Fog, holy crap the dense fog. Visibility down to barely 100' in some places. And yet I saw MANY motorists, as much as 1/3 of all motorists encountered on my 15 mile commute, driving with NO LIGHTS on at all!!! Absolutely incredible to me. Do people not THINK as they roll out of the driveway in the morning?
Had mostly no problems except the @$$hole riding a couple feet off my rear tire as I was climbing a hill. The fog and oncoming cars with no headlights made it worse because he couldn't really see very well to pass. But he passed anyway. Oncoming traffic had to slow down. I yelled some obscenities. And then life went on.
*edit* fog looked about like this (Google search image from India):
