Along with thunder and lighting, fog is among my favourite atmospheric phenomena. The best thing about it is the way in which it reveals the characteristics of light: the diffusion around omnidirectional sources and the elegant linearity generated by point sources and sharp edges. The fact that it makes trees look atmospheric and intriguing is of considerable benefit.
The fog today is apparently so bad that they are canceling flights out of Heathrow. I find that a bit surprising, as I thought commercial jets had RADAR guidance systems for takeoff and landing, to use under such conditions. They are justified in being concerned about takeoffs and landings. Along with Controlled Flight Into Terrain, approach-and-landing accidents have accounted for 80 percent of fatalities in commercial transport-aircraft accidents from 1979 through 1991. Given how crowded the airspace around London must be, extra caution is probably warranted; I imagine they would not be taking huge financial knocks for canceling flights without good cause.
[Update: 22 December 2006] Many thanks to Tristan Laing for setting me up with another year of Photo.net hosting.