This is where picking a bike that suits your climate/circumstances come in.

I commute on a bike (at least, if I'm not going out after work) in London. I have a foldable Brompton bike fitted with an e-bike conversion kit from Swytch. I can ride in the middle of summer (ok, maybe not in those 40C days we just had) and arrive with no sweat. And if it rains, no matter, I stop, fold up my bike and jump on the Tube, or a bus.