Question about Freezing Fog

[Atleastitwillbemild]

Freezing fog is probably my favourite type of winter weather phenomena. I love the sight of it even more than snow but it seems to be a pretty rare occurrence in my area (Thames Valley/Marlborough Downs) and only happens for one morning out of the year about once every 3 or 4 years. 
We do very well in most cold snaps for frost and fog and I know how both weather features form, but it seems to be very tricky to get both of them to occur at the same time.
My question is: under what conditions are freezing fog patches more likely to form than just normal frost? Is it more likely when the cold is from certain sources or wind directions? Is it dependent on absolute stillness of wind? Or temperatures below a certain threshold? The last time I witnessed a particularly spectacular display of rime was during the Dec 2010 freeze when there had already been snow on the ground for a good fortnight and the temperature had been fluctuating between minus teens and a few degrees below zero for days on end. I wondered what could have been so different about the conditions that caused the freezing fog to form on that night only, during a run of seemingly (to my untrained eye) pretty identical weather conditions to the other days of the week. Any explanation would be helpful. Thanks