18 degrees C = ~65 degrees F. That's HOT to people? Wow. They shouldn't move to the south of the US! In the summer, our low temperatures will be more than 25 degrees C and our highs will exceed 35 C.

I don't ride below 65 without something over my knees. I have enough cranky knee problems - I don't need to make them worse. But - if it works for you and you suffer no ill effects, great! I have ridden in knickers and a sleeveless jersey, so go figure.