Of course its all relative. Here in Central Texas it's not hot till it gets over 100°F. Over in West Texas even higher. Last week at 19:00 Hrs when ridding my short route it was 116°F three feet above the asphalt. I was feeling it. Luckily most of my ride is over torn up asphalt and light gravel and not as hot. Halfway through my ride I passed a house with a bunch of elementary school kids playing with the hose in the front yard. I turned and motioned for them to spray me down and they got a big kick out of it. Especially when a motor cycle rider stopped for a spray too. Do what ya can to beat the heat. As far as training is concerned I guess it might be as hard to get some people to ride in the heat as it is ta get me to ride when its 40°F...
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No matter where you're at... There you are... Δf:=f(1/2)-f(-1/2)