible, especially if you avoid the hilly routes that involve slowly grinding up the climbs.

Thanks. I was familiar with wet bulb temp and heat index as the "feels like", but I was definitely not as familiar with what it means as far as evaporation. I had to read up on the details, and found this excellent summary which includes solar irradiation estimate. https://www.weather.gov/ama/heatindex BTW, increased convection helps with evaporation, but evaporation is different from pure convection. I would still like to see a separate chart that takes airflow into account. All the heat index stuff basically assumes you're in the shade and not moving. Looking at this from the engineering side, we always have to take into account: airflow, ambient temp, humidity, and altitude. All those affect conduction, convection, and evaporation. For sure when I ride in the heat, I hate going up a slow hill which usually also means head wind is getting blocked by the hill, or I have a tail wind (the worst). This is why I stick to the flatlands. It feels way different even in high heat index conditions.

The "Heat Index" provided by weather sources is based upon calculating for evaporation based upon current temp and humidity. This is what it feels like outside to one of us hairless monkeys. Currently I see 87F air temp, 95F heat index, and 60% humidity. Normal skin temp range is 91 to 98 degrees. If the heat index (sweat effectiveness) is higher than current skin temp the heat will move from the air to the skin and increase core temperature. Add to this the heat the body is producing and is unable to shed and the rate of core temp increase steepens. This will be true with heat transfer via conduction, convection, and radiation. So, sweat evaporating from the skin is a convection path, the sweat-soaked shirt unable to evaporate fast enough and holding heat (water is efficient at holding heat) is a conduction path, and the sun shining on bare skin is a radiation path for heat. Let's say skin temp is 95F and the heat index is 97F. A 2 degree spread probably isn't enough to remove all the heat being produced by a rider. Add to this heat gain from sunshine, and the stored heat in soaked clothing and it is unlikely there is any other outcome than for the core to increase in temperature.

I was getting dressed to ride yesterday evening and checked the weather on my phone. It said the air quality was crappy and I’ve been having a lot of headaches lately that I thought were caused by heat but may be allergies, or dust, or a combination of all three. So instead of riding I took the dog to the Personal Trail Network for a hike and some trail work. He was all about it.