For the most part, Fall is the best season for commuting by bike.
1) Nice cool mornings make the pre-work shower unnecessary
2) Fair weather Freds begin their hibernation...as the season goes on, the trail is less and less crowded
3) Not as much rain as the spring
There's one thing that sucks, especially on days where it's 40 degrees in the morning and 70 degrees in the evening. Thos are the Gnats of the Potomac. Along the Mt Vernon Trail, near the Pentagon I've been attacked by a swarm of these pests twice in the past week. It's a coordinated attack with hundreds, maybe thousands landing all over my arms and legs. I swallow a few, causing me to cough uncontrollably for a mile or so. The gnats have a particular fondness for cyclists...yesterday, I ran in the same area and was ignored by all the gnats. An hour later, I rode through and my arms were covered.
Not sure why it happens in only one short section, nor why they don't attack me while running. Did a couple of googles and couldn't find an answer, nor a mention of the Potomac River Gnats anywhere on the Interwebs. Maybe there's a gnat expert out there that can give me an explanation.