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Ventoux is only 1900m high, unless there's bad weather you'll be fine.
We've been high on Croix de Fer and Galibier which are higher (Galibier a lot higher) and they were fine. I did use a wind jacket for the descent down to the Lauteret but took it off then as I was too warm.
Bainbridge-
Both of those I've been up in miserable weather in summer kit.
I think we rode the Galibier the day after or the day before people had to be carried off it during that Tdf stage sportive thing. It was snowing. CdF back in 2009 was pretty fucked - I was the only one that went up in a rain storm (obvs, I'm a fucking idiot) and, guess what? It's a LOT colder coming down than riding up. Something I didn't really think about as a fat flat track specialist with little experience in the mountains. I had borrowed someone's jacket/gilet but still had summer gloves, etc. That stands out as being properly grim descent but fun to think about because I didn't (quite) die. :D
hippy
Is googling 'temperature at mont ventoux' an acceptable way of predicting the weather at the top? I'm nearby, with my bike, but only summer kit. Reading online people say it's proper cold even in Summer, but forecasts seem quite mild using the aforementioned method