As the earth gives up its heat to the longer, cooler nights, fog forms in the low areas. A body of water provides even more moisture for the atmosphere.
I have to escape from the Pine River valley every morning. In the middle of summer it may only be pleasantly cool, but right now the first three miles of the ride to work have been shrouded in murk. A please-don't-kill-me-yellow vest seems like a good idea, unless the morning is cold enough for me to wear the nicer yellow Sugoi Stealth windbreaker.
Days are short enough and my commute is long enough for me to bring lights for the ride home. On a clear evening I don't really need them unless I stay late at work, but each day the sun sets a little earlier, while home gets no closer. And a few clouds can bring dusk even more quickly.
I warm up more slowly in the mornings and ride more deliberately in dusk, so the trip takes a little longer at both ends of the day. It's all part of the natural arc of the commuting season.